Activities Log.

This page last updated Friday, October 27, 2017.

In Good Bear Day.
See also: More Changes.

This is my daily log page. It is what web users in their never ending assault on the English language have named a blog. I hate that word and that's the last time you will see it on my site. Instead of taking up bandwidth on the email list I will post my activities here so those who are interested can keep up and those who don't care don't have to be exposed to my more mundane writings, ravings, and rantings. I will report on things I am doing that may or may not become a part of the permanent site. Entries older than a certain number of weeks or months will be deleted. I haven't decided as yet on how long things will last here.

I often run projects in parallel so I won't jump from one to the other. At the beginning of each project there will be a centered heading with the title of the project followed by left justified dates for each entry concerning that project. The centered headings will help you jump from one project to the next. The date below the centered title will be the date I start a project. I will enter the entire project under that date in order going forward in time. When a project becomes mature it will either be given a page of its own on this site, or will be deleted as not important enough to warrant that honor.

Good Bear Day.

Friday, October 27, 2017

Someone or some group has declared today as "Good Bear Day" in honor of all teddybears. So Sue and I let four of our teddybears have a teddybear picnic in our side yard. Here are some pictures that we took. I am the one human shown. Trees have begun to turn here in Kentucky so one is featured in one of the pictures.

Teddybears enjoying themselves.

Our beautiful maple tree behind the bears' picnic.

I briefly joined the bears' picnic.

More Changes.

Wednesday, October 25, 2017.

In the two days less than 3 months I have been rather busy. Among the things I have done are wrestling with a printer which I should never have carried from the store to the car, trying to figure out why one of my motion sensing lights stopped working, trying, unsuccessfully, to find the same model so I could use the same mounting bracket, and learning how to operate a new o'scope and then writing a review of it.

My wife and I walked into Staples looking for a new printer because the old one was streaking quite badly and replacing the ink cartridges didn't fix it. She fell in love with a monster that turned out to be almost too tall to fit on my computer table. It was so heavy we had to destroy the box in getting it out. That meant we couldn't take it back. The thing has been trouble from the start. I won't go into the details but in a nutshell they are Trouble installing the software because one of the install screens didn't draw correctly, not knowing that the printer had two paper trays because the salesman didn't tell us, failing to print until the computer is rebooted, and the cost of ink cartridge replacement being in the neighborhood of 250 dollars. Yesterday I told her that I was going to take it out on the car port and bust it up with a sledgehammer. I do have one. She talked me down but I can still imagine how satisfying it would have been to lite into that printer with a 9 pound hammer. Or is it 15. This is a printer designed for a professional office and the salesman should have never sold it to us. But he doesn't care as long as he gets a nice commission. I have ordered a printer more suitable for a home office from Amazon and it should be in tomorrow. This monster is a combo scanner printer and it will function as a standalone copier. It's going into her office to serve in that capacity or into the storage shed where it will stay until it rusts away. And she can buy the ink carts out of her money.

Over the years I have tried several solar power motion sensor lights. The first ones were ruined by getting the sun directly in their eyes. I mounted the first one on the ceiling of the car port where it was impossible for the sun to shine directly on it. Other than that there is no place where a sensor can be protected from direct sunlight at every season of the year and every time of day. You may be thinking "put it on the north side of the house." But in the summer the rising and setting sun is north of an east west line and it will shine on the sensor. More recent models seem to have solved that problem and the sensors will stand up to direct sunlight. One of the lights stopped working and it turned out to be a defective battery. I couldn't find a replacement. It meant a trip back to Lowe's where I found one that looked about the same but wasn't. The battery is different but they look like something that will be around 3, 4, or 5 years from now. They look like shorter fatter C cells. Two of them. Even the plug on the end of the solar panel cable is different so I will have to install the new panel instead of using the old one.

If you have been keeping up with this site you will know that I wrote a review of a Hantek DSO that was not totally complementary. Back in May I bit the bullet and bought a Siglent scope which is on a par with Tek scopes. So is its price. It was on sale for a lower price and I grabbed it while I had the chance. I just finished writing a review and in the process learned a lot about it. The sale included a 4 channel 100 MHz scope and the 2 channel 200 MHz scope I bought. They were essentially the same price. After giving it a lot of thought I decided that I would be likely to use the extra bandwidth more often than I would use the two extra channels. About two weeks ago I saw a 4 channel 50 MHz scope on Amazon for about 300 dollars and on a whim bought it. I fired it up with all engines running to make sure it worked and haven't done anything with it since. I'll get around to reviewing it eventually.

And then there is the matter of my workbench. I found that the power supply sitting on top of the distortion analyzer was inducing interference into it and moved it. Here is a picture of my bench as it now stands, or sits whatever.

Latest version of my ever changing workbench.

The big scope, on top of the distortion analyzer, and the spectrum analyzer, on top of the Fluke power supply,, are not used in those locations. When one is needed it is lifted down to the bench top.

Changes To My Workbench.

Thursday, July 27, 2017.

If You scroll down to the heading with the date Tuesday, March 07, 2017 you will see the rolling cart I built to hold a number of pieces of test equipment. (Note: It might be quicker if you do a search for the date. Be sure to type it exactly as you see it here.) Because the spectrum analyzer was quite expensive and sitting on the very top I went to great lengths to tie it to the cart so it couldn't fall off. But the analyzer was very inconvenient to use when up there. Because I had gone to great lengths it was difficult to unhook and remove from the cart. Also I have replaced the Hantek oscilloscope with a Siglent model SDS2202X which is somewhat larger. I decided that some changes were necessary.

You can see the previous state of the equipment shelf by scrolling down to Monday, October 03, 2016. The shelf above has changed since then but the bench's equipment shelf is shown exactly as it was before the current changes. Here is how it looks now.

At first glance it looks as though I have 4 oscilloscopes. The two smaller screen units that are stacked are programmable digital function generators. The large scope on the left that sits on top of the Fluke power supply is the spectrum analyzer. Although it is high off the floor it is sitting on something that doesn't move so it should be secure. It is not meant to be used there. I take it down and place it on the bench for use. To the right is the Siglent scope which is a good one. You will be seeing a review of it in time. To make it fit I had to remove the Heathkit power supply and move the low voltage high current supply and the DMM to the top of the HP distortion analyzer. It also gets lifted down to the bench for use. These changes have made a bit more space on the equipment shelf. The end items no longer have their feet right on the edge. The Heathkit power supply is not needed now that the Fluke is working properly. The black case between the Fluke power supply and Siglent scope contains the directional coupler which works with the spectrum analyzer to make return loss and VSWR measurements. The Heathkit frequency counter now sits on top of the two function generators.

Case Closed.

Spectrum Analyzer Impedance Buffer.

Monday, July 17, 2017.

Looks like it's been almost another month. I suspect you know how it is. You get to working on a project and it is so much fun that you don't want to stop and document it. Well, it happens. The power meter is finished and now lives on a page of its own. It also lives on my equipment shelf where it looks good. Even Sue likes its appearance. This permits my return to the blind fisherman's buoy. To make some tests on that project I need to use the spectrum analyzer but the input impedance is too low to do what I need. And so it goes.

That's right, I now own an RF spectrum analyzer. In February I decided to spring for it. Not a Keysite, not an Agilent, not even an old HP, but a Siglent. Probably made in China but isn't everything these days? It goes up to 2 GHz although I would have been quite happy with 200 MHz. It isn't useful for audio work because it only goes down to 9 kHz and the tracking generator is specified to go down to 100 kHz although it is usable down to 50 kHz. That makes it worthwhile having for receiver IF and RF stages and filter work. Its main limitation is that the input impedance is 50 ohms. Well I hear the manufacturer saying, it is intended for RF work. True. But when I need to sweep out an IF filter at 80 kHz a 50 ohm input impedance is a little low.

My mission then is to design and build a buffer amplifier that will have an input impedance equivalent to that of an oscilloscope so I can use a standard 10 meg ohm probe with it and a bandwidth wide enough to match my needs. Also I want it to be very compact and battery powered, no tubes, sorry.

I don't expect there will be huge demand for this circuit because there might be one other person on the FWT list who owns a spectrum analyzer. When I finally come up with a circuit you can duplicate I'll leave it up for a while. BTW, if anyone knows of a good spectrum analyzer that does audio frequencies please let me know.

Bandwidth?

When I started out I had no idea just how much bandwidth I could get with transistors on hand. I had to decide what would be the absolute minimum I could live with and the answer came back 30 MHz. What would I really like? 100 MHz came to mind. Anything above 100 would be chocolate sauce on the ice cream.

I started digging through my store of transistors and found a couple of dual FETs and a few BJTs all with Heath part numbers. They must be replacement parts for a Heathkit oscilloscope. As such they probably have some pretty respectable Ft values. So I thought up a circuit using one of the FETs and one BJT. It looked like I could get 30 MHz out of it but it wasn't all that far down at 100 so I decided to try for it. Here is the original circuit.

I tried adding series RCs to the emitter with smaller capacitors but couldn't get the curve right. Guess I need to sleep on it as it is getting late.

Tuesday, July 18, 2017.

I must admit that I was not using the best techniques for broadband work. The tracking generator in the analyzer has a 50 ohm output but I didn't have it terminated. In fact I was using cables with clip leads on the ends for both input and output.

Also, these socket breadboards are not the best for RF work. I do have a version I built that incorporates some features that should make it a little better. When I mounted the socket I put a piece of Plexiglas under it to cut down capacitance to the chassis. It has BNC connectors on the left and right and a lid that can be closed. There is no power supply which means that power must be introduced from the outside or in this case included inside, a 9 volt battery that sits inside with the circuit board. Eventually you will see a picture of it farther down this page. There is an older one WAY on down. It is just after the heading "Thursday, December 01, 2016"

I snapped a 50 ohm through termination on the left hand BNC and ran the shortest possible cable from the TG output to the breadboard input. Since the amplifier has a 51 ohm resistor in its output I just ran a cable from the right hand connector to the analyzer's input. Of course. Signals go in on the left and come out on the right. Right?

That was a much greater improvement than I had expected. Next I investigated the effect of changing the value of the source resistor in the FET source follower. A large value, 10 k ohms, gave more gain while a smaller value, 2.2 k ohms gave lower gain but better high frequency response. This is what I would expect. The wide dynamic range of the analyzer means that high frequencies are more important than gain.

Oscilloscope vertical amplifiers that I have seen schematics of show emitter followers between every stage no doubt to isolate the miller capacitance. I think I'll add an emitter follower between the source follower and the BJT common emitter stage.

Monday, August 14, 2017.

The blind fisherman's buoy is as good as I can get it with parts from China. Any improvement will require a considerable outlay of cash. I will have to consult the end user about that. Meanwhile I can get back to the impedance buffer for the spectrum analyzer.

I eventually came up with this circuit. It gives a really good looking sweep from 0 to 100 MHz as shown below.

To be continued.

How Time Flies.

Tuesday, June 20, 2017.

It has been almost a month since my last entry. I have been working on the audio power meter while buying a minimum of new parts. That has meant recycling enclosures and using parts on hand. Something I would like to do more of because it saves money. It will without doubt make it harder for others to duplicate. The power meter will reappear on a page all its own within a short time.

I have also been discovering that the noise from power lines and neighbors security lights is at such a high level as to make radio astronomy impossible. Even ham radio operation will not be under the best of conditions. And then there was our wedding anniversary.

Sue asked me what I really wanted and I told her "a Fender Stratocaster". That was back in February and she said she would buy it right then. I suggested it might be more appropriate if we wait until June. On the second of June she said "Let's go buy your new guitar. Our anniversary is on the 19th. She bought me a brand new Stratocaster and a 6V6 amplifier to go with it. It has been several years since I played because my aging bones protested when I tried to play my big acoustic guitar. I had two problems. One was that my right arm went numb when I Reached around it to play for more than a few minutes and my left hand was in pain when I tried to form chords that were more interesting than C, D, E, or G. Yesterday was our anniversary and I bought her a cameo necklace. She likes them but they are very hard to find and expensive because they are out of style. Neither one of us worries about what's out of or in style. We wear what we like.

Here are three pictures of me with a captive audience, the Stratocaster, and amplifier.

Case Closed.

Radio Jove.

Monday, May 08, 2017.

The largest planet in our solar system, Jupiter, gives off intermittent radio emission in the HF band. Sorry, it's not little green men or women trying to make contact, It is totally natural. It is caused by electrons spinning around in the strong magnetic field of Jupiter and colliding with its largest moon IO pronounced I O. This became of interest to me when I was an undergraduate at the U of F. One of my friends from my home town and a fellow ham arrived at the university a year ahead of me and he found employment with the radio astronomy department as one of those lucky people who got to stay up all night to operate the radio equipment to catch the occasional, and at that time unpredictable, radio Jove events. The observatory wasn't complicated. It consisted of about two dozen Collins 75s1 receivers and some paper chart recorders that used an ink pen writing on a moving roll of paper.

After I entered graduate school the Jupiter program became more important to me as I began to see it as a possible connection to employment after I graduated. Long story short, I did get a job with the department which led directly to my position with Western Kentucky University where I did various jobs including teaching for 33 years before retiring.

Meanwhile the cause of the radio emissions from Jupiter has been figured out and the events, often called storms, can be predicted with more accuracy than weather on earth. Most of the research in this area has fallen into the amateur scientist area and it is being coordinated out of NASA's educational program. Anyone who is interested can buy a radio Jove kit for just a little over 300 dollars. The receiver is now all silicon based and confined to a single frequency of approximately 20 MHz. Long range radio propagation on this frequency is so rare that is little interference from terrestrial stations and the ionosphere is highly transparent to radio signals of this frequency even in the daytime. As a result the little receiver when connected to a simple half wave dipole antenna can pick up radio emissions from the center of the milky way galaxy and the sun. The latter is what has revived my interest in the project. In August there is going to be a total eclipse of the sun and the shadow of the moon is going to pass over my house. NASA wants to collect data from as many amateur radio astronomy stations as possible. They want data from in the path of totality, near the path and far from the path. Since the radio Jove program gave me my life's employment I thought I would offer a little payback by participating in the experiment.

So I have spent last week building the receiver which is pictured below just before putting the cover on it. The kit includes the coax, wire for the antenna, and insulators. I have to buy the masts myself. The best thing is that because it looks at the sky it doesn't need to be 50 feet in the air to work DX. 10 feet is about right to use the ground as a reflector for an extra 3 dB of gain over a dipole in free space. I need to get going on the antenna because NASA wants us to collect data of the normal sun over as many days as possible prior to the eclipse. I'll keep you posted on how things turn out.

To Be Continued.

Updating Two Heathkit Resistor Substitution Boxes.

Sunday, April 23, 2017.

Back in the distant past I bought and built two Heathkit resistor substitution boxes. My thinking then was in some circuits resistors come in pairs. My intended purpose for the boxes was circuit development rather than repair which was actually their original intended purpose. They were made with 10% 1 watt carbon composition resistors. Most of the resistors were near the edges of the tolerance range and over the years many of them have drifted out of tolerance. I decided it was high time to replace the original resistors with 5% carbon film resistors.

As with most jobs of this kind it turned out to be more work than I had anticipated. I started on Thursday the 20th with a little time out to marvel over my wonderful new camera. Since I have two boxes I can easily show before and after.

Having finished with one I intend to start on the other eventually but I have another iron in the fire that is awaiting my attention. I won't bother to write about the second one.

Case Closed.

Learning a New Digital Camera.

Saturday, April 22, 2017.

So why do I need a new digital camera? Isn't the one I have working just fine? I'm not one who will buy something I don't need, unless I want it. But in this case it is a need because of the nature of the old camera.

It's a Kodak. If you have been asleep or living in a cave or survivalist commune for the last 5 years or so you may not know that Kodak declared bankruptcy and shut down their operation. It's a shame because they did make a good camera that didn't cost an arm and a leg. I have been taking pictures with it since 2004. Most of the photography on my websites was done with that camera. So what's wrong with it?

It uses a very special rechargeable battery that is made out of unobtainium. It's charge discharge cycle is getting shorter and shorter. It won't be long before that camera will become useless.

Also the software that transfers the pictures and allows me to process them a little to fit and look good on my website won't install in Windows 10. An updated version is not available. My wife's computer is an XP box which is where I have the program installed and where I have to take the camera to upload pictures. That computer is on a home network with this one and after I get the pictures in the shape I want I transfer them to this Windows 10 box. I have a duplicate set of picture files on the two boxes witch doesn't hurt but I would like it better if I didn't have to run from one room to the other and Sue would like it better if I didn't keep messing with her computer.

My Brother-in-law is a member of Costco in Nashville and he bought me a Nikon outfit which on special for a couple of hundred dollars less than the price for the same one on Amazon. Here is a picture of it.

Obviously this was taken with the Kodak. It is probably the last picture taken with that camera that will be seen on any of my sites. The outfit came with two zoom lenses. The one mounted on the camera is an 18 to 55 mm zoom and the other is a 70 to 300 mm zoom lens. Not shown are a camera strap, carrying case, two rechargeable batteries, and charger. And by way of saying goodbye to the old Kodak which has served me well here is a picture of it taken with the Nikon.

As you might imagine the Nikon has lots of features which make it very complicated. I think I have learned the ones I am most likely to use but there are many more that I am only aware of but haven't mastered yet. By the way, the model is D3400.

Case Closed.

Repairing an old capacitor substitution box.

Wednesday, April 19, 2017.

Many years ago, before I was married, I had recently finished building a Heathkit capacitor substitution box. Its range is, I still have it, from 0.0001 uf (100 pf) to 0.22 uf. Almost within a month I needed values larger than 0.22 uf and so I built one of my own in a 3 x 4 x 5 aluminum box. It started at 0.2 uf and went up to 1000 uf in a 1 2 5 sequence.

A few days ago I needed it again and it took a bit of looking to find it. When I did find it not all the capacitors tested good. I accomplished my purpose while avoiding the capacitor values I had judged to be too leaky to be connected into a circuit.

Set the wayback machine for Monday, April 17.

I decided I needed to replace some of the capacitors and took it apart. What I found was appalling. The capacitors were not only leaky they were leaky. Almost all of them were exuding some kind of sticky oily substance. I took them out and threw them away. It took a good part of Monday to unsolder and remove the old capacitor leads from the switch lugs. I recovered the switch without breaking any of the lugs. Then I started selecting the capacitors I would use in the new version of the box. With values on hand I could make up 0.2, 0.5, 1.0, 2.0, and 5.0 uf with voltage ratings of 630 volts. That was a considerable improvement over the old version. It required some paralleling of capacitors to obtain the values within 2% or less. I can't really justify this especially now that capacitors come in the standard 10% values, for example 0.22 uf and 0.47 uf.

I was able to find or parallel 50 volt electrolytic capacitors over the range of 10 to 100 uf. This is also an improvement. For the 200, 500, and 1000 uf capacitors I was stuck with 25 volt parts.

I printed out a label on photographic printer paper, soldered the capacitors to the switch, and installed the switch in the box. I didn't give as much thought as I should have to how long the capacitor leads should be and what capacitors went where around the switch. I managed to squeeze everything into the box and tested the box. Two of the switch settings measured 7.5 pf on the digital RLC meter. I had broken two of the switch lugs during the assembly.

Tuesday, April 18.

The switch was already ruined so I cut the tops off the lugs to make it easier to remove the capacitors from the switch while preserving the caps. I soldered lengths of hookup wire to each capacitor to avoid putting stress on the lugs of the replacement switch. Luckily I had a replacement on hand. This time I carefully planned out what capacitors would go where so the largest parallel combination would have plenty of room. I soldered the wires that now served as capacitor leads to the switch lugs and things seemed to be a lot better.

Then I found that the label had been damaged and I scraped it off and printed out a new one. As I had done before, although not noted, I turned the printed label over to Sue to cut just inside the outer lines. She can do this with much greater precision than I can.

I sanded off the remaining dried glue from the front of the box. Then I glued the paper to it. In the past I have used and recommended the use of rubber cement but time has shown that it deteriorates over time and the edges of the paper start to curl up. Now I am using something called Aleene's tacky glue. It dries clear but doesn't get hard as a rock. It does a good job of sticking paper to metal. Ask me in 5 or 10 years how it holds up over time. The important thing is to spread it evenly over the surface of the metal panel all the way to the edge of where the paper will go. The product is available from Hobby Lobby and probably other arts and crafts stores.

Some have reported problems with the paper wrinkling under the heads of screws when they are tightened. I have not experienced this problem with good quality photo paper and glue that has had plenty of time to dry. I use flat washers under the retaining nuts on controls and switches. When possible I hold the screw still on the label side and turn the nut tight on the inside of the panel. But when all this is impossible I rely on the strength of the paper and glue and don't use all my manly strength to tighten the screws.

Wednesday, April 19.

Now we are up to this morning. I used an Exacto knife to cut away the paper from the holes where the switch and grounding post go. I installed both and started soldering capacitor leads to the common wire as shown in the photo below.

This time there were no broken lugs or other open circuits.

So I put it together and back on the shelf where it is ready to be used the next time I need a large capacitor for some kind of test.

Case Closed.

Swapping an HP 608D for a 608A.

Wednesday, April 12, 2017.

Now I am helping a totally blind friend who is also an avid fisherman. He needs to place a buoy at his favorite fishing spot and be able to keep track of where it is when on his boat. We are going to plant a low power transmitter on the buoy and use a receiver on the boat with a directional antenna to track it. After some discussion we settled on 433 MHz because transmitters and receivers for that frequency are easily found on eBay.

I did a series of distance tests using my Arduino to turn the transmitter on and off and powering the receiver from a 9 volt battery. It looked promising. I needed to test the receiver with a known and variable input level in order to locate a point on the board that had a DC voltage that was proportional to received signal strength. So I fired up my HP 608Dpictured here.

Imagine my surprise when I found it only went up to 420 MHz. I had been sure that It went up to 500. Fortunately I hadn't gotten rid of the first piece of used HP gear I ever owned, a 608A. Right after buying it I wrote a letter to HP explaining my situation and asking if they could supply a manual and any parts I might need. I received a manual by return mail and a very nice letter dated October 22, 1973. I'm sure the man who wrote it is long ago retired and very likely is no longer living. The 608A does go up to 500 MHz. Pictured below.

I opened it up for the purpose of reforming the electrolytic capacitors only to find that it had oil filled capacitors. I tested them under voltage anyway just to be sure but all was well. I found that it worked well in CW mode but when modulation was switched on the carrier level dropped to zero. I put it back together anyway and did the test on the receiver. There was no analog voltage proportional to signal strength. It is a transmitter and receiver set intended for data transmission by amplitude on or off and that's all it would do. A search on eBay turned up a superhet receiver that did have an analog output of signal strength. The interesting thing is that it costs more to ship than it costs. Both prices were less than two dollars. How do they do that and make any money?

While waiting for the arrival of the slow boat from China I removed the 608A from its cabinet again and started tracing out the modulator. Looking at the schematic I picked out a capacitor that if shorted or even a bit leaky would cause the symptom. It was one of those metal cased rectangular block capacitors that were used mostly in military equipment from the 40s and early 50s. I replaced it with a modern 630 volt capacitor that I could solder directly to the wafer switch and tube socket. That fixed it.

A look at wave forms on the scope was in order. The envelope looked good up to about 90% modulation but the meter seemed to saturate at about 75%. The meter read about right at 50 % which is probably where I calibrated it the last time I worked on the generator. I decided that was good enough so I put it back together.

Right after I got the generator, back in 73, the RF output level was quite low and I had replaced both of the pencil triodes in the RF box. This threw off the frequency calibration a little but I needed it for a project and accurate frequency readings were not necessary as I had access to a frequency counter. I'm pretty much in the same situation again. I have a project to do and there isn't time to get involved in frequency realignment. I have a means of measuring frequency with sufficient accuracy to do what I need to do. So the signal generator that is anywhere from 60 to 65 years old will have to wait a little longer for a frequency alignment. Although the last time I calibrated the output amplitude was 20 or 25 years ago when I checked it, it was accurate to within 1 dB. I wonder if the products that Keysite is making today will still be operational 65 years from now.

Case Closed.

A Nuvistor Preamplifier.

Sunday, April 02, 2017.

Another item I brought in from the shed was a Nuvistor preamplifier. This device gave me something to test with my recently acquired spectrum analyzer. Before being moved to the shed it had spent many years in the attic. I removed its case and reformed the electrolytic capacitor as with the tube tester from yesterday below.

This one formed up and did a good job of filtering the B+. Since it is obviously intended for use by hams I tested it on the traditional ham bands of 160, 80, 40, 20, 15, 10, and 6 meters. The results are shown below.

About the Spectrum Analyzer.

It is a Siglent SSA3021 100 kHz to 2.1 GHz spectrum analyzer. I ordered the tracking generator add on and that is what I used to make the above plots. The generator only has a 20 dB range of adjustment from -20 to 0 dBm. Even at the lowest setting it proved to be a little too much for the preamp. I inserted 20 dB worth of fixed coaxial attenuators into the line between the generator and the preamp. Now with the adjustment range of -40 to -20 dBm I could investigate the gain compression which I suspected was going on. Sure enough the compression (reduction of gain) was about 3 dB at -20 dBm input to the preamp. In all the pictures above the input to the preamp was -40 dBm and as you can see the gain on most bands was pretty close to 20 dB which is as specified on the Ameco website..

I will do an extensive review on this analyzer soon but for now I can say the most disappointing thing about is the lack of labels on the frequency scale as there are on the amplitude scale. It is possible to trick it in to revealing the frequency units per division as you see in the pictures. As the analyzer is set up, "frequency step" is equivalent to frequency per division. Without labels the frequency span must be kept a whole number so the arithmetic is easy enough to do in the operator's head. Values of start frequency and stop frequency may be entered via a keypad and so can take on any value unlike traditional analyzers that have switch selectable ranges and so are restricted to what the manufacturer provided. I'm not sure the new way is advantageous.

The little preamp works well and can be of benefit when using an older tube receiver especially on bands of 20 meters and higher frequency. Unless you are using a real antique it probably wouldn't help much if at all on 160 and 80 meters.

Scared the ____ out of me.

I did a Google search for the schematic and followed a link to BAMA. I received a warning from my anti virus program that the site was attempting to download militias software onto my computer. Sometimes a site that isn't part of the establishment will give this warning. While I was reading the information from my program and deciding if I would trust BAMA the malware must have found a way in because the page came up that said my computer would be deactivated in 5 minutes unless I called a phone number. Attempting to close the page failed. I have had this happen before and I didn't call and my computer was not deactivated. I decided to take a chance and unplugged the power cord. When it rebooted it worked and I did a virus scan. Nothing was found. I suppose the BAMA site has been hacked and I wouldn't go there for a while.

Case Closed.

Playing With an Old Tube Tester.

Saturday, April 01, 2017.

I'm waiting for some parts to come in for the Arduino project. So I thought I'd do a couple of things that have been languishing at the bottom of my to do list for at least 15 years if not longer.

When my older brother passed away, as the only surviving member of our immediate family, I wound up with a lot of his stuff. I passed some of it on to his friend in Texas but I kept a few things. One was the tube tester pictured below.

He was mainly a mechanic but he dabbled in electronics a little. This is an interesting little tester because it takes advantage of the fact that many tubes have the same base diagram. It is well known that the 12AU7 etc all have the same pinout. The 6AU6 is another with a large number of pinout companions. So I took it apart and used my bench power supply to apply 150 volts across the filter capacitor with no other power applied. It showed a just detectable amount of movement of the current meter so I disconnected the power supply and plugged it in. I measured 120 volts between the negative of the filter capacitor and electrical ground. Oops? Although it has a power supply it seems as if the B+ is derived directly from the line. So I plugged it into my isolation transformer and scoped the voltage across the capacitor. It showed peaks rising to 170 volts with a DC between peaks of about 50 volts. Hmmm. High ESR in the capacitor. So I replaced it. While I was getting ready to button it up I notice some broken insulation in the power cord. "Wake up stupid" I told myself. I was lucky not to have joined the great balls of fire club. Again. I replaced the cord and did some continuity testing. Sure enough the B+ was line connected but it was totally isolated from the chassis. There is a transformer in it. That's where they get the different heater voltages.

After getting it back together I grabbed a tube and tested it. There was no difference between the tube being plugged in and not plugged in. I guess I had better look for the instruction manual to get the schematic diagram. To my surprise and delight a Google search found it right away. Reading the operating instructions was an eye opener. No change in the indication of the eye tube means a good tube. As it turns out this is not a cathode emission tester. It is a grid emission tester. The correct operating procedure is to adjust the "eye adj" knob for just a thin sliver of dark in the eye tube with the tube under test not plugged in. Then set the heater voltage and plug the tube in. If there is grid emission the eye will open. So no change means a good tube. The manual discusses TV set symptoms that are caused by grid emission and stresses how fast a service man, (this was the 1950s after all", can find the problem tube using this tester. In the back of the manual the company is advertising its full blown tester, cathode emission, mutual conductance, and grid emission. The advertised price is $139.50. It would be worth having a time machine just to go back and buy things at the prices of 60 years ago.

Case Closed.

Learning to speak Arduino.

Thursday, March 09, 2017.

Everything new has its own lingo or as some call it bafflegab. The Arduino is no exception. I had bought the beginner's kit from MCM Electronics about 3 months ago but my incentive to open it up was a request from a totally blind friend who also is a ham for a talking SWR meter. It came in a nice plastic box and the lid is scanned below.

My years of experience with Heathkits taught me that the first step is to check the parts against the parts list. As you can see I have checked off every part except one. OK, there is one part left over and it has to be the "Prototyping Shield for Arduino" but some identification pictures would have increased my confidence.

I went to Arduino.cc and downloaded the IDE (Integrated Development Environment). I started it knowing that the board was not connected and the first words I saw in the window were "void setup". "Well" I reasoned "the Arduino isn't connected" so I connected it. No change. Now to me "void" means empty or invalid something is wrong. Roget's Thesaurus gives me the following synonyms.

I soon figured out that void had something to do with the programming language, I'm not quite sure what as yet. But I can't get the IDE to talk to the Arduino over the USB cable. If anyone can help I'd like to hear from you.

Friday, March 10, 2017.

The day after posting the appeal for help in this space I had help from Chuck who worked with me for more than an hour, maybe it was closer to two, and the IDE made contact with the Arduino. Following installation instructions on the IDE download page I ran a sample program named "Blink" and a green LED that was soldered on the Arduino board began to turn on and off. At this stage I was willing to settle for even small successes.

So on to project 1. There are 15 individual project cards. Below you see scans of both sides of the first one.

I have to admit that now that I see the picture blown up on my computer screen I understand what to do to connect the experimental circuit. However, I know enough about computers to know that without a program it's just a useless lump of silicon. There is no other literature with the kit than what you see above. It would be pointless to scan the other 14 project cards. What you see is what I got.

So I went to the DFRobot website and became frustrated because what I was looking for didn't jump off the screen at me. I admit, it is a fault of mine that at times I get short of patients and give up too quickly.

Sunday, March 12, 2017.

So I went to Amazon and searched on "Arduino for Dummies" in books. Remember books? There is such a book but it has a date of 2013 so I looked down the page for more recent releases. I settled on 3 recent books all listed as best sellers on Amazon. They are; I didn't make a mistake. That's exactly what is in print on the front of the third book. The first book listed is very thin and DaVinci read through it in less than an hour. That gave me just enough information to understand what came next. And what came next was, I had the good luck to see an email from someone with some Google search hints. One was to enter what you are looking for on a given site followed by "on website.com". Many websites have search engines that range from slightly helpful to totally useless. As I found the first time I tried it letting Google do the walking works a lot better than the pathetic search engine on the site itself.

If you click on this link you will see exactly the kit that I have. Probably the first thing you will notice is that it is out of stock. The email from MCM said it was a closeout price which was considerably less than the price from the linked site above. I suspect this is a discontinued product.

On this page was a U-tube video and some files to download. The most useful file was a zip file named "Starter Code.rar". I couldn't make it unzip correctly and I had to in effect put everything back together. There were a bunch of folders which were empty and the files listed below them which belonged in them.

I have now performed experiments 1 and 2. The wiring was identical except for the pin connections and the color of the LED. The color didn't make any difference but the pin connection did.

As you see above the LED and resistor were connected between pins "GND" and 13. When I loaded the program, which for unknown reasons is called a sketch, and compiled it the led didn't flash. Fortunately I had learned just enough from reading through the first book to find the trouble. The variable declaration "int pin = 10" placed the output on pin 10. Changing the line to "int pin = 13" made it work. You would think that the person who drew the figure and the one who wrote the program would have agreed on which pin to use. My first impression was that the kit was put together by a committee that never met.

Project two was exactly the same circuit but using ground and pin 10. The program was in agreement and it worked. It flashed the LED to send SOS on the flashing LED. The programmer didn't know the rules of Morse code so the timing was a little off. I decided to correct it. I found I didn't exactly know the syntax for this programming language. Several error messages later I finally got it to work but I need to do some reading in the second book listed above. Note: I didn't just change the numbers imbedded in the code, I defined new variables and entered their names instead of the numbers. I am learning.

If you look at the U-Tube video on the kit page you will see there are some rather sophisticated experiments.

Monday, March 13, 2017.

Project 3 was a traffic light, or half of one. Three LEDs, red, yellow, and green, were for the cars and two, red and green represented the walk/don't walk pedestrian lights. A pushbutton switch initiated the pedestrian crossing sequence. Sue was totally fascinated with it. It might be fun to put together a toy with several options. Perhaps a light detector so when someone's shadow fell across it, it would cycle.

It seemed as if project 4 was a step backward because it went back to a single LED and was called flashing light. Didn't we do that already? It wasn't on or off, it was a brightness modulator. I didn't understand the code at all. I looked at the voltage on my scope and it was a 490 Hz square wave that was being duty cycle modulated from 0 to 100%. So it's time to stop playing with prepackaged circuits and programs, I'll never get used to calling them sketches, and study the book I have on programming. There really won't be anything to report day to day. When I get to the point of writing my own code I'll let you know.

Saturday, March 18, 2017.

I'm not quite to the point of writing code from scratch but I am big into modifying someone else's code. An example from Monk's book was a morse code generator. It did it by flashing a L E D but the beginner's kit included a "buzzer". It generates a click every time the state of the input changes so if it is fed a square wave at an audio frequency it will produce a steady tone. The built in functions are tone(pin, frequency) and noTone(pin). This is admittedly a simple conversion but in spite of my best efforts it generated a good many error messages. One was caused by a Basic habit. The function in Basic chr$() returns the character for the ASCII code argument. In Arduino char* declares a string variable. Consequently there were a few chr* declarations.

There are still some unanswered questions. For example the Arduino syntax of the for - next loop familiar from Basic is

for(int I = 0; I < 20; I++)

Contrast this with the Basic syntax,

For I = 0 to 20 step 1.

In Arduino can the step be different from 1? In what is similar to a let statement it uses i++ as shorthand for I = I + 1. Can the last part of the for statement be I = I + 2? I don't know. Arduino has eliminated the need for a next statement by placing the contents of the loop inside braces sometimes known as curly brackets { }. Well, this seems to be turning into an Arduino language tutorial. That is not my intention.

The language seems to be alright but I have many complaints about the IDE (Integrated Development Environment). In short it's lousy. In long, It's too small, hard to read, hard to manage, and very picky.

Too Small. When it pops up on my 22 inch monitor it is 5-3/4 by 5/3/4 inches. Only 7 lines of code are visible at any time and if there are comments on a line the horizontal scroll bar must be used to see them. But I hear you say "use your mouse to stretch the window or maximize it. That would be a practical solution if it had a memory. But it doesn't. When you select New from the menu to start a new program a new instance of the IDE opens and it is the same size as the original startup.

hard to read. The text is small and low contrast. It appears that the programmer used a text color of gray rather than black. If it were black that would help only a little. IT'S TOO SMALL DAMIT! No doubt it was programmed by someone in his 20s who probably has vision a little better than the so called perfect 20/20. OK mister twenty something, come back in 50 years and tell me if you can read the text in your program. You'll have to tell it to my grave stone because I won't be on the green side of the grass to hear you. In case you still don't get it people's vision gets less sharp as they grow older. So by programming as you have you are discouraging the older part of the population plus the visually impaired and legally blind.

It won't work with JAWS either. I searched for scripts that could make it work but they don't seem to exist. I don't know how to write scripts or I would write one. I received a suggestion that I look up something called java access bridge. I downloaded the zip file but I can't figure out what to do with it. I read the installation instructions and all I got was a headache. Apparently it must be manually installed by copying files to certain directories but there are so many alternative choices I couldn't make heads or tails of it. I doubt if there are many fully sighted people in the world who could successfully install it. Someone who is blind? Forget it. Someone told me that the Arduino language is similar to java. That doesn't convey any information because I don't know java from cappuccino.

Hard to manage. Every time you open a new program * it a new window opens. If you have stretched or maximized the window the new one is hidden behind it. You have to use Alt Tab to find the window that should have appeared but didn't. You have to manually close windows that you are no longer using even empty ones. If you get behind in closing unused windows the latest one slowly moves down to the right and eventually creeps off the screen. Trying to move them creates such a mess that it's easier to close every one and start over.

* (I refuse to call them sketches. Anyone who knows English knows that a sketch is a rough drawing not a set of lines of program code.)

Picky. I moved a program from one directory to another and when I tried to load it I received an error message which I will paraphrase because I don't remember the exact wording. This "sketch" must be in a folder with the same name as the "sketch". Create the directory and move the "sketch" to the folder. PICKY PICKY PICKY!

Well I'm struggling along as best I can which is all any of us can do. I have taken to composing code in WordPad and copying and pasting it into the IDE. After finding and correcting errors I copy and paste back to WordPad. However it seemed as if earlier corrections weren't making it back to WordPad. I may have forgotten to do something. I was working later than I should have been. If the problem persists I'll try Note Pad and see where that gets me.

What about void? The word "void" precedes the name of a procedure that does not return a value to the procedure which called it. Undoubtedly a poor choice of words but probably too late to change it. Beginners should be warned in advance to prevent panic attacks.

The text to Morse code converter works and I amused myself and my wife with it this morning. It needs my PC to create the text and transfer it to the Arduino over the USB line. This begs the question why not just program the whole thing on the PC? Well, I'm having fun with Arduino. And by the way it looks like there is enough interest on the fun with tubes email list to make it the fun with tubes and Arduino email list. If that's what approximately 750 people want who am I to stand in their way.

Thursday, March 23, 2017.

I am now getting help with the Java bridge. Darrel is helping me by phone and email. He tells me that it works but so far it isn't working on my computer. It's too bad we can't get someone to write an automatic installer. It is done for all other programs, why not this one?

This issue has distracted me from learning Arduino. I hope I can get back to it soon before I forget what I have learned to date.

Monday, March 27, 2017.

Well the IDE is a total disaster. Darrel has given up and so have I. We seemed to have arrived at this point totally independently. The IDE was written in the JAVA language and it is totally inaccessible with the software known as screen readers or text to speech converters. So the folks who made java came up with something called the java access bridge to help the blind operate java programs. But there's just one catch. The access bridge does not come with an automatic installer as 99.999999999999% of all programs do. The program must be manually installed. Apparently figuring it out is largely a matter of chance and I have not been as lucky as Darrel.

What were you java people thinking. It's as if someone was providing seeing eye dogs to the blind but to qualify to get a dog they had to read an eye chart. This goes beyond stupid. Beyond callas. I have no words. I curse you to a horrible death being eaten by fire ants. I curse you to the hottest place in hell. I curse you for ever and ever and ever.

That did nothing to make java bridge work but it did make me feel a little better.

Windows has had a built in magnifier since 98 and maybe since 95, I'm not sure. Anyway it looks as if it was left out of windows somewhere between XP and 7. I am now running 10. I found if I call up the Run box and type in magnify it starts. I did a search and found magnify.exe. Should any one wish to duplicate what I did the search will turn up at least 3 instances of magnify.exe. Select the one with the shortest path (least number of characters) and write it down. Then create a desktop icon to start it. I went one more step. I put magnify.exe in the startup menu so it starts automatically when the computer boots up. When I set the magnifier to 1.5 it enlarges the IDE sufficiently to allow me to use it without the help of jaws (my screen reader). I'm not happy about the other problems but I can live with them

Meanwhile back at the ranch, what's going on with Arduino? I wouldn't recommend the beginner's kit for anyone past the sixth grade. The canned programs teach nothing about programming. The only thing to be learned is wiring circuits on a breadboard. The experiments do have a fairly high wow factor but that's not enough for those who really want to learn how to program.

However, I have been working my way through Monk's book (listed above) and the parts in the kit have been very useful for that purpose. It might be worth the money if it ever comes back in stock. I have bought and started Monk's second programming book, Programming Arduino Next Steps. It seems to be more about the C++ language which the language used by Arduino seems to be a subset of. I have some ideas using the parts in the kit and writing some programs, sketches for all you conformist, to do some things that are more interesting then turning on a light.

To Be Continued.

Where have I been and what have I been doing?

Monday, February 27, 2017.

For years I have had a Specific Products model SR7F WWV receiver sitting in my rack unused. It has been unused because its power transformer was burned out. I finally got tired of tuning in WWV on the Drake 2B with some special crystals I had installed. I removed the WWV receiver from the rack and examined it. I removed the power transformer which required removing a set of audio filters. Someone had told me the transformer was bad but I decided to test it. I connected line power to the primary and measured secondary voltages. Hmmm, everything looked normal. I let it run like that, no load, while I did something else. After about 20 minutes I began to sniff that hot smell. Not a burning smell but the smell that metal gives off when it is very hot. I unplugged the transformer and touched it. "OUCH!" It was really hot and with no load confirmed that it was bad. I found a replacement on the AES website and ordered it.

Mounting it wouldn't be easy. I really didn't want to drill any new holes in the receiver's steel chassis. The original transformer was one of those that mounted with the core laminations parallel to the chassis with a large cutout for the windings and end cover. The replacement was a conventional tab mounted type with the core perpendicular to the chassis. Two of the four tab holes did line up with the original mounting holes but the leads exiting the transformer from the bottom would have been squeezed by the chassis opening. I had to devise another way of mounting it. So I cut an adapter plate from a piece of aluminum plate I happened to have. Its shape is not important because this is an ancient receiver and it seems unlikely that anyone reading this page has ever seen one of these receivers let alone has one.

Meanwhile I am working in my wood shop to build a rolling cart to house the new spectrum analyzer as well as the Boonton Q meter and the Boonton/HP RX meter. These are true boat anchors that currently I have to lift down from a shelf and carry to the bench and then return them to the shelf when I am finished with them. Both my wife and my doctor are telling me that I'm getting too old for all that heavy lifting so a cart on casters seems to be the best answer.

Meanwhile a totally blind friend who is also a ham wants my help to adapt an Arduino speech synthesizer for a talking SWR meter. I have an Arduino starter kit but I haven't had time to do anything with it other than open the box and look at the parts and wonder "What is that for?"

And if all that wasn't enough I bought a spectrum analyzer and although I could easily get started with it based on my previous experience with an HP analog spectrum analyzer some of the more esoteric functions required reading the manual. The analyzer came with a program that will control it over a USB line from my computer. I installed the software and the next time I tried to use Corel Paint Shop Pro X it wouldn't start.

Why is this important? Paint shop is, was, the link between MaxCAD and the internet. All those schematics you see on my website were created in a drawing program I wrote myself that I modestly call MaxCAD. The printer interface was easy to write for so it has printer output. I was not able to figure out how to write a gif, jpg, or bmp output. By examining the HPGL output of AutoCAD which is an ASCII file I was able to figure out how to write an HPGL file. HPGL (Hewlett Packard graphics language) was designed to drive a plotter directly and is very straight forward. The file extension used is PLT. But HPGL is not recognized as a graphics interchange format on the internet. I was using Paint Shop to scale the files and convert them to GIF format which is universally recognized by web browsers. This was a crisis that took my attention away from the new toy as well as the WWV receiver.

I tried reinstalling the program but it would come to a point where the process would just stop dead in its tracks and no matter how long I waited it wouldn't go any farther. This is a problem I have seen before so I uninstalled the program, that is I tried. It would run for a while and then go dead in the water. This is the definition of being stuck between a rock and a hard place. I can't go forward and I can't go backwards. I bounced back and forth between these two boulders several times before giving up. I don't know for certain that the spectrum analyzer software was the cause of this problem. The appearance of my windows 10 screen changed slightly at about the same time. A windows update may have been the cause. That version of Paint Shop was originally intended for windows XP. Microsoft has of late shown an increasing disregard for compatibility with older programs and that is likely what killed my only link to the internet.

So - I starting Googling and eventually found a program called reaConverter 7 Pro. I downloaded the trial version and found it did exactly what I needed. So I bought it. Problem solved so you can take your paint shop and shove it.

With that panic moment taken care of, and believe me it was a panic situation, I can get back to the other alligators that are snapping at my heals. I have so many digital clocks that I really need the WWV receiver. Being able to turn it on and select the best received frequency by the turn of a rotary switch is a real time saver. So fixing it and getting it off my bench is number one at the moment. The equipment cart will solve a number of other problems and then I can tackle those other problems as well.

Monday, March 06, 2017.

I got the transformer installed and managed to get the filter assembly back together including the terminal where a wire had broken off. I called in Sue and she spotted the broken end in a minute. So the receiver worked but it didn't seem to be performing very well. I checked it with a signal generator and found that all frequencies were 5 kHz low. There are individual trimmer capacitors on all the crystals but the consistency pointed to the IF being off frequency. All the same I tried correcting the frequency with the trimmers but they didn't have the tuning range to correct the error. So I retuned the second IF to match a couple of frequencies I had not mess with and that brought everything into alignment. The receiver covers 2.5, 5, 10, 15, 20, and 25 MHz. I don't think they are transmitting on 2.5 and 25 anymore. I know you are curious so here is a picture of it.

Now with that problem solved I can get to the other alligators that are snapping at my heels. Glue up on the equipment cart started today and the first two subassemblies are drying. Tomorrow they go together to make the framework which will have to dry overnight and then the shelves and casters will go on. I find it much easier to apply the stain and varnish before assembly so once it is together all that will remain is to mount a power strip and the voltage regulator for the Q meter.

The cart will hold the 10 drawer tool box which is now taking up floor space, a file box that holds equipment manuals and really doesn't have a home, the Q meter, RX meter, Spectrum analyzer, filter choke analyzer, the power supply it needs to work, and the analog scope which in spite of advances in digital technology is needed often enough to be kept ready for immediate use. It may also hold a few odds and ends if I can place them somewhere they will not be in the way of using the other equipment.

Tuesday, March 07, 2017.

The equipment cart was finished yesterday and I put a few finishing touches on it last night and today. What follows are four thousand words on the subject.

Case Closed.

Modifying the HP334A RF Detector Circuit.

Tuesday, January 17, 2017.

Getting at the RF detector board turned out to be easier than I thought it would be. I did have to remove the main shield in order to get at it. It wasn't hard. Then there was a small shield around the board which was held to the rear panel by two screws and to another part of the internal shielding by two more screws. The circuit board was held to the rear panel by two screws. Just to make things interesting the HP designers had routed the output wire from the detector board through a hole in the detector board shield and the cable did not have removable connectors. The input cable did. So I had to work with the board and shield still connected to the main unit. It was difficult but not impossible.

Below is the unmodified detector circuit in figure 1 and the modifications drawn in by hand in figure 2.

Figure 1 Unmodified RF Detector in the HP 334A.

Figure 2 Modified RF Detector.

I replaced the diode on the board and also the spare which was held to the outside of the RF shield by two farnstock clips. I tacked on a 5.6 k ohm resistor to inject bias and reinstalled the board and shield. The resistor lead exited the shield through an extra hole which may have been put there for the purpose of biasing the diode. If so the hole would have been left in because there would have been a cost associated with removing it which over the production run would have been greater than the cost of leaving it in. At least that's my speculation. I used a shield mounting hole and screw to fasten a terminal strip to hold the two resistors and capacitor for biasing the diode and filtering the bias voltage to avoid adding noise from the power supply to the signal under test. See figure 8 under the heading "Low Distortion AM Detectors" below. I stole a bit of -24 volts from a nearby edge connector and I was ready to trial and error the resistor values. First I connected a substitution box between the power supply connection and the resistor lead coming out from the detector shield. I found a value that gave a minimum distortion reading at 100% modulation of my test generator and put a resistor of approximately 2/3 of the value to the detector and the filter cap that goes to ground. Then I tried the sub box between the negative lead of the capacitor and the negative power supply connection. This value came out larger than I had planned on for good filtering so I increased the value of the resistor going to the detector and found a new value for the other resistor. After several iterations of this procedure I found the following values. For the vertical resistor in figure 8 36 k ohms and for the horizontal resistor that connects to the voltage source 18 k ohms.

I put the whole thing back together and made some measurements. I was initially disappointed with the results. It was late and I was very tired and I probably read the wrong scale. Upon testing today I find satisfactory readings. I have flipped the axes of the chart so I can test at several different frequencies.

Signal source, SiglentDSG810.

Modified HP334A RF Detector.
Frequency (MHz) THD (%) @ 90% Mod THD (%) @ 60% Mod
0.54 0.147 0.098
0.8 0.165 0.122
1.2 0.182 0.140
1.6 0.190 0.150
2.0 0.197 0.161
3.9 0.242 0.218
7.2 0.274 0.234
14.3* 0.340 0.249
21.3* 0.430 0.257

* Using a Siglent SDG1025 which may have more distortion.

Wednesday, January 18, 2017.

There are no readings for 30% modulation because the meter was reading carrier leak through and noise.

Thursday, January 19, 2017.

I find it interesting that I was able to get better carrier frequency rejection in my test circuit than the HP engineers got in a device that was sold under the HP brand name. This is mainly due to the layout of the PC board. The three capacitors C1, C2, and C3 were all in the center of the board, parallel to each other, and close together. I found when setting up my test breadboard that putting the capacitors in close proximity allowed the carrier frequency to be coupled around the filter by parasitic capacitance.

But before I break my arm patting myself on the back I need to remember that the diode available to the HP engineers in 1965 was nothing close to the 1N6263. I'm sure they thought of adding a small amount of forward bias to the diode, after all they weren't stupid, but because in 65 every diode that came off the semiconductor line was different from all the others the bias would have needed to be adjustable and this would have added one more step to the final calibration procedure and probably one more piece of test equipment to the alignment station. For mass produced equipment economics drive the decision and it is likely that improving the filter was found to be economically unfeasible.

Case Closed.

Low Distortion AM Detectors.

Wednesday, November 16, 2016.

This investigation was started when I received an email from John Wise who has developed a modification for the Williamson amplifier. At this time he is interested in building a low distortion AM transmitter. He sent me some references from the WWW in which a pentagrid converter is used as the modulator and the RF output is detected and the recovered audio sent back to the audio amplifier to apply negative feedback around the modulator and detector. The catch is, and there is always one, that if the detector is not perfectly linear the feedback might well make the distortion worse rather than better. My goal in the following is to test the basic voltage doubler AKA peak to peak detector with several different diode types including a vacuum tube with various combinations of source and load resistances to determine how they will perform.

Testing the test instrument.

The danger in such a project is that the instrument being used to generate the AM signal that is being fed to the detector may itself have some distortion. To measure the distortion of an AM signal generator it is necessary to detect, rectify, the AM signal and pass the recovered audio to a distortion analyzer. To test a detector you need a low distortion AM signal generator. To test the signal generator we need a linear detector. That's a catch 22 if there ever was one.

At some point we have to rely on Mr. Hewlett and Mr. Packard to provide us with a linear detector. The HP 334A harmonic distortion analyzer has a built in AM detector for testing AM signal generators and transmitters. That seems like a good place to start. It's getting late and there is something else I need to do or Sue will be very unhappy in the morning because the cable TV box is locked in some mode I can't get it out of. So it's a call to tech support then to bed.

Friday, November 25, 2016.

Let's begin by testing the HP-606B signal generator. It has a modulating signal output so we can look at the distortion of the internal sine wave oscillator. The internal oscillator has two selectable frequencies of 400 and 1000 Hz. I will test it on both frequencies.

Distortion of 606B internal modulation oscillator.
Frequency (Hz) Amplitude (Volts RMS). THD (%)
400 3.95 1.8
1000 3.85 0.68

We will use 1000 Hz as the modulating frequency and make measurements with the RF detector in the 334A.

606B with amplitude at 3 volts 90% modulation. Output unterminated. RF = 1.6 MHz.

Distortion = 1.23%.

Substituting the low distortion function generator for the internal oscillator in the 606 gives,

Distortion = 0.93%.

I'd call that a significant improvement.

When the RF amplitude is reduced to 1 volt the distortion goes up to 2.26%.

This indicates that the linearity of the detector in the 334A is dependent on input amplitude as expected.

Now we will test the function generator under the same conditions as the 606, 90% modulation and 1.6 MHz carrier frequency. Just one little problem. The maximum output amplitude of the FG is 20 V peak to peak (P-P).

The distortion is 1.6%. <-p> When the output amplitude of the 606 is adjusted to give the same recovered audio level as the FG the distortion is 1.46%. No. I didn't reverse those numbers. I just checked them again.

The 606 when modulated by an external low distortion audio generator does a little better than the FG but not by much.

Now here's a surprise. I didn't know if the RF input was terminated in 50 ohms or high Z. That sent me to the manual to read the specifications. The input is high Z which means that I was getting twice the output from the 606 as indicated on the meter. The wavelength at 1.6 MHz is 187.5 meters. For the metrically impaired that's 615.16feet. The 3 foot maximum cable length is not going to have any significant effect on the impedance. But that's not the surprise. It is that the distortion in the detector is specified at 30% modulation. I knew that 30 % was the standard for signal to noise ratio measurements and now I know it is also the standard for distortion measurements.

With both generators operating at 30% modulation and the levels matched as above the 606 read 0.325% and the digital function generator read 0.32%. The "distortion added" by the detector specification on the 334A at frequencies in the AM broadcast band and input levels between 3 and 8 volts RMS is 0.3%. Given the inherent precision of digital circuitry I think it is safe to assume that the distortion in the DFG is less than 0.1%. The 606 uses analog circuitry and vacuum tube to boot so it may run higher levels of distortion at higher percentages of modulation.

I think I can account for the seeming contradiction in the numbers above for 90 % modulation. When you connect a nonlinear element in the feedback loop of an amplifier the output is the inverse of the nonlinear function. The 606 and 334 have the same detector circuit so the modulation envelope of the 606 is being predistorted with the inverse of the detector function. Then the detector in the 334 gives an output which has better linearity than the original modulation envelope thus giving a lower distortion reading.

Thursday, December 01, 2016.

I think I can rely on getting low distortion figures from the digital function generator. So now I will breadboard a standard detector to see what distortion figures I can get and how, or if, I can alter them by some circuitry tricks.

One thing that is necessary when making distortion measurements is to be sure that the carrier frequency output from the detector is way down. 0.3% distortion means that the sum of the harmonics is 50 dB below the recovered audio fundamental level. The filter in the circuit below shows a theoretical attenuation of 91 dB at 1.6 MHz. .

Figure 1 Basic Detector With Filter.

As constructed on the shielded breadboard shown in the photograph below the -3 dB frequency is 9.5 kHz and the attenuation at 1.6 MHz is 98 dB. I'll take that. The top on the breadboard's lid was closed for the measurements.

Test conditions. Input voltage = 20 V p-p at 100% modulation. (10 volts p-p unmodulated carrier).
Carrier frequency = 1.6 MHz.
Modulating frequency 1000 Hz.

The residual noise measurements may seem strange. We are accustomed to thinking of the noise floor as being constant. It is but it's the gain of the analyzer that has changed. The modulation percentage of the function generator is set and the "set level" is adjusted for a reading of 100%, not to be confused with 100% modulation. Then the modulation percentage is set to zero and the range switch on the analyzer is cranked down to the lowest range which is 0.1%. So the reading in the first column of 0.008% is actually 0.008% of 90%. 0.011% is 0.011% of 60% and 0.021% is 0.021% of 30%. These numbers aren't quite consistent but remember we are not just pushing the limits of accuracy we have totally knocked them over. If we strictly follow the rules of measurement we wouldn't report a reading that is less than 0.03 that was read on a range setting of 0.1. Some may say that values can be read down to 1/10 scale but this is pushing very hard on the limits of accuracy.

THD Values for Figure 1.
Diode # Recovered Audio (V)
@ 100% Mod.
THD (%)
@ 90% Mod.
THD (%) @
60^% Mod.
THD (%) @
30^% Mod.
Residual (%)
No Mod.
-- 0.008 0.011 0.021
1N270 (1) 2.40 0.605 0.295 0.130
1N270 (2) 2.38 0.65 0.32 0.148
1N270 (3) 2.40 0.600 0.285 0.120
1N933 (1) 2.47 0.63 0.294 0.119
1N933 (2) 2.49 0.555 0.240 0.094
1N34A (1) 2.42 0.812 0.400 0.170
1N34A (2) 2.18 0.88 0.37 0.158
1N34A (3) 2.42 0.74 0.342 0.143
1N34A (4) 2.44 0.78 0.382 0.163
1N4148 (1) 2.55 0.58 0.194 0.076
1N4148 (2) 2.55 0.582 0.199 0.076
1N5819 (1)
Schottky
1.79 4.1 2.3 1.05
1N5819 (2)
Schottky
1.79 4.0 2.32 1.05

Well, that produced a few surprises. The 1N4148 silicon diode did much better than expected while the Schottky diode did much worse than anticipated. You may be wondering where I got some of these diodes. The germanium 1N270s came to me on a computer board I got from somewhere. There were originally 40 of them in 10 resistor-diode gates. The number is now down to 21. I have a plastic drawer full of the 1N933s also germanium. I don't remember where I came by them. The 1N34As are brand new. That seems to be the type that everyone is using in these linearizing detector circuits.

The older germanium type seem to be more closely matched while there is a lot of variation among the 1N34As. The Schottkys have good repeatability but they have a serious linearity problem. If the modulation percentage were to be limited to 90% the 1N4148s could be used but the voltage applied to the detector needs to be relatively high, 20 volts p-p at 100% modulation.

The oscilloscope tells the story.

Figure 2 Detector Input and Output with 90% Modulation.

Figure 3 Detector Input and Output with 100% Modulation.

Note: The channel two range is actually 1.5 volts per division. As you will observe on the right of the picture this channel is still set for the X10 probe when in reality the scope was connected to the output of the shielded breadboard by a length of Coaxial cable.

The silicon diode gives the most recovered audio and best linearity except at that last little bit near 100% modulation. I remember reading something about adding a little DC to bias the diode just short of conduction for use as a detector. I think I'll try it. But that's all for today.

Wednesday, December 07, 2016.

Figure 4 Modified Detector with Biased Diode.

All data below is for a 1N4148 silicon diode. The FG was set to 100% modulation and the bias adjusted for minimum distortion. Then distortion was measured at a few intervals concentrated just below 100%.

Some investigation which is not included showed that the optimum amount of bias is somewhat dependent on the percent modulation. The percentage change is small and when the bias is optimized at 100% modulation the distortion at 90%, 60%, and 30% modulation is still less than with no bias.

THD Values for Figure 4.
All Diodes = 1N4148
Bias = 15.00 V
Modulation (%) THD D1 (%) THD D2 (%) THD D3 (%) THD D4 (%)
100 0.803 0.810 0.795 0.795
98 0.603 0.630 0.620 0.620
96 0.540 0.545 0.540 0.540
94 0.485 0.495 0.485 0.490
92 0.445 0.450 0.445 0.440
90 0.410 0.415 0.401 0.405
60 0.174 0.177 0.174 0.172
30 0.0745 0.0730 0.0725 0.0705

I am amazed by the repeatability of these numbers. Or am I? Maybe the detector is not adding any significant distortion at each point and I am measuring the distortion of the function generator. Right now I have no way of knowing. I'm going to start experimenting with different detector circuits with the hope that I am able to find one that will give either consistent or better figures than the simple diode detector I have been experimenting with. As I think about I am not encouraged. The best EEs in the world worked on a detector for the HP334A and the best they could was a simple half wave diode detector. The only thing that gives me hope is that they would have been working to stay within a budget. I have no concerns about large quantity production costs and can try very complex circuits such as the phase locked loop. Now, what did I do with those NE561s I ordered 35 years ago.

Friday, December 09, 2016.

Well, I'm not quite ready for that just yet. I'm still perfecting the detector circuit itself. John Wise pointed out to me that the detector circuit in the 334A uses the diode in shunt rather than in series. So I modified the circuit accordingly and found that different values were required for the biasing resistors and the distortion was somewhat lower. When I put this article up as a permanent part of this site, and I will, I will leave out the false start with the series diode. But for the daily log it stays just to show that lab work doesn't always lead to the best circuit on the first try.

Figure 5 Detector with Shunt Diode and Bias.

Recovered audio for this detector at 100% modulation is 3.00 volts.

THD Values for Figure 5.
All Diodes = 1N4148
Bias = 12.00 V
Modulation (%) THD D1 (%) THD D2 (%) THD D3 (%) THD D4 (%) THD D4 (%)
With Values
Shown in Fig 6
THD D1 & D4 (%)
Doubler, Biased
15.9 V Fig 7
THD (%) 1N6263
Small Sig
Schottky
100 0.700 0.710 0.690 0.690 0.720 0.830 0.430
98 0.545 0.555 0.540 0.540 0.565 0.699 0.321
96 0.475 0.480 0.470 0.465 0.495 0.600 0.281
94 0.430 0.435 0.420 0.420 0.450 0.545 0.258
92 0.395 0.400 0.390 0.385 0.410 0.405 0.234
90 0.360 0.370 0.360 0.355 0.380 0.470 0.210
60 0.164 0.166 0.160 0.160 0.180 0.227 0.108
30 0.055 0.056 0.055 0.054 0.062 0.072 0.060

I found that the output of the function generator has a small DC offset and there doesn't appear to be any way to adjust it to zero except to add a small offset from the control panel. This was adding some extra bias which had to be made up when a DC blocking capacitor was used. I cannot account for the fact that all distortion levels are less than with the series diode but I'll take them.

Half Wave or Half Wave Doubler, That is the question.

Every AM transmitter circuit I saw used the voltage doubler. I suspect this circuit will give more distortion than the half wave rectifier but I'm going to keep an open mind and test the circuit on another day. I think I see the rationale for using a doubler but I also think it is unnecessary. It isn't needed for the extra voltage, there is plenty of RF voltage available. I assume that those who design such circuits are thinking that there is a possibility of asymmetric modulation and in that case the feedback loop needs to know the peak to peak voltage rather than the peak voltage on just one half of the RF cycles. If such asymmetrical modulation was taking place applying this information back to the audio amplifier in analog form would not correct the problem. The envelope would remain distorted and the detector might well make the distortion worse.

Actually I think it is difficult if not impossible to create asymmetrical amplitude modulation. Scope patterns can be deceiving. If some audio becomes mixed with the RF output it can give the appearance of modulation of only, say, the upper half of each wave and none or very little on the lower half of each wave. The audio cannot be transmitted through the ether along with the RF. At the reception point the audio is gone and the modulation envelope is once again symmetrical.

When I was at the university of Florida there was an EE student who was a couple of years ahead of me who thought that he could transmit stereo on AM by modulating the upper halves of each RF cycle with the left channel and the lower halves of each cycle with the right channel. He thought he had it until he tried to send the signal to a receiver across the room. He was seeing mixed audio and RF at the antenna connection but the audio part wasn't getting to the receiver. If such a transmitter had been possible receiving a monaural image would require the receiver to be equipped with a peak to peak detector. A common AA5 which uses a half wave detector would only reproduce one channel. As it turned out nobody had to worry about those things.

Saturday, December 10, 2016.

Before I move on I was wondering what would happen at a higher impedance level. So I changed the circuit as shown below.

Figure 6 High Impedance Detector Circuit.

I have added a column to the table above. I no longer see the need to plow through many different diodes when they all give pretty much the same results. The bias supply voltage for the new values is 15.00 volts. All the values are a little higher than they were but they are still less than when the diode was series connected.

I can't postpone it any longer. I must face the music and connect a voltage doubler circuit.

Tuesday, December 13, 2016.

Today I added the 7th column in the table above. It is for a voltage doubler aka peak to peak detector. The bias was set at 15.9 volts. The distortion is higher as one might expect. Such a detector might be necessary in a crystal set but as I stated above it seems unnecessary as used in the feedback loop of an AM transmitter.

Figure 7 Voltage Doubler With Bias..

Also I finally got hold of some small signal Schottky diodes. The ones I tried before were intended for use as rectifiers in switch mode power supplies. They had forward current ratings ranging from 1 to 30 amps and looked like rectifiers. It was tested in the circuit of figure 6 with one small mistake, a 220 k ohm was used in place of the 470 k ohm resistor. The diode and bias supply were turned around.

Wednesday, December 14, 2016.

In case you are confused by the entry above figure 8 below should clear things up.

Figure 8 Shunt Half Wave Schottky With Bias.

I think I have struck gold this time. I'm going to run 4 of these small signal Schottky diodes (1N6263).

THD Values for Figure 8.
All Diodes = 1N6263
Bias = 10.60 V
Modulation (%) THD D1 (%) THD D2 (%) THD D3 (%) THD D4 (%)
100 0.420 0.420 0.420 0.420
98 0.320 0.320 0.320 0.320
96 0.276 0.276 0.276 0.276
94 0.248 0.251 0.250 0.250
92 0.230 0.229 0.229 0.229
90 0.208 0.209 0.206 0.207
60 0.107 0.107 0.109 0.105
30 0.059 0.059 0.060 0.060

Wow! I am totally knocked out, blown away, and have had my socks knocked off. I see no reason to search any farther for a low distortion detector. These diodes are not at all expensive. At Jameco they cost $0.29 in single lots, $019 in lots of 10, and $013 in lots of 100.

I think I'll build an AM transmitter, at least on a solder breadboard, to see how it sounds. I doubt if I own an AM tuner that will give distortion figures this low but it should be enlightening to do some testing.

I am still curious about how vacuum diodes such as the 6H6 and 6AL5 will perform. I have some of each so it's just a matter of wiring a socket and plugging the wire ends into my shielded breadboard pictured above. I'm not quite ready to pronounce "case closed" but for those interested in the bottom line on low distortion detectors you have just read it.

Friday, December 16, 2016.

Thoughts About Detectors and Holidays.

The common uses for circuits like this are,

  1. Audio only detector, recovers the audio from an amplitude modulated radio signal in a receiver.
  2. AGC detector, recovers a DC level proportional to the average carrier level in a receiver.
  3. AC to DC converter in a voltmeter, starting with an AC input it recovers a DC level which is proportional to either the average, peak, or peak to peak, level of the AC input.

When bias is applied to linearize a diode detector the DC output is not zero when the AC input IS zero. This means that the circuit as shown is suitable only for number 1 above. A DC blocking capacitor must be present in the output circuit to prevent the DC level from upsetting the bias level on subsequent amplifier stages.

It is possible to remove the DC offset from the output if uses 2 or 3 are desired. There seems to be little point in using a highly linear detector for AGC.

As an AC to DC converter in a voltmeter linearity is absolutely necessary if printing special scales on the meter face is to be avoided. If the meter is to be entirely silicon based the offset can easily be removed by a simple op amp circuit. If all vacuum state is desired the whole circuit could be floated to remove the offset. These investigations will be performed after the Christmas and new year's holidays are passed.

The next two weeks are the busiest time of the year in most households and this one is no exception. As if Christmas and new year's weren't enough my birthday occurs three days before Christmas. Sue and I will be entertaining and visiting the homes of friends and family members. This is most likely to be the final entry in 2016.

Thursday, January 05, 2017. A digression.

And so it was. I'm back now and ready to face the new year, NOT! Well, I'll do the best I can.

While playing with my new Christmas toy, a hand held digital storage oscilloscope, I stumbled onto something. The math function has a number of functions that take the ch1 + ch2 from the analog scopes and add the other 3 functions from the small calculator plus FFT. After I got tired of playing with FFT I decided to see what the multiply and divide functions might do. I got the biggest surprise when I started with two triangular waves at a phase angle of 90 degrees and multiplied them.

The yellow and blue traces are the two waves and the pink or purple wave is the product. I haven't yet figured out exactly why it looks so much like a sine wave. There doesn't seem to be any way to get the calculated wave out of the scope for processing by other instruments. I may have to set up an analog multiplying circuit using a 595 four quadrant multiplier to see just how good a sine wave it is.

Thinking about it in the time domain I think each half cycle may be made up of two parabolas one a mirror image of the other and connected.

Thinking about it in the frequency domain I can't see how harmonics could be canceled leaving only the fundamental frequency.

If you have any thoughts I hope you will post them to the list rather than directly to me. It might start a good discussion.

End of Digression.

Monday, January 09, 2017. And Now, Back to our Story.

All that is left is to investigate the vacuum diode. Here is the schematic diagram of the circuit.

Figure 9 Biased Detector Using Vacuum Diode.

We will begin with the 6AL5 since the miniature version is more likely to be used by experimenters. Here are photographs showing the wiring of the socket and the tube in socket in breadboard.

THD Values for Figure 9.
All Tubes = 6AL5
Bias As Noted
Modulation (%) THD V1 (%)
Bias = 19 V
THD V2 (%)
Bias = 19 V
THD V3 (%)
Bias = 19 V
THD V4 (%)
Bias = 15
100 1.08 1.07 1.27 1.14
98 0.810 0.810 0.960 0.900
96 0.700 0.690 0.860 0.780
94 0.605 0.600 0.765 0.700
92 0.540 0.540 0.685 0.640
90 0.490 0.490 0.625 0.585
60 0.196 0.202 0.266 0.275
30 0.069 0.072 0.098 0.10
Tube Tester
Diode 1
2/3 Red 2/3 Red 3/4 Red 3/4 Red
Tube Tester
Diode 2
2/3 Red 1/2 Red 3/4 Red 3/4 Red

The meaning of the tube tester results is as follows. 2/3 Red means 2/3 of the way up the red part of the scale. 3/4 red means 3/4 of the way up the red part of the scale. It appears that an old weak tube will give less distortion than a new one. I knew there was a good reason not to throw away those used tubes.

Comparing results at 90% modulation between the 1N6263 and 6AL5 shows that the tube gives about 2.5 times the distortion even for the best case. Looks like the 6H6 will have to wait for another day.

Tuesday, January 10, 2017.

In searching my old tubes I found 9 6H6 tubes. One is an RCA and it looks to be the oldest of the lot. Another is a GE which although the black paint is shiny there is a paint chip. Several others are RCA and are labeled JRC-6H6 and on the next line VT-90. I selected two of these for the test below. V1 is the old RCA, V2 and V3 are the military VT-90s, and V4 is the GE.

THD Values for Figure 9.
All Tubes = 6H6
Bias As Noted
Modulation (%) THD V1 (%)
Bias = 8.0 V
THD V2 (%)
Bias = 9.5 V
THD V3 (%)
Bias = 7.0 V
THD V4 (%)
Bias = 25 V
100 1.28 1.35 1.54 2.79
98 1.00 1.10 1.35 2.50
96 0.905 0.945 1.22 2.20
94 0.810 0.860 1.13 1.98
92 0.745 0.780 1.05 1.81
90 0.685 0.720 1.01 1.67
60 0.320 0.330 0.540/td> 0.700
30 0.130 0.130 0.234 0.285
Tube Tester
Diode 1
2/3 Red 2/3 Red 2/3 Red 3/5 Red
Tube Tester
Diode 2
2/3 Red 2/3 Red 2/3 Red 3/5 Red

The results here contradict those for the 6AL5. It appears as if I pretested the tubes and ordered them from best to worst. I did nothing of the kind. The probability of that happening is small but not zero. The order in which the tubes were tested was just the luck of the draw.

I and others were curious about how tube diodes perform as detectors. Now we know. If someone who has heard about this page but not seen it should ask me "did you test tube diodes?" I can answer yes and they scored pretty low. And by the way I did some undocumented testing of the effect of reduced heater voltage. The effect was very small, not enough to be worth it.

Once again I want to point out that biasing the diodes can only be done when AC modulation is to be recovered. The bias adds a DC offset to the output which is blocked by the capacitors in an audio detector. If the circuit is to be used for converting AC to DC for measurement purposes or detecting the carrier level of a received radio signal some means must be found to remove the DC offset. That's easy to do with a silicon based op amp but somewhat difficult with tubes.

What Now?

For one thing I plan to look into modifying the detector circuit in my HP334A distortion analyzer. It may prove to be inaccessible without disassembling half of the unit. If so I will build a small box containing a diode and 9 volt battery to do the detecting.

Next I am going to build a low power AM transmitter that is similar to many that are posted on the www. Most of these use a 6BE6 as the modulator oscillator. That will be a separate article on this page. So as porky pig would say "a the a a the a a the a a that's all folks".

Case Closed.

Updating Used Tube Storage.

Wednesday, September 21, 2016.

Ever since I installed the swingout parts storage cabinet the storage for my stash of used but still good tubes has been in a state which is not to my liking. I have been wanting to improve it but I just haven't gotten around to it. Well, I found a round to it in the yard the other day so I can make the desired improvements. Here is how my used tube storage looked on July 15, 2011.

It took about three weeks with time off to replace the decaying shop ramp, which is another story. Here the new drawers.

Case Closed.

Filter Choke in the Negative Lead.

Monday, September 05, 2016.

Why would you want to connect a filter choke in the negative lead? We all have been taught to build circuits that look the same as the schematic diagram. All schematics show the choke in the positive lead.

Hammond makes a 20 henry 500 mA choke that would work nicely in a transmitter power supply for a pair of 813s or 4-250As. Such a power supply should be 2000 volts for the 813s or 2500 for the 4-250As. Just one little problem. If you read the fine print on the Hammond site you will be made aware that the maximum voltage between the winding and the frame is 800 volts. You put that choke in a 2 kV power supply and BANG! There goes a 50 dollar choke. Before you start thinking about ceramic insulators, read on.

The trick of course is to wire the choke in the negative lead as shown below.

The top diagram shows the conventional way of wiring a choke. The one below that shows how to put it in the negative lead. Notice there is only one ground point which is after the choke. The center tap of the transformer winding experiences a relatively small voltage. Even if a choke input filter is used simply by removing the capacitor on the left the voltage at the center tap should not exceed the breakdown ratings of the transformer.

The third diagram shows a trick that might be employed if the circuit has already been wired up. Reversing the diodes will of course reverse the polarity of the power supply so the ground point will have to be moved and connections to the powered circuit reversed. The reversed diodes alternative has been shown to keep someone from saying "you didn't show what happens if the diodes are reversed".

When I was a teen-aged ham operating 40 and 75 meters in Florida there was a case of a Globe King 500 that went off the air. A couple of weeks later it came back on and the owner reported that the filter choke had shorted to ground. He had solved the problem by mounting it on ceramic insulators. That presented a real hazard to whoever came into possession of it after he became a silent key. Placing the choke in the negative lead wouldn't have made it work because some point of the winding would still be shorted to ground. But it would have made it safer. At that time I didn't know enough to give him advice on the matter but even if I had I wouldn't have offered it because he wouldn't have taken it. He was well known among Florida hams for not taking kindly to any advice which he always interpreted as criticism and became highly offended. At my age at the time I stayed out of his way as much as I could. I never knew what happened to his equipment when he went silent but I and lots of others would sure have loved to have that Globe King even with its hot filter choke.

Case Closed.

Fluke 407 Power Supply.

Friday, August 12, 2016.

Two members of the email list have Fluke 407 power supplies and they may have set a record for the amount of traffic generated. I am one of the two and my faulty memory has been the reason for a good part of the traffic.

The original post rectifier filter parts in the fluke were two twist lock can capacitors which were 90 uf at 500 volts in series. On July 27 2010 I discovered that one of the two capacitors had failed open. I know the date with such accuracy because I had the good sense to take pictures of the original configuration.

And my repair.

Apparently, the nylon tie passed through the grommet partially visible above the top capacitor and back through the hole now vacated by the original capacitor. I must have had the 40 uf 500 volt capacitors on hand. Although the combination made only 80 microfarads that is within the tolerance range of the originals and the replacement. I wasn't concerned about any possible imbalance in charge due to unequal values because of the tolerance and the robust balancing resistors used by Fluke.

The next part of the repair is not as well documented but it seems as though it was a relatively short time later, say a year. The other capacitor failed open and left me with nothing to do but replace with two more 40 uf 500 volt capacitors. These are recently taken pictures of the 4 capacitor repair.

I mounted a piece of Plexiglas to the top side of the chassis using the former capacitor plate mounting holes. A small hole drilled in the Plexiglas in the center of each large chassis hole permits the wire to pass through and keeps it well away from the chassis.

Two terminal strips, one mounted to the chassis but with the intervening lug removed, and another one mounted on a ceramic insulator hold the capacitor bundle and the two balancing resistors.

In the bottom picture the Plexiglas is rendered visible by the reflection of one of the bench lights.

In my faulty memory I thought I had used two 220 uf 250 volt capacitors in series with their own balancing resistors to replace each 90 uf 500 volt capacitor. Also I didn't remember how careful I had been to make a somewhat robust installation. True, it wouldn't survive a rocket ride to the space station but it should survive normal usage in my shop. However, I am now committed to replacing this fix with one that is much closer to the original. It makes sense to do that since it isn't really my power supply. If it were mine I would just button it up, lift it back to the shelf and declare "case closed".

Monday, October 03, 2016.

The Fluke is finished as far as I am concerned. After much flailing about I settled on a pair of 220 uf 600 volt capacitors and clamps along with replacement rectifier diodes supplied by Mike Grant. Sorry the picture is a little blurry. I must have forgotten to press the close up button. The power supply is back together and on the shelf as you can see in the picture below. I'm not going to haul it down and take it apart to take another picture.

Case Closed..

Swinging Choke.

Tuesday, August 09, 2016.

Remember this?

I clicked "buy now" on this item and wound up with it. The seller had listed it as an eight henry choke in spite of what was printed on the side. It turns out that what is printed is right and the seller didn't have a clue.

With no direct current in the choke it reads 52 henrys on my Filter Choke Analyzer.
At a current of 20 mA it reads 32 henrys and,
at 40 mA it reads 16.5 henrys.

Just right off I would say that aint too bad. Tomorrow I will try it in a small power supply with my load box to see what happens. There have been predictions on the FWT list that when the current falls below the critical value for the inductance that the ripple will increase. We'll see.

When I was back at the University of Florida someone said that a breadboard is a form of truth table.

Tuesday, August 16, 2016.

The power supply pictured below may be a bit rusty because it goes back to 1958 when I started my DeVry course. That was prior to, and I suppose a test run for, my enrolment in the University of Florida's EE program.

In its original form the four screws along the front were insulated from the chassis and held spring clips which I don't know how to spell. Phonetically they are call Farnstock clips. A clip carrying 300 volts isn't very safe but in 1958 people were credited with more intelligents than they are today. I don't recall ever getting a shock from it.

After the DeVry course was finished, I got my general class ticket, and bought a Viking VFO, I modified the power supply to power it. The octal socket was where the VFO plug went and the 7 pin mini socket was for a VR tube. The VFO had its own VR tube but I decided that it wouldn't hurt to have two bow strings. The VFO soon got a more compact power supply of its own and the next job for the little supply was powering a dedicated headphone amplifier. It can be seen in pictures on the page for that project.

This is the underside of the chassis. Look closely at the tube socket next to the transformer. That's right, those are top hat diodes soldered to what once was a socket for a 5Y3. The red capacitor is a dual 8 uf at 450 volts. The big resistor is a 1000 ohm. It is not marked with a power rating but I think it is a 10 watt. You can see how DeVry saved the cost of a bayonet lamp socket. The grommet is hard and would probably disintegrate if I tried to remove the lamp.

For this experiment I removed the old rectifier tube socket and filter elements. The modern 1N4007 diodes, swinging choke and filter capacitor are on the breadboard. Here is the schematic diagram of the test setup.

And here is a picture of the setup. I used the decade resistance box for low values of current which were impossible to hit exactly with the load box. When the power was above 1 watt I switched over to the load box. In most cases I was able to hit the stated current within 4%.

Here are the data I collected for the above circuit.

Load Current (mA)

Output (VDC)

Ripple (mVP-P)

0

328

20

2

250

370

4

214

580

6

213

610

8

213

620

10

211

660

12

211

700

14

209

720

16

208

740

18

207

760

20

206

790

22

204

830

24

200

860

26

200

920

28

199

960

30

198

1000

32

197

1100

34

195

1180

36

193

1260

38

192

1380

40

189

1500

Thanks to a gentleman whose name is not in his email address and not in his message either I have learned the well kept secrets of Microsoft Excel. Here is a graph of the above data.

One interesting thing. When the load was "zero" mA, the scope and voltmeter in parallel for a load of 5 megohms, the ripple waveform look like this.

For all loads of 2 mA and higher the ripple waveform looked like this.

You will also note that the sawtooth ripple had a much lower amplitude than the sinusoidal ripple.

I STILL HAVE SOMETHING TO RANT ABOUT.

I was given some very good and useful help on the Fun With Tubes email list and managed to produce the graph above. The kind of graph I want is called a "Scatter Chart". I would never have thought of looking at that one in a thousand years. Just shows how important communication is. If you called a spade a digging tool most people would probably figure out what is meant. But if you call a spade a hydraulic jack who would figure that one out?

The problem is that there is no way to independently find out how to work with Excel. The help screens which were never very good have gotten much worse in the 2013 release. I have a conspiracy theory as to why. Microsoft sells courses in using products such as word and excel. For a "small?" fee you can get tutorials from them. If they make it too easy to get help from the built in help they can't sell as many tutorials.

Do I really believe that? Sometimes I do and sometimes I don't.

RANT OFF

Unless there are questions, Case Closed.

Monday, January 16, 2017. There Were Questions.

Someone on the email list asked what the waveform looks like if the capacitor is omitted and just the choke and the load are present. The three scope screen shots are the answer.

Load Current = 1 mA.

Load Current = 10 mA.

Load Current = 30 mA.

Schematic of Updated Power Supply.

This is the power supply as updated from the original DeVry version. The 15 k ohm resistor across the switch makes sure that the capacitor is always charged to the average voltage to minimize the magnitude of the pulse of charging current when the switch is closed. When the switch is open the full unfiltered 370 volts peak to peak output of the rectifier is coupled by the 100 uf capacitor and applied across the resistor. That is why it needs to be a 1 watt. When the switch is closed there is likely to be a pulse of current depending on where the line is in its cycle at the instant of closure.

The photo below illustrates how I make holes in a chassis to fit parts similar to the choke. I measured the choke with digital calipers to accurately determine the locations of the four mounting studs and terminal insulators. I made a drawing using my graphics program and print it out. Then I taped the paper drilling template to the chassis with double stick tape. The template covers the hole left when the tube socket was removed. Refer to the photo of the power supply just after the first picture of the choke at the beginning of this article. Next I had Sue, whose vision is better than mine mark the holes to be drilled with a marker punch. The terminals pass through the tube socket hole. The punch marks are a bit small so I enlarge them with a 1/16 inch bit held in the chuck salvaged from a electric drill that died decades ago. The hammer, punch, chuck with bit, chuck key, choke and power supply with template in place can be seen in the photo below.

This is the power supply after the holes were drilled and the wiring completed. One of the 100 uf capacitors can be seen peeking out from under the chassis.

Then it had to be tested. Here are the data for both choke and capacitor input modes. Notice that in choke input mode the critical current is somewhat higher than the value obtained above using a 22 uf capacitor. The circuit is the same except for the capacitor value. The 100 uf cap and 15 k ohm resistor in series across the input have no effect. Values did not change when this part of the circuit was opened. The 150 k ohm bleeder resistor was not present during the data collection.

The final test was to power my original headphone amplifier that I made for the first Koss headphones. When in the choke input mode the 120 Hz ripple was somewhat audible in the phones. It disappeared when the switch was set to capacitor input mode. There is some 60 Hz hum being coupled from the heater wiring. I don't remember this from when I used to listen to it regularly. True, that was more than 50 years ago but I would have taken measures to eliminate it if it had been that audible back then. My music source then was a turntable with a magnetic cartridge and now I am using one of those little music players that is smaller than some of the parts in the amplifier. The output of it is a lot less than line level so maybe I never heard the hum because I didn't need to turn up the volume that high.

Case Closed, again.

A New Radio with Old Tubes.

Monday, August 01, 2016.

I have had the idea of building a superhet radio around tubes that have only a 2 digit number for some time now. My first iteration will be with tetrodes and pentodes but I hope someday to build one entirely with early triodes. This action was precipitated by an offer from J. Ed to sell some 2.5 v two digit tubes. My plan is this.

35 - RF Amplifier.
24A - First Detector.
27 - Oscillator.
35 - IF Amplifier.
75 - First Audio and Second Detector.
47 - Audio Output.
80 - Rectifier.

I have used terminology from the early days of the superhet. Ever wondered why the diode detector is called the second detector in some radio data? In the early days the mixer was called the first detector.

There is no pentagrid converter in the two digit family so I will have to use a separate mixer and oscillator. The 35 is what we know today as a remote cutoff tube. The old manuals refer to it as a super control tetrode. I think the 75 was a late addition to the two digit group partly because of its relatively high number but mostly because it has a 6.3 volt heater.

I have ordered most of the parts or I already have them on hand but I had to order a special power transformer to get the 2.5 volt filament windings. Then a separate 6.3 volt transformer for the loan tube that requires that voltage. Also an output transformer. I haven't worked out the chassis layout yet. I'm going to wait until I have the transformers in hand before I do that. I think I'll use those old Meissner IF transformers. At least some of the passive parts will go with the tubes.

Friday, August 05, 2016.

Upon checking my parts on hand I found I had plenty of Hammond antenna and RF coils but no oscillator coils. I have corrected that oversight.

The three transformers came in today so all I need is an oscillator coil. I'm still working on the schematic diagram. I will post it here as soon as I have done all I can without breadboarding. I am anticipating the power supply, mixer, and oscillator, will require some circuit testing and modification.

Saturday, August 06, 2016.

The oscillator coils and 6 pin tube socket came in today. I also have done all I can on the schematic without breadboarding. I'll do the chassis layout next and get it drilled and punched. Since this is a "one of" the chassis will serve as a breadboard.

Figure 1 First Schematic of Radio.

Tuesday, August 09, 2016.

I had an assortment of 5 pin sockets but decided it would look best if all sockets were the same. To that end I have ordered some 5 pin ceramic sockets. They should be in shortly. Meantime here is a chassis layout.

Figure 2 First Chassis Layout of Radio.

This is for a 17 by 10 inch chassis. I haven't worked out in my mind as yet how I am going to do the dial. I haven't even decided between airplane or sliderule. Sliderule style is easier to make calibrations for but I think the airplane style will be easier to make.

To Be Continued.

Building a Better Monitor Stand.

Wednesday, July 27, 2016.

I'm not making a habit of posting my woodshop projects here unless they bear directly on an electronics project. For example when I was cutting printed circuit boards for the IF transformers I documented two crosscut sleds for my band saw. One for square cuts and the other for 45 degree cuts. But most of my woodshop projects will go undocumented on this page.

This one may be of interest although it will not include enough information to permit duplication unless there is demand.

The Problem.

The problem is a rather special one for me because of my extremely limited vision. I have a screen reader to do the heavy lifting when there is text to be read. There are hot keys used by the totally blind to deal with menus and ribbons. That's a lot of memorization which the totally blind are accustomed too. I used to be able to memorize like that but I find as I grow older I either can't or don't want to do all that memorization. So I do a lot of looking at the screen. I have a pair of special glasses that have a magnifying lens mounted in regular glasses frames which allows me to view items through the magnifier hands free. Note: That's how I solder. Looking at the screen through this lens places the end of my nose about half an inch from the screen. That means if I want to look at something near the top of the screen I have to stretch up as tall in my chair as I can, or to look at something near the bottom of the screen I have to scrunch down. Neither position is especially comfortable and it would be nice if the monitor stand permitted up and down adjustment.

My HP computer I bought in 2004 came with a flat screen monitor that had such adjustment built in. Shortly after that monitor makers decided that a majority of computer users didn't need that feature so they discontinued it and to hell with the minority. That now 12 year old computer is still functional and is the weather station computer. There are still some programs that run on it that won't install on this one so it is on a network with this one so I can transfer files. My excuse is the weather station and Sue. I call it her computer.

The solution.

After stretching and scrunching for about a year and a half I decided it was time to do something about it. So I used my own graphics program to design a system that would allow me to raise and lower the monitor. I stretched and scrunched until I had a good design and then I went to my shop and built it. I started on July 6 and finished today. That was working 5 days a week. On Saturdays and Sundays I watch movies with Sue. No matter how long a marriage has lasted one can't totally ignore ones wife. You know you've been married a long time when she says "Hand me that thing over there on top of that thing." And you know what she means.

Here is a picture of it. It is based on two physical principles, the teeter totter, and the parallelogram. OK, so the parallelogram is a principle of geometry. That doesn't keep it from working. The forward and back movement of the monitor is only about half an inch which is not even noticeable in use.

At the rear I constructed an open top box to hold the counterweight. The white box you see sitting in it is a new Hammond 1650K. Without the monitor and counterweight the rack is balanced. With help from Sue I brought it into the house and borrowed the scales from the bathroom to weigh the monitor. It was 5.6 pounds. I opened the drawer where I store new transformers and grabbed one at random. When plopped onto the scales they read 5.6 pounds. I snapped the monitor onto the front, placed the transformer in the counterweight box and there they sit. When the day comes that I want to build an amplifier around that transformer I will have to find another counterweight. Meanwhile there is no reason to change it.

Case Closed.

Using the Pentode-Triode Tube as the Amplifier-Phase Splitter..

Sunday, June 12, 2016.

Over the decades many different ways have been developed to produce the two out of phase signals to drive push-pull power output amplifiers. The one we will examine here appears in RCA tube manuals numbers RC20 through RC30 spanning the years 1960 through 1975.

The basic circuit appears in the 15 watt amplifier driving the output tubes directly and in the 50 watt circuit with a pair of 6CB6s between the splitter and the output tubes. It appears with a variation in the 30 watt amplifier.

Both the original circuit and the variation employing a 7199 tube will be analyzed followed by substitution of several pentode-triode tube types. It needs to be noted that the 7199s that are in my possession are in white boxes marked "SOVT". They all have a defect. If AC is used to warm the heater and the heater transformer secondary is at ground potential there is considerable coupling between the heater and the cathode of the pentode. The induced AC is 20 dB below typical output level. If the heater supply is raised to 20 or more DC volts above ground the coupled signal is reduced to 60 dB below typical output. This in my opinion is not sufficient for use in a quality amplifier. For these tests I am operating the heater from DC. I knew that old transistor power supply would come in handy someday. The 7199 was supposedly designed to have minimum coupling between heaters and cathodes. Apparently SOVT didn't get the memo.

Figure 1 Breadboarded Circuit for 7199.

The distortion is excellent and so is the gain. The value of f2, the upper -3 dB frequency might be a little disturbing but it can all be accounted for. The output capacitance of the pentode section is 2 pf, the input capacitance of the triode section is 2.3 pf, and the plate to grid capacitance of the triode is 2 pf. The miller capacitance is Cpg x gain and gain is unity. So we have 6.3 pf of tube capacitance. Add that to the 10 pf capacitor which was added to prevent instability and we have 16.3 pf of capacitance just in the tube. That gives a -3 dB frequency with the plate resistor of 220 k ohms and we have a frequency of 44.4 kc. Add a couple more pfs from breadboard capacitance and you have the f2 of 40 kc. The equalizing network usually hangs on the grid of the splitter so the value of f2 will be moved down to make the global feedback loop stable.

Well that's pretty good performance. When the circuit was used in the 30 watt amplifier the designers made a change which I find intriguing. They derived the screen voltage for the pentode from the cathode of the triode. This introduced DC negative feedback to regulate the plate voltage of the pentode. Sounds good in theory but in practice the tubes made by SOVT just didn't match up to the RCA tubes the circuit was designed for. I may come back to it if other pentode-triodes work well in it.

Going back to the original circuit the other three 7199s I own did not measure up to the first one I tried. The worst one measured 4% THD at 25 volts and about 1.3% at 17 volts. The gain and bandwidth were essentially the same for all 4 tubes. The 7199 is unique in its pinout so I will have to rewire the breadboard before trying anymore tubes. Oh, by the way, here is what the breadboard looks like.

Tuesday, June 14, 2016.

I have spent all of the time since Sunday gathering information on what tubes I have in my collection and their characteristics. I have summarized the data and general circuit diagram in the figure below.

Figure 2 General Circuit with Table Listing Tube Parameters.

I don't think I'll test all of them but the ones that seem most promising. Such as those with low amplification factor that I have more than 1 of.

Friday, June 17, 2016.

Figure 3 Small Signal Equivalent Circuit of Split Load Phase Inverter for Gain Derivation.

Small Signal Gain of the Phase Splitter.

The split load phase inverter has been with me so long it is an old friend. However, I have never seen a derivation of the gain and output impedance of it and I have never done it myself. I am going to correct that oversight here and now.

First step for writing loop equations is to assign polarity of voltage sources. Second assign current directions in all loops where current is flowing. Third, assign polarity across passive components.

The first loop starts at the cathode and goes up through VGK, down to ground through V voltage source, and the only way to get back to the cathode is through RK.

VGK - V - IbRK = 0     (1)

Starting at the cathode and writing the equation around the output loop gives,

- μVGK - Ibrp - IbRP - IbRK = 0     (2)

Now we solve (2) for - μVGK and (1) for VGK.

VGK = V + IbRK     (3)

Substituting (3) into (2) gives,

- μ(V + IbRK) - Ibrp - IbRP - IbRK = 0     (4)

Now we factor -Ib out of the three terms on the right of (4).

- μ(V - IbRK) - Ib(rp + RP + RK) = 0     (5)

We expand the term on the left,

- μV - μIbRK - Ib(rp + RP + RK) = 0     (6)

Now we put the term - μIbRK inside the parentheses on the right while factoring out the Ib.

- μV - Ib(rp + RP + μRK + RK) = 0     (7)

Saturday, June 18, 2016.

Now we are ready to go for the output voltages.

VO1 = IbRP (8a)     and     VO2 = - IbRK (8b)

We will work with these one at a time. Solving (8a) for Ib,

Ib = VO1 / RP   (9a)

Substituting (9a) into (7) and moving the equals sign,

- μV = VO1 / RP (rp + RP + μRK + RK)     (10a)

Simultaneously at the same time we will, divide by V, multiply by RP, and divide by the contents of the parentheses. Also remembering that gain A1 = VO1 / V.

A1 = VO1 / V = - μRP / (rp + RP + RK(μ + 1))     (11a)

Now returning to equation (8b) and performing the identical set of algebraic gymnastics we jump directly to equation (11b)

A2 = VO2 / V = μRK / (rp + RP + RK(μ + 1))     (11b)

And to refresh everyone's memory, VO1 is the voltage at output 1, VO2 is the voltage at output 2, V is the input generator voltage which is also the voltage applied to the grid of the triode, μ is the amplification factor, rp is the plate resistance, RP is the physical resistor connected to the plate, and RK is the physical resistor connected to the cathode. Where a voltage at a point is referenced to circuit common.

It is obvious by inspection that if RK = RP then A2 = A1. This is as it should be.

Output Impedance.

To obtain the output impedance we short the input and connect a signal generator to the output as in the figure below. The grid is an open circuit so even if there were a resistance or impedance in series with it there wouldn't be any current flowing through it to cause a voltage drop.

Figure 4 Small Signal Output Impedance at plate.

So we write the loop equations and solve as we did above for gain.

Sunday, June 19, 2016.

VGK + IbRK = 0     (1)
and
V - Ibrp + μVGK - IbRK = 0     (2)
solving (1)
VGK = - IbRK    (3)
Substituting (3) into (2)
V - Ibrp - μIbRK - IbRK = 0     (4)
Moving the = sign.
V = Ibrp + μIbRK     (5)
Dividing by Ib, factoring out RK,
V / Ib = R1 = rp + RK(μ + 1)    (6)

While we now have a value for R1 we have not accounted for RP. It is directly in parallel with the generator V and the current flowing through it is Ia which has not been included in any of the above. We must calculate the parallel combination of R1 and RP. We will use the product / sum formula.

RO1 = RP( rp + RK(μ + 1)) / (RP + rp + RK(μ + 1))     (7)

< b>Figure 5 Small Signal Output Impedance at Cathode.

Now we find the output impedance for output 2. The loop equations are as follows.

VGK + V = 0
and
V - μVGK - Ibrp - IbRP = 0
Skipping some familiar steps,
V + μV = Ibrp + IbRP
Skipping,
V / Ib = R2 = (rp + RP) / ( μ + 1)
Combining R2 and RK in parallel gives.
RO2 = (RK(rp + RP)) / ((μ + 1)(rp + RP + RK))

Where RO2 is the output resistance or impedance of the cathode output, RK is the physical resistor connected to the cathode, rp is the plate resistance of the tube, RP is the physical resistor connected to the plate, and μ is the amplification factor of the tube.

Numeric Calculations.

Now I am going to put these equations in a spreadsheet. For the 7199 the gain came out to be 0.8700. For the plate output the resistance was 14232 ohms and the cathode output was 505.25 ohms. Now I need to attempt some measurements to see how these calculated values measure up.

Monday, June 20, 2016.
A different tube type.

AES is listing the 7199 at 53.95 and says they are out of stock. In its stead they are selling an adapter socket to change to the 6GH8. Comparing tube parameters shows quite a lot of difference. Well let's see how they perform. I only have two of them. One is NOS but the other is of questionable origin. The quiescent values show that the triode is operating very close to grid current. The output had to be reduced to about 1 volt RMS to obtain anything like a good distortion value. The calculated value of gain is 0.9512.

I'm not going to buy any more tubes at this time. I'll try a 6U8 next and then a 6LN8. Fortunately they both have the same pinout as the 6GH8.

Wednesday, June 22, 2016.

My intended line of research was to try to find circuit values that would place all individuals of a given tube type on the sweet spot. That is the operating point which yields the minimum, and it is hoped, low distortion. This proved to be a dead end.

The empty dates below indicate days on which all work was done in vain.

Thursday, June 23, 2016.

Friday, June 24, 2016.

Saturday, June 25, 2016.

Something that distinguished the 6BL8 and 6LN8 from the 6U8 and 6GH8 is the amount of headroom in the triode section. Even the 7199 was not as good. I am defining headroom as the difference between the cathode voltage and the grid voltage in the triode section at the quiescent point.

Sunday, June 26, 2016.
Optimizing the Circuit for the 6U8.

The worst distortion was found in those with the least amount of headroom in the triode. I decided to ease up on the poor little thing and changed the cathode and plate resistors in steps until I arrived at 68 k ohms.

Tuesday, June 28, 2016.
A Discovery and a Realization.

My 6LN8s are all NOS RCA. My 6BL8s are 4 Raytheon, 1 Sylvania, and 1 Edicron, the word London in small print follows the manufacturer's name. Here is what I discovered. All 6LN8s but the Edicron are dual branded LCF80. All of the 6BL8s are dual branded ECF80.

I discovered that when the circuit from the 30 watt amplifier was used positive feedback was introduced. The RCA engineers must have known this which is why I suspect that the 15 and 50 watt amplifiers did not use this variation of the circuit.

Figure 6 Slightly Modified Circuit From 30 Watt RCA Amplifier.

I'll do a bit more mind reading on the RCA engineers later. But for now let's look at the behavior of the circuit in figure 6. Don't worry about the 1.5 meg ohm resistor to B+. I'll explain it later. For now just pretend it isn't there.

When used in the circuit of figure 1 the 6BL8/6LN8/LCF80 tubes have a lot of headroom in the triode section. That means virtually all of the distortion is coming from the pentode section. I needed to find a way to make every tube hit the sweet spot regardless of its particular set of parameters. I wired the circuit of figure 5 on my breadboard. I decided to decrease the value of the 180 k ohm resistor to give tighter control over the operating point.

Applying DC feedback from the cathode of the phase splitter back to the screen grid of the pentode is negative feedback. If the cathode voltage of the triode is too high the screen grid voltage is increased which increases the plate current of the pentode decreasing its plate voltage and causing the cathode voltage to change much less than it would have without the feedback.

I changed the resistor to 82 k ohms and turned on the B+. I had an oscillator. Puzzled, I turned off the power and checked the circuit again. Everything was right so I started increasing the value of the resistor. At 150 k ohms it stopped oscillating so I measured the gain and distortion. It showed a couple percent distortion and a gain in the low 800s??????? That can't be! I checked the circuit again and the settings of the generator and range of the voltmeter on the output. Everything was right. What the hell is going on?

I changed the resistor back to its original value of 180 k ohms and repeated the measurements. The DC cathode voltage of the triode was higher than I wanted so I connected a resistor from the screen grid to B+ to bring it down some. A 1.5 meg ohm got it where I wanted. The distortion was down some but the gain was 452 which is still outrages.

I studied the circuit again and it hit me like the proverbial ton of bricks. The 180 k ohm resistor couples AC signal from the cathode of the triode to the screen of the pentode along with the DC. The capacitor then couples those signals from the screen to the cathode. The cathode resistor is unbypassed so the coupled AC signal effects the cathode current of the pentode. There is no inversion from the cathode to the plate of the pentode and no inversion from the grid of the triode to its cathode. That's positive feedback no matter how you cut it.

Wednesday, June 29, 2016.
The Situation of Positive Feedback.

When I derived the feedback equation in my textbook I decided to assume that the feedback is negative and derived the equation from that starting point. It comes out with a positive sign in the denominator. All that is needed is to plug in the numbers and the answer comes out right.

In standard electrical engineering texts no prior assumption of the sign of the feedback is made. The equation is derived for the general case of signal from the output of an amplifier being fed back to its input. The equation which results is this.

A' = A / (1 - AB)

Where A' is the gain of the amplifier after the feedback has been applied, known as the closed loop gain, A is the gain of the amplifier before the feedback was applied known as the open loop gain, and B is the fraction of the output which is fed back to the input.

Note: All texts use the greek beta (β) for the feedback fraction. Making a β appear on your screen requires me to type 6 characters. That's why most web pages that cover this subject use the capitol B as I intend to do.

Using this equation requires just a small amount of thinking. If the feedback is negative the user must plug in the value of A as a negative number. If the feedback is positive the sign of the open loop gain A must be positive. Even though I delt with negative feedback only some students were sufficiently confused so as to never get a feedback problem right. Eventually I switched to teaching negative feedback using the form of the equation with a positive sign in the denominator.

Because we have positive feedback I need to use the general form of the feedback equation. We have two readymade examples in the data from yesterday. When using the circuit of figure 1 which had no positive feedback the overall gain for the 6BL8s came in around 151. Let's apply the general form of the feedback equation. First we must calculate the value of B for the 150 k ohm resistor. The positive feedback path is through the resistor labeled as 180 k ohm in figure 5, through the 22 uf capacitor to the pentode cathode. There is an 820 ohm resistor from the cathode to ground. I have not included the 10 ohm resistor on my breadboard but I have shown it to make a point later. For AC the 1.5 Meg ohm resistor is in parallel with the 820 ohm one and can easily be neglected. So,

B = 820 / (820 + 150 k) = 5.437 x 10-3

A' = A / (1 - AB) = 150 / (1 - 150 x 5.437 x 10-3) = 813

Well, that's pretty good agreement. In the next example I remembered to write down the gain value. So you won't have to go scrolling back up the page it was 452. Let's repeat the calculation.

B = 820 / (820 + 180 k) = 4.535 x 10-3

A' = 150 / (1 - 150 x 4.535 x 10-3 = 469

Since we can predict the gain with considerable accuracy we can safely say that there really is positive feedback in the circuit. Let's see what happens when we bypass the cathode resistor. The gain should go down.

It did but not as much as I had expected. With the screen bypass connected to ground to avoid any unforeseen feedback and the cathode resistor bypassed the gain changed to 382 and the distortion to 0.84%. Removing the screen bypass cap from the cathode eliminated the positive feedback which would have lowered the gain but bypassing the cathode increased it again. The distortion was improved. Removing the cathode bypass while leaving the screen bypassed to ground brought the gain down to 119 but the distortion was increased to 1.15%.

What Were the RCA Engineers Thinking?

Starting with the 15 watt amplifier the circuit of figure 1 was used and NFB from the output transformer secondary is taken through a parallel RC to the cathode of the pentode of the 7199.

Then we move up to the 30 watt amplifier. NFB is not taken to the cathode but to a 10 ohm resistor in series with the cathode resistor. The usual reason for doing this is because the cathode resistor is bypassed and an attempt to inject feedback at the cathode would meet with failure. But there is no cathode bypass cap. It seems possible even likely that the design engineer found exactly what I have found in regard to positive feedback. He, there were only a tiny number of women in EE in 1959, decided to bypass the cathode resistor to remove the positive feedback thus improving the distortion figure somewhat but not reducing the gain very much. After a prototype was built and the design presented to the other members of the design team he was probably asked if positive feedback inside the global feedback loop was detrimental. There is lag compensation C2 and R10 across the plate resistor of the pentode in addition to the usual lead compensation C6 across R18 which indicates that they had some trouble taming it. They probably found that removing the cathode bypass capacitor had no effect or an insignificant effect on the performance of the amplifier. Like good engineers they didn't include any unnecessary parts.

In static testing with the input shorted the variation in pentode plate voltage was greatly reduced by the DC feedback. Distortion in the amplifier stage is reduced be it triode or pentode as the plate voltage is increased but this puts more stress on the triode that is used as a split load phase inverter. The cathode and plate resistors will need to be either higher wattage or higher resistance. The tube itself may be forced into its positive grid region which causes a lot of distortion. The only cure for this is to either lower the grid voltage or increase the resistance values of plate and cathode resistors.

I should note here that the Harmon Kardon A300 has positive feedback which was consciously and deliberately introduced. Some testing I did several years ago revealed that the gain increased more than the distortion. Increased gain results in more negative feedback and a reduction of overall distortion. I think it is safe to say the engineers at HK knew what they were doing so maybe we should credit the RCA engineers with the same degree of wisdom.

What Happens With More Positive Feedback?

In the two examples worked above the value of AB is less than unity. As you can see the application of this much positive feedback increases the gain and does the opposite of what is done by negative feedback.

If AB = 1 the denominator goes to zero and the value of A' goes to infinity. The amplifier delivers output with no signal input. That's commonly known as an oscillator. The amplitude of oscillation is constant. If the value of AB is slightly less than 1 the amplitude will slowly decay. If the value of AB is slightly greater than 1 the amplitude of oscillation will slowly increase. But what happens in a real circuit? There aren't any easy answers in simple algebra. What the advanced forms of mathematics tell us is that the amplifier oscillates and the amplitude keeps building up forever. What happens in the physical world is that the amplitude of oscillation builds up until the amplifying device begins to saturate. This leads to a decrease in gain and an equilibrium is reached in which AB is maintained at exactly unity.

In a low distortion oscillator the amplifying device is not allowed to saturate. A sensing and control circuit adjusts either the value of A or B at a certain output level. This control circuit holds the value of AB at exactly 1 without the device saturating.

I have included this information because it was not taught to me well and I didn't understand it until I was in graduate school.

End of digression.

Thursday, June 30, 2016.
So where do we go from here?

I'm giving up on the screen grid feedback circuit. The negative DC feedback is good and if it could be increased a little it would make the operating point very repeatable. But the side effect of positive AC feedback sets an upper limit on the amount of control feedback and is very likely to make the global feedback loop hard to stabilize. Before I leave this topic I am going to try a circuit in which the control feedback is taken to the control grid. That should preclude any incidental positive feedback.

But for now I am going back to the open loop circuit and see what is to be found by juggling the values of cathode and screen resistors. This too proved to be a dead end. ,

Tuesday, July 05, 2016.

Meanwhile back at the ranch I have realized that I have been too conservative with power supply voltage. If I just push it to the Max, that is my name after all, 400 volts, I can get lower distortion figures than I have been getting. The output voltage is set at 25 volts which will drive a pair of EL34s to 55 watts. For example, running through my samples of 6BL8/ECF80 tubes I get distortion figures ranging from 0.06% to 1.5%. I am using the values from the 7199 circuit. There is no single set of values that will give the lowest distortion from each tube. I'm going to try some closed loop control of the operating point.

Wednesday, July 06, 2016.
BINGO!!!

Figure 7 Amplifier/Inverter with Closed Loop Q-Point Stabilization.

The -100 volt supply is derived from the adjustable bias output of the Heathkit IP17 power supply. Adjusting the voltage of this supply changes the cathode voltage of the triode section.

I ran this circuit with the 6BL8/ECF80 and 6LN8/LCF80. All but one of the 6BL8s gave distortion figures between 0.3 and 0.4%. The one that fell outside this range was the Sylvania which was at 0.78%. It also showed a distinct dip in the distortion as the operating point was adjusted. The 6LN8s showed the same distortion range with the same setting of the bias control. This setting led to a triode cathode voltage of approximately 120 volts. This is right at the limit of the 15 k ohm 1 watt resistors in the plate and cathode of the triode section. I don't have any 2 watt resistors. Note: I have no hesitation about running resistors right up to their limit as they are on a breadboard and exposed to free air. Also the operation is intermittent with short on times and off times that are at least as long and often longer.

For tomorrow night's testing I am going to increase the triode plate and cathode resistors to 22 k ohms. This will allow operation up to 148 volts. Also I will reduce the value of the 100 k ohm resistor so I can get to this voltage with the bias supply in the IP117. Note: the resistor which is labeled as 3.0 k ohms in the diagram was actually 2.2 k ohms. It will have to be changed when the 22 k ohm resistors are substituted. The calculated value is 4.94 k ohms which is almost halfway between 4.7 k and 5.1 k. Although it is just a bit closer to 5.1 k ohms.

Thursday, July 07, 2016.

Here's what the workbench looks like.

I could overwhelm you with data but I think at this point data for one tube type will make the point. I ran through the six 6BL8 tubes using the following procedure.

A. Adjust the input signal so the output is 25 VRMS.
B. Adjust the Set Level on the distortion analyzer for 100%.
C. After these two steps have been completed the 100% reading on the analyzer can be used to set the output to 25 volts.

  1. Adjust the input level so the HD analyzer indicates 100% in the Set Level mode.
  2. Measure the distortion and while it is being indicated adjust the bias control for minimum distortion.
  3. Return to the Set Level mode and correct the input level for a 100% reading.
  4. Once again measure the distortion and while it is being indicated again adjust the bias control for minimum distortion.
  5. Record this value as the distortion value.
  6. Record the DC voltage at the Triode Cathode.
  7. Read the output voltage of the signal generator and divide the value into 25 volts to obtain the gain. Record this value.
  8. After these columns have been filled in select the tube that gave the highest distortion.
  9. Install that tube and repeat steps 1 through 4. All readings should be very close to the ones obtained before for that tube.
  10. Do not change the bias control as you reinstall each tube in turn and record the results.

The bias control adjusts the value of the source that is labeled -100 volts in the diagram of figure 7.

I do not intend that you are to follow the instructions above. These are the steps that I took to fill in the table below.

TABLE 1 6BL8
B+ = 400 V, output voltage = 25 VRMS.
First Run Through
VKT Optimized for Each Tube
Second run through
VKT set for Worst Tube
Tube %THD Overall
Gain
DC VKT %THD Overall
Gain
DC VKT
6BL8 1 0.118 148 138.1 0.220 150 125.3
6BL8 2 0.222 138 115.3 0.225 137 122.1
6BL8 3 0.058 149 130.4 0.070 152 126.2
6BL8 4 0.105 152 111.4 0.215 147 124.7
6BL8 5 0.72 121 125.9 0.72 121 124.8
6BL8 6 0.070 152 125.5 0.070 154 124.0

As far as I am concerned this is it. The final word. Each tube type will have its own sweet spot but it appears if the operating point is selected for the worst case of any type the ones that showed up much better than that one will be a little worse than they were but still better than the worst tube. I guess it is up to me to determine the sweet spot for each tube type. I'll do my best. Meanwhile an edited version of this page will soon appear on the fun with tubes site.

Friday, July 08, 2016.
P. S.

TABLE 2 6LN8
B+ = 400 V, output voltage = 25 VRMS.
First Run Through
VKT Optimized for Each Tube
Second run through
VKT set for Worst Tube
Tube %THD Overall
Gain
DC VKT %THD Overall
Gain
DC VKT
6LN8 1 0.13 128 119.8 0.13 130 118.5
6LN8 2 0.0635 156 131.6 0.066 158 124.9
6LN8 3 0.090 149 120.3 0.105 148 123.3
6LN8 4 0.265 138 123.6 0.265 138 123.6
6LN8 5 0.220 138 123.0 0.215 137 123.7
6LN8 6 0.194 151 104.7 0.265 147 121.5

Sunday, July 10, 2016.
Case Reopened.

In the last paragraph of the entry for July 7 I state that I feel responsible for determining the sweet spot for each tube type that is likely to be used in this application. That means that I can't close the case until I have determined it for enough tubes to satisfy most amplifier builders.

Although 7199s are no longer available at a reasonable price there may be some out there for whom money is truly no object. Here are the results for the ones I have.

TABLE 3 7199
B+ = 400 V, output voltage = 25 VRMS.
First Run Through
VKT Optimized for Each Tube
Second run through
VKT set for Worst Tube
Tube %THD Overall
Gain
DC VKT %THD Overall
Gain
DC VKT
7199 1 0.090 144 129.8 0.090
0.116
144
146
130.4
125.3
7199 2 0.083 156 126.6 0.115
0.071
155
157
132.4
127.4
7199 3 0.073 146 121.9 0.140
0.076
144
146
128.4
123.9
7199 4 0.078 155 116.4 0.195
0.145
152
154
131.0
126.0

After making measurements on other tube types I was amazed by the distortion figures for the optimized operating point. With all four tubes being less than 0.1% I really didn't have any worst tube to use as the standard. Selecting tube 1 as being least best I collected the data in the first row in each cell of the second set of data. That gave distortion figures ranging from 0.090% to 0.195%. Any sane person would be content with that but I am a perfectionist. For the second row in each cell I averaged the DC cathode voltage and used that as the sweet spot. The voltage reading changes quite a lot for a small movement of the bias knob so I got as close as I could. With distortion figures ranging from 0.071 to 0.145 there really isn't a lot of difference in the worst case but the average comes out looking better.

Building an Amplifier around the Circuit of Figure 7.

If you are building an amplifier of 25 watts or more most likely you will have a B+ supply of at least 400 volts. You will be using fixed bias so you will have a negative power source. Do I need to say that the negative voltage needs to be very low noise?

Probably the easiest way is to connect the end of the 100 k ohm resistor to the wiper of a pot that allows the voltage to be adjusted from 0 to -100 volts. The voltage at the triode cathode will be approximately 1.5 times the voltage at the end of the 100 k ohm resistor. If you don't have that much negative voltage in your amplifier you can decrease the value of the 100 k ohm resistor. 1 k ohm per volt at the setting farthest from zero should come out about right.

I have adjusted the RC time constants of the input and the DC feedback so there is no infrasonic peak. You may think that 0.1 uf coupling into 100 k ohms is a bit small but this and the low end of the output transformer will be the only significant poles at the low end. When the global feedback loop is closed up you will probably have to decrease the values of the two 0.1 uf coupling caps to eliminate a low frequency peak which will likely appear.

Wednesday, July 13, 2016.
Something Else While I wait.

Because they deserve a fair test I ordered some 6GH8s from AES along with some 6AN8s because its triode section has an amplification factor of 19. Experience has shown that a low μ makes the best split load phase inverter. The 6GH8 with its triode μ of 40 will have to be checked out.

While I wait for the delivery service to come through I decided to test a combination of individual tubes. Specifically a 6C4, 1/2 of a 12AU7, and a 6BH6. The 6C4/12AU7 has been used successfully for many years in many amplifiers both commercial and home brew. In some testing I did several years ago the 6BH6 proved to have the lowest distortion of the 7 pin miniature family of sharp cutoff pentodes. One interesting problem arose. Some of my stock of 6BH6s are ringers. They test OK on a tube tester but in reality they are remote cutoff pentodes. Needless to say I eliminated them from the test.

I also became aware that the voltage on the control grid of the pentode may be positive or negative. Obviously what I need is a nonpolar electrolytic cap but two polarized ones back to back or belly to belly will have to do for the moment. Note the change in Figure 7. Although I have collected the data it's getting late and I don't have time to type it in tonight.

Thursday, July 14, 2016.
Separate Tubes For the Pentode and Triode.

My new breadboarding system has given me a great deal of flexibility. The electrical connections are very reliable and will withstand at least 600 volts because everything is doubly insulated. Building it took a lot of time and effort but I am glad I did it.

TABLE 4 Separate tubes, 6BH6 and 6C4
B+ = 400 V, output voltage = 25 VRMS.
Changing 6BH6
First Run Through
VKT Optimized for Each Tube
Second run through
VKT set for Worst Tube
Tube %THD Overall
Gain
DC VKT %THD Overall
Gain
DC VKT
6BH6 1 0.081 154 122.9 0.080 154 125.1
6BH6 2 0.113 152 118.0 0.163 149 123.8
6BH6 3 0.081 149 124.2 0.068 150 124.2
6BH6 4 0.105 148 124.0 0.128 148 124.6
6BH6 5 0.088 152 123.5 0.10 152 124.5
6BH6 6 0.116 153 108.6 0.165 144 123.2

When testing pentode triode tubes the two are inseparable. When using separate tubes it is possible to change one at a time. Thus I have two tables. I didn't try to find a new sweet spot for the different triodes although in retrospect maybe I should have.

As I increase the voltage the distortion falls off gradually and then rises precipitously. I attribute the gradual fall to the pentode and the precipitous rise to the point where the triode starts to draw grid current. Last year I did some testing on triodes which revealed that low plate voltage increases distortion and as the plate is operated at a higher voltage with the same plate load resistor the distortion decreases. Until proven otherwise I will assume that the same applies to pentodes.

TABLE 5 Separate tubes, 6BH6 and 6C4
B+ = 400 V, output voltage = 25 VRMS.
Changing 6C4
Tube %THD Overall
Gain
DC VKT
6C4 1 0.081 155 120.2
6C4 2 0.11 153 120.0
6C4 3 0.075 152 124.0
6C4 4 0.145 152 124.1
6C4 5 0.209 149 124.6
6C4 6 0.153 148 127.7

Saturday, July 16, 2016
6AN8.

I ordered six 6GH8s and six 6AN8s from AES. Although I said earlier that I wasn't going to order anymore tubes I really need to test the 6GH8 as a replacement for the 7199. I ordered the 6AN8s because its parameters looked promising.

The 6AN8 appears to have a unique pinout among the pentode triodes. It required a total rewire of the breadboard. It had to be done but the results were disappointing. The tubes fell into two distinct groups. One group featured higher distortion than the other and also a higher sweet spot voltage. There were 4 tubes in the first group branded Dumont. Their distortion ranged from 0.398 to 0.93 but the sweet spot voltage ranged from 129.4 to 132.1. The second group of 2 tubes were a Raytheon and an RCA. Their distortions were 0.19 and 0.25 respectively with sweet spot voltages of 111.0 and 119.4. The Dumont's scattered their gains from 128 to 146 while the Raytheon and RCA were 140 and 146. In the latter case a larger sample would have helped but what I have is what I have. I just realized that the 6BR8 which I have six of is the tube that Tim E. Smith used in his contest winning amplifier. I should also test it. It's been a while but I think he gave them to me. If so, thanks Tim.

TABLE 6 6BR8
B+ = 400 V, output voltage = 25 VRMS.
First Run Through
VKT Optimized for Each Tube
Second run through
VKT set for Worst Tube
Tube %THD Overall
Gain
DC VKT %THD Overall
Gain
DC VKT
6BR8 1 0.099 146 115.5 0.29 144 119.4
6BR8 2 0.38 140 122.8 0..38 140 119.1
6BR8 3 0.078 139 111.9 0.32 137 118.5
6BR8 4 0.283 130 117.6 0.29 130 117.0
6BR8 5 0.345 117 122.7 0.36 120 115.6
6BR8 6 0.25 120 121.4 0.29 123 117.4

About all that can be said is that DC feedback makes the distortion uniform from tube to tube and presumably would stabilize it as the tube ages. The level of distortion is that of the worst tube. This tube would be quite acceptable inside a global feedback loop but not so good for a feedback free amplifier.

The next step is to go back to the 6U8 pinout so I can test the 6GH8. After that I am going to test a 6AU6 as the split load phase inverter and then take another look at the 12DW7 which is 1/2 of a 12AX7 and 1/2 of a 12AU7 in one tube. An amplifier without NFB would not need nearly as much gain in the amplifier stage as is needed in one with feedback.

Wednesday, July 20, 2016.
6GH8

TABLE 7 6GH8
B+ = 400 V, output voltage = 25 VRMS.
First Run Through
VKT Optimized for Each Tube
Second run through
VKT set for Worst Tube
Tube %THD Overall
Gain
DC VKT %THD Overall
Gain
DC VKT
6GH8 1 0.44 139 122.1 0.495 140 119.2
6GH8 2 0.62 132 119.4 0.705 132 119.6
6GH8 3 1.54 75.8 125.9
6GH8 4 0.123 127 119.4 0.108 130 117.3
6GH8 5 0.66 134 120.2 0.66 136 118.8
6GH8 6 0.495 109 117.1 0.50 112 116.4

Tube 3 appears to be defective. Every time I plugged it in it gave different results. I made a graph of distortion and gain versus triode cathode voltage and found that the distortion changed very little while the gain changed drastically, by a factor of 2 between 110 and 125 volts. This foiled my technique of adjusting the voltage for minimum distortion. This is unlike other tubes in this series of tests in which the gain changes only slightly while the distortion changes drastically passing through a distinct minimum.

Saturday, July 23, 2016.
A Pentode as the Split Load Phase Inverter.

Figure 8 Amplifier/Inverter Using Separate Tubes.

On this date I tested a 6AU6 and some other pentode tubes as the phase splitter. This work depends on the 6BH6 and 6C4 combination. Tables 4 and 5 are repeated here so you won't have to scroll back and forth to compare the data.

TABLE 8 (TABLE 4 Repeated) 6BH6 and 6C4
B+ = 400 V, output voltage = 25 VRMS.
Changing 6BH6
First Run Through
VKT Optimized for Each Tube
Second run through
VKT set for Worst Tube
Tube %THD Overall
Gain
DC VKT %THD Overall
Gain
DC VKT
6BH6 1 0.081 154 122.9 0.080 154 125.1
6BH6 2 0.113 152 118.0 0.163 149 123.8
6BH6 3 0.081 149 124.2 0.068 150 124.2
6BH6 4 0.105 148 124.0 0.128 148 124.6
6BH6 5 0.088 152 123.5 0.10 152 124.5
6BH6 6 0.116 153 108.6 0.165 144 123.2

When testing pentode triode tubes the two are inseparable. When using separate tubes it is possible to change one at a time. Thus I have two tables. I didn't try to find a new sweet spot for the different triodes although in retrospect maybe I should have.

As I increase the voltage the distortion falls off gradually and then rises precipitously. I attribute the gradual fall to the pentode and the precipitous rise to the point where the triode starts to draw grid current. Last year I did some testing on triodes which revealed that low plate voltage increases distortion and as the plate is operated at a higher voltage with the same plate load resistor the distortion decreases. Until proven otherwise I will assume that the same applies to pentodes.

TABLE 9 (TABLE 5 Repeated) 6BH6 and 6C4
B+ = 400 V, output voltage = 25 VRMS.
Changing 6C4
Tube %THD Overall
Gain
DC VKT
6C4 1 0.081 155 120.2
6C4 2 0.11 153 120.0
6C4 3 0.075 152 124.0
6C4 4 0.145 152 124.1
6C4 5 0.209 149 124.6
6C4 6 0.153 148 127.7

Some on the Fun With Tubes email forum have been urging me to test a 6AU6 as the split load phase inverter. Here you go. I also examined the 6BH6, 6CB6, and 6AQ5 as the pentode. I chose the 6BH6 out of curiosity because it showed low distortion when operated in other modes. I selected the 6CB6 because of its use by RCA in its 50 watt amplifier design, and the 6AQ5 out of more curiosity.

TABLE 10 Separate tubes, 6BH6 and 6AU6
B+ = 400 V, output voltage = 25 VRMS.
Changing 6AU6
Tube %THD Overall
Gain
DC VKT
6AU6 1 0.25 168 111.1
6AU6 2 0.325 170 109.0
6AU6 3 0.34 171 108.8
6AU6 4 0.34 170 109.2
6AU6 5 0.51 171 103.5
6AU6 6 0.35 171 107.2

TABLE 11 Separate tubes, 6BH6 and 6BH6
B+ = 400 V, output voltage = 25 VRMS.
Changing second 6BH6
Tube %THD Overall
Gain
DC VKT
6BH6 2 0.26 170 109.4
6BH6 3 0.265 170 108.4
6BH6 4 0.29 171 107.8
6BH6 5 0.288 171 107.6
6BH6 6 0.259 170 109.2
6BH6 7 0.281 170 107.7

TABLE 12 Separate tubes, 6BH6 and 6CB6
B+ = 400 V, output voltage = 25 VRMS.
Changing 6CB6
Tube %THD Overall
Gain
DC VKT
6CB6 1 0.34 171 109.1
6CB6 2 0.405 170 107.9
6CB6 3 0.355 171 109.2
6CB6 4 0.38 172 108.2
6CB6 5 0.33 171 107.4
6CB6 6 0.262 170 110.0

TABLE 13 Separate tubes, 6BH6 and 6AQ5
B+ = 400 V, output voltage = 25 VRMS.
Changing 6AQ5
Tube %THD Overall
Gain
DC VKT
6AQ5 1 0.174 128 142.1
6AQ5 2 0.155 134 135.3
6AQ5 3 0.177 132 135.4
6AQ5 4 0.0405 131 139.2
6AQ5 5 0.135 132 136.0
6AQ5 6 0.159 140 125.1

When given sufficient B+ the 7199 really shines. Even with its heater to cathode problems I would still recommend it if it were not priced as if made of gold with an envelope made of diamond. If I were the one selecting the substitute in existing equipment I would go with the 6BL8 with the 6LN8 as second choice. The 6GH8 is only so so. However, I suspect that the management of AES made the choice based on information not available to me. I am thinking of number of tubes in stock. The audible performance of an existing amplifier that has a properly designed global feedback network would probably not be effected by the substitution.

In new homebrew equipment I would recommend the circuit with two 6BH6s. Second best in this group seems to be the 6BH6 6C4 combination. In a stereo amplifier the two 6C4s could be replaced by a single 12AU7. A home brewer who has the test equipment to measure distortion would be well advised to make the bias reference adjustable and tune up the inverter circuit for lowest distortion before closing up the global feedback loop. Even if you must use the values I have provided you will do quite well.

The Miss Consistency award goes to the small signal pentodes. The 6AQ5 looks interesting. Although I haven't tested it I think it would output considerably higher voltage than the other tubes. Higher drive voltage is needed in such circuits as cathode loaded and circlotron output stages.

Sunday, July 24, 2016.
Triode Triode, the 12DW7/7247.

Figure 9 Amplifier/Inverter Using Triodes.

Usually when I order tubes I receive an assortment of NOS from various manufacturers. When I ordered 6 12DW7s from AES I received all GE which are in military boxes. You know the type, a label that reads

ELECTRON TUBE
1 EACH
DLA900-85-M-W290
A 3/85
GE 7247
GENERAL ELECTRIC CO.

All 6 are in identical light gray boxes that show not the slightest tendency to disintegrate upon being opened. Six out of many tubes that were supplied to the military in 1985, if the fourth line is indeed a date, are likely to have a degree of similarity not even found in tubes for the civilian market. Therefore table 14 below may be pointless. But it is presented anyway.

TABLE 14 12DW7/7247
B+ = 400 V, output voltage = 25 VRMS.
First Run Through
VKT Optimized for Each Tube
Second run through
VKT set for Worst Tube
Tube %THD Overall
Gain
DC VKT %THD Overall
Gain
DC VKT
12DW7 1 0.28 71.2 122.4 0.275 71.2 123.4
12DW7 2 0.40 72.0 124.0 0.425 72.0 123.2
12DW7 3 0.435 72.9 124.0 0.44 72.7 123.8
12DW7 4 0.385 72.9 122.4 0.38 72.7 123.2
12DW7 5 0.32 72.0 124.1 0.335 72.0 123.2
12DW7 6 0.325 72.0 123.8 0.33 72.5 123.3

I found the upper -3dB frequency to be 119 kHz. Pretty good considering the large value of plate load resistor. I did try it with an unbypassed cathode resistor. As you might expect the gain was reduced and by a factor of almost 1/2. However the distortion was increased. I think I can explain that one. The -3 dB frequency was reduced to 34.6 kHz. This indicates an increase in the impedance level at the plate of the high μ triode. This is to be expected as an unbypassed cathode resistor introduces current feedback which increases the output impedance. Increased output impedance can make the voltage at the plate of the first triode and the grid of the second triode, they are connected, more sensitive to small changes in grid current as the grid to cathode voltage approaches zero. So the circuit shown in figure 14 appears to be the best.

I wonder what bright young graduate engineer decided to place section 2 of the tube on pins 1, 2, and 3 with section 1 on pins 6, 7, and 8. I guess we'll probably never know the answer to that one.

What Can We Conclude From All This Data?

Before answering that question I must admit that I am being much too picky. Any of these circuits would be quite at home in a negative feedback loop. Remember that distortion adds not directly but as the square root of the sum of the squares. We want to know how much distortion a driver circuit can have before it effects the overall distortion by a certain percentage. Percent change is translated into a multiplier by

M = 1 + %change

The Percent Distortion of Driver PDD is given by

PDD = sqrt(M2 - 1) x Percent Distortion of output tubes.

So if you will only except an increase of distortion of 5% over the distortion of the output amplifier stage the allowable percent is

PDD = sqrt(1.052 - 1 ) x Output distortion.
PDD = 0.32 x output distortion.

Taking the tube manual data for the 6L6 as an example the distortion for a pair in push pull at 25 watts is 2%. a driver circuit must have a distortion of 0.64% to increase the overall distortion by 5% over that of the 6L6s themselves. Which is 2.1%. Only one of the tubes in one of the circuits came close to this value. So what am I in such a lather about. Damned if I know. I guess I just like to have things as good as they can possibly be.

Some time ago I tested a pair of 6550s and found their actual distortion was considerably below the tube manual value of 3%. . Unfortunately I don't remember the number but I am sure it was less than 1%. Let's say it was 1%. To avoid boosting this value to 1.1% the distortion in the driver must be less than 0.458%.

That leads us to the question "can anyone really hear the difference?" Although the difference in the numbers seems so small as to be inaudible direct experience says that many people can hear the difference. My own wife Sue has a hearing loss that requires her to wear hearing aids. In spite of this as I try out different amplifiers with similar but not identical numbers she can hear the differences. She is a singer and her musical training may account for this ability. I find that if I just don't think about it I can hear the difference. If I try too hard to hear a difference they all start sounding the same. I have no explanation of this phenomenon.

Where that leaves me is to try to get the numbers as low as I can because I am an engineer first but then listen and ask Sue's opinion as a backup.

Case Closed. I hope.

AM Radio Stations Heard.

Tuesday, May 03, 2016.

After Sue goes to bed and I have nothing to do I fire up the 5 tube superhet which now has transistor IF transformers doing the work of tube IF transformers. The receiver seems dead above about 1000. I think I need a small capacitor connected from primary to secondary of antenna coil as I often see in older radios that do not have a built in antenna. Here's the list of what I have heard so far.

Entries are nighttime unless otherwise noted.

610 WRUS Russellville KY. Local daytime.
620 WRJB
630 WLAG or WLAP
650 WSM Music City. Day and night
660 WFAN
670 WSCR Sports
700 WLW Cincinnati
720 WGN Chicago
750 WSB Atlanta
760 WJR Detroit
770 WABC New York
780 WBBM Chicago
830 WCCO
840 WHAS Louisville
860 Speaking French
870 WWL New Orleans
880 WCBS New York
890 WLS Chicago
930 WKCT Local. Bowling Green. Can see towers from kitchen window.
1000 WNBP
1020 KDKA Pittsburgh
1040 WHO Des Moines Iowa. I grew up listening to this one.
1120 KMOX St. Louis
1130 Speaking Spanish.
1340 Sports, local station
1450 sports, local station.

I have a lot of interference between about 1100 and 1500. It sounds like sync pulse buzz as in old analog TV. I think it's the internet data leaking out of my cable modem box which is in a plastic case. I need to string up a wire antenna that goes out away from the house. Now I am using a wire strung the length of the house in the attic.

Slim possibility of updates.

Another Breadboard.

Monday, April 25, 2016.

Solder breadboards are reliable and stable but they do take a bit more effort to build and change. The DeVry modular breadboard system is probably the best one ever developed for tubes and I was fortunate enough to have stocked up on the pieces and kept them. I know this doesn't do anyone else any good which is why this will never become a "how to" page on this site. This is my report on what I am doing and this is what I am doing.

Here is a photo of the original DeVry system that I acquired in the 1960s.

The sloping panel platform originally had a small power supply that delivered 6.3 VAC, 18 VDC and 150 VDC. It was born out of the period of transition between tubes and transistors. The system is suitable for discrete transistors but totally unsuitable for integrated circuits. I have changed the power supply to +/- 1.2 to 15 volts regulated. I also added the edge connector for troubleshooting the plugin modules in my home brewed quadrophonic preamp and control center.

For breadboarding tube circuits it's still a bit limited. Space is the most obvious. Ease of use is another. Because the tubes sit upright the pins must be counted backward and there have been times when I became confused and couldn't figure out why a given circuit just wouldn't work when it should.

The nicest thing about the solder breadboards is that the tubes are operated base up which means that pins are counted forward and it gets the tubes out of the way. I have acquired more than a few blisters from working on a power amplifier on the DeVry system. I am working on a plan wherein I will remove two of the red panels from the chassis and mount them individually on legs with the tubes underneath. Wires will come up through a center hole to connect tube elements to the modular connectors and the rest of the circuit. The underpinning of the panels will be aluminum to provide a ground plane for good high frequency performance of audio amplifiers and even some low end RF. ,

I will add to this when I have some actual pictures to show. Right now all I have are some elevation views of the planned devices and they are rather confusing with their many hidden lines.

Monday, May 09, 2016.

I did a very careful design and then started mass producing the pieces. I have nine of the red panels so I will have 3 with 7 pin mini, 3 with octal, and 3 with 9 pin mini sockets. The design calls for 10 aluminum angle pieces for each panel. That's right, 90 pieces to be cut.

First step, cut each piece to length.

The process employs a stop block, woodworking term, to set the length of each piece. The end of the angle stock is pushed against the stop and the toggle clamp closed. Then the sled is pushed through the band saw.

Some of the pieces need a 45 degree angle cut but I neglected to take pictures of this part of the process. I used the same jig as that used to make 45 degree cuts on the circuit board for the IF transformers.

Some of the pieces needed to have 1/8 inch ripped off the edges so that piece will snuggle inside a full sized angle piece and not hang over.

The blue tape was to fine tune the amount of aluminum being removed.

The second stop block, in this case it may be more accurately called a rip fence, was added without removing the other block. Also the clamp had to be shifted. The extra holes are more visible in the previous picture.

The second block had to be removed and replaced several times as I needed to go back and correct mistakes.

The last pieces cut were the legs on which each assembly will stand. They are approximately 5-1/2 inches long. I say approximately because I didn't get the stop block mounted exactly in the right place.

Although I am working in metal I am applying woodworking philosophy. For example if a table is to be 29 inches high it really doesn't matter if it is actually 28-7/8 inches or 29-1/8 inches high. The important thing is that all 4 legs be exactly the same length.

The legs are a little off from 5-1/2 inches but they are all off by the same amount. Here is a picture of all 90 pieces of angle aluminum next to the 9 red plastic panels.

Tomorrow I start drilling and counter sinking in the plastic. Wish me lots of luck. I am aware that this must be done slowly to avoid melting the plastic instead of cutting it. The purpose of the angle cut pieces will become clear when you see tomorrow's pictures.

A digression, the source of the angle aluminum.

Last year a friend called me and asked if I would like a tube AM KW ham transmitter. At first I said I didn't think I had room for it. He said he didn't either and he had to get rid of it. A friend of his had given it to him many years ago and he had kept it for those years. I suspect his XYL had laid down the law but I don't know that for a fact. Anyway I agreed to take it off his hands but I stated that I would probably strip it for parts rather than try to put it on the air. He said that was alright with him.

He and one of his sons delivered it in a pickup truck. It was too tall to fit in my storage shed. We put it in my shop where it did fit but the door was too low and it had to be brought in horizontally and then stood up. With it taking up space in my shop I had a strong incentive to start breaking it down.

It hurt to do that but I had no choice. The rig was a pair of 100THs modulated by another pair. I saved the HV power supply which came without rectifier tubes, the modulator which did come with tubes, the VFO, and all transformers, relays, tubes, switches, pots, and RF parts.

I also saved the angle aluminum which the rack was made of. That's right, he made his own rack. He used 1 x 1 inch angle stock for lighter parts of the rack but 1-12 by 1-12 inch stock for the load bearing parts. Some of the pieces were 1 x 1-12 inch stock.

I used up all of the 1 x 1 stock in this project. While cutting the legs I ran out of 1 x 1 stock and had to use some of the 1 x 1-1/2 stock. My first thought was to rip the 1-1/2 side down to 1 inch but I decided that it didn't really make any difference if about half of the legs were different.

His call, which I don't know, hasn't been heard on the bands for several decades. His rig which I am sure he was proud of will never be heard again. But a part of it will live on in the service of tubes. Maybe that will give him a little consolation if he is up there watching.

End of digression.

Tuesday, May 10, 2016.

The holes in the red panel are where the white connector modules are inserted. Each connector module has 5 holes for the insertion of wire and all 5 holes are electrically connected. When a mounting screw uses up a hole a connector module can't be placed there.

My original plan was to drill and counter sink holes between the holes intended for modules. I decided that this was just too much precise work and I would go ahead and use up an existing hole.

DeVry had mounted the panels to the base shown above using #4 self tapping screws. These were pan head screws and often caused modules to be placed in an undesirable position. Using flat head screws will minimize the impact of the locked out holes.

Now you see why the 45 degree cuts were necessary. An aluminum plate will be mounted to the bottom which will hold a tube socket. Four legs will be mounted at the corners. Two down seven to go.

Friday, May 13, 2016.

Making these small parts is going much slower than I had expected. When making parts on a mass production basis precision is required or they won't fit. This is mass production as opposed to fitting each part individually. Custom fitting takes even longer and results in parts that are not interchangeable.

I call these parts the tube socket plate brackets. The tube socket plate will mount across them. The holes that you can see are tapped to make changing the plate and socket easy. This is my plan ahead strategy in case I need to build a circuit with more than 3 tubes having the same number of pins. When I quit at the end of the day I had only two more brackets to make. The legs are already mass produced so all that remains are the nine tube socket plates, three nine pin, 3 octal, and 3 7 pin. Once again precision is required so the mounting holes in the plates will align with the tapped holes in the brackets.

Monday, May 16, 2016.

I spent most of today tapping holes. While working on the next to last hole the tap broke. After extracting the broken end of the tap I was able to thread a screw into the hole. Sue wasn't home so I couldn't go into town to get another tap. I went looking for and found a self tapping screw. For those who may not know they are made out of harder metal than the average screw and have a cut in the end of the screw. Also it is tapered at the end. This allows it to be threaded into a hole and it will cut its own threads. It was a hex head which made it easy to drive. I ran it in, took it out, and threaded in one of the intended screws. It worked. I dropped the self tapping screw into the small box where I keep my taps. It's not a recommended way of tapping holes but it worked. A self tapping screw will wear out if used to tap many holes. It is only intended to cut threads for itself. The idea is that the screw will be left in place after it cuts the threads.

I have started cutting holes in the red plastic panels for wires from the tube sockets to pass through to the working side of the panel. I cringe a little at defacing the panels but this will probably be their last use. Film at eleven.

Monday, June 06, 2016.

I have spent most of the time drilling and countersinking holes. It's a slow process when each hole has to be exactly in the right place. I am now in the assembly phase. I must admit that in spite of my best efforts such as using drilling jigs to insure repeatability, Pete has not come to the party. Maybe I would have been better off to drill the holes in the legs and then use each leg as the drilling guide for the holes in the particular module. Oh well, there seems to be no end to learning from experience.

The large holes have been drilled in the red panels and the tube socket plates have been drilled, punched, and the sockets mounted to them.

This picture shows one tube socket plate with socket temporarily mounted on one of the breadboarding modules and another plate and socket with wires soldered to the socket pins. The hole in the red panel is visible. I am using the numerical color code used throughout electronics. Black is ground and that wire is soldered to a solder lug that is mounted on the plate. After that brown is for pin one of the socket and so on to white for pin nine. Sue used some of her artist paint to add color to the connector blocks as seen in the picture below.

The connector blocks will only take solid wire which will break if bent repeatedly. My approach is for the connections to the tube socket to be unchanged once installed. If it ever does become necessary to change a tube socket as I mentioned earlier it will be avoided, if possible, by making a socket adapter.

I gave considerable thought to the placement of the connector blocks. The 9 pin tube most likely to be used in this breadboarding system is the generic dual triode such as the 12A_7 series. Pins 1, 2, and 3, are all together. These are the plate, grid, and cathode, of triode 1. Pins 4 and 5 are together and separated from the others. These are the heater end pins and are indeed the heater pins for virtually all 9 pin tubes. This should reduce induced hum caused by the use of an AC heater supply. Then again pins 6, 7, and 8, are together for access to plate, grid, and cathode, of triode 2. Pin 9, heater tap or internal shield in some tubes is next to the ground connection. The picture below shows the legs attached and an octal tube plugged into the socket of one of the modules which is upside down. Or as my dad used to say when I was very small "downside up".

The module which is right side up is one for a 7 pin tube. The heater pins, 3 and 4, are away from the rest of the pins also for hum reduction. I was not able to apply any similar logic to the octal tube because there is so much variability among these tubes.

In the picture below you will see that I have changed the placement of the connectors for the 7 pin tube. The revised placement allows wires from the blocks to the tube socket pins to run more directly and not get bunched up in the middle. A similar adjustment was made on the octal socket but there are no pictures to catch me out.

Assembly is proceeding. Four down, five to go.

Saturday, June 11, 2016.

All 9 are finished. Here is a picture.

They are locked together in groups of 3. Left to right they are, 7 pin mini, 8 pin octal, and 9 pin mini. It is very easy to separate them for single use. The next step is to use them for something.

Case Closed.

Medium parts storage.

Monday, April 18, 2016.

After I graduated and got my first real job I could afford to buy some of the things I wanted. One of them was a method of storing medium sized electronic parts. I found in the allied industrial catalog a system that consisted of a set of metal shelves and cardboard boxes to store the parts in. I had seen these before. They were used by many parts wholesalers.

The cardboard boxes were shipped flat and the buyer had to assemble them by inserting tab A into slot B.

After marrying and moving into a home of our own I acquired several more of the boxes but could not find the metal shelves. I used a combination of light metal bookshelves and pigeon hole modules bought from a bargain store . When I remodeled my electronics shop I expanded this storage by a considerable amount. I built a set of sturdy wooden shelves to hold the boxes.

As you can see I am a few boxes short and have quite a mess at the bottom. So, I asked myself "If you call yourself a woodworker why don't you build boxes out of wood? That should be more durable than the cardboard boxes which are becoming a little shop worn."

I had been contemplating this question for a couple of years when an email from Shop Smith informed me of a finger joint jig made by Incra. The name Incra in the woodworking world is roughly equivalent to HP in the electronics world.

Then I got sick. Some of you know the story, others know parts of the story but I won't burden you with it here. Suffice it to say when the package arrived in the mail I didn't have the energy to put it together much less use it.

I am doing much better now and have gotten back most of my endurance.

Today I put it together.

Then I used it to demonstrate to myself that I could make a good finger joint with it. It's the perfect way to make boxes out of 1/4 inch plywood.

Friday, April 22, 2016.

Case not closed after all. Tuesday I spent part of the day designing a box on my computer. I always do that so when I get to the shop I have a much better chance of getting it right. I went to the shop and started cutting parts to make 3 large boxes. If you look carefully at the pictures above you will note two different sizes of boxes. The smaller ones are 4 inches wide, outside dimension, and the larger ones are 8 inches wide.

Wednesday and Thursday I spent cutting parts and did the glue up on Thursday afternoon. One of them is shown below.

Today I started cutting out pieces for 3 more large boxes, 8 small and one extra-large, 12 inches wide. As time passes I will be slowly replacing the cardboard with plywood boxes. My shop machinery is precise enough that I can make pieces and they will fit together without custom fitting. Building with interchangeable parts goes much faster than custom fitting each part. SWMBO drafted me to mow the lawn or I would have gotten more done today.

Monday, April 25, 2016.

I am continuing to manufacture boxes. Not much to report on and even less to take pictures of.

To be continued.

Attempting to make tube IF transformers out of the transistor type.

This article has been given a page of its own on this site.
IF Transformers for New Construction.
IF Transformers for Older Radios.

Motorola Single Ended Stereo Amplifier Chassis.

Saturday March 5, 2016.

I am presently working on an amplifier that came unexpectedly from my late brother Lee. He gave me a number of things before he passed and I just got around to opening one of them which is a suitcase phonograph. It was a lot heavier than such a phonograph should be and I wondered why. Upon opening it up nothing looked out of the ordinary, at first glance. Here are two pictures with the lid removed.

What's wrong with these pictures? Give up? There aren't any controls. I removed the sloping panel in front of the changer and found an amplifier that was bigger than one would expect in a phonograph such as this. It came out amid a tangle of wires that connected the control strip to the main chassis. In retrospect I was a little too aggressive with the wire cutters so to reconnect the control strip to the amplifier will require a bit of new wiring.

When this picture was taken I had already added a set of spring clip speaker connections which can be seen mounted on standoffs between the two output transformers and the can filter capacitor. The lugs on the transformers seem a bit fragile and I didn't want to risk damage caused by repeated soldering and unsoldering. The first thing I did was to reform that capacitor with all the tubes removed and the B+ point connected to my variable voltage power supply. All caps in the can reformed well.

I wanted to fire it up but examination of the connections to the 12AX7s revealed that the grids had no return to ground within the chassis. All that happened in the control strip. Unfortunately I had cut all the wires between it and the octal plug which goes into the socket at the right end of the chassis. So I temporarily soldered four resistors to the proper pins of the plug to make return to ground.

The order of things as I have deduced them is phono pickup - volume control - first half of 12AX7 - tone controls separate bass and treble, with balance control - other half of 12AX7 - 6BQ5 - output transformer - speaker. The label shown below helped in this.

When I connected a clip on power cord and fired it up ;with 8 ohm dummy loads I could only get a few milliwatts out before serious distortion set in, identically in both channels. I checked the voltage on the grids of the 6BQ5s and sure enough it was about 6 volts positive. The capacitors used were considered good for the day. They were enclosed in little white ceramic tubes and everybody, myself included, thought they were among the best. The amount of leakage I measured indicates they must have been wax paper and foil the same as the cheapies that were available at the time. Replacing them improved things a lot.

I got about 1.4 watts out before clipping became visible on the scope. The distortion was about 3 %. At 1 watt I measured 2 % in one channel and 1% in the other. I have yet to have an amplifier on my bench in which the distortion is the same in both channels.

To Be Continued.


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