Hardly a week goes by that I don't get an email asking "Where can I get IF transformers for tube circuits?" Although tubes, sockets, high voltage capacitors, and power transformers, are still being manufactured it seems that IF transformers are not. I usually suggest that they watch their local antique malls for a radio with a badly damaged plastic case, which has no antique value, and junk it out for the parts. I have found one site on the web where someone suggests that you wind your own. Even with my coil winding experience, that's a little more than I want to tackle right now. Maybe later.
I have thought about adapting transistor IF cans for use with tubes but I'm not sure they would stand the voltage. One day I was looking at an IF transformer I had removed from it's can and realized that a small RF choke might work. That led to this.
This is the one using the 680 microhenry coils. Note the fixed capacitors in parallel with each trimmer. It has a bandwidth of 15 kc. The one using the 1 millihenry coils has a bandwidth of 13 kc. That's from testing one using a signal generator and scope in a bench test.
Murphy Strikes Again.It seems that Ocean State Electronics has gone out of business. The link below won't lead you to anything useful. Also AES (Antique Electronic Supply) has discontinued the trimmer capacitors. The enclosure boxes are still available but that's about the only good news there is as of July 9, 2013. I'm going to leave the page up as is for now but revising the transformer project is now on my to do list. Would someone do me a favor and help me escape from under this long strip of paper labeled "Max's to do list"?
Parts list.Well, Antique Electronics Supply, in accordance with Murphy's Law have discontinued the RF chokes used in this project. I have found some chokes from Ocean State Electronics. Their ordering process is a little slower than what we have become used to as they frankly admit on their web site. I think they are doing things the old fashioned way. I placed an order, over the internet, and it arrived two weeks to the day later. I have been told they are reliable. I placed a large order and nothing was missing or damaged. The chokes leave something to be desired but they are all I can find at the moment. If anyone out there knows of a source of the pie wound chokes like those used previously, pleas let me know.
The parts have come in, I have rebuilt the transformers, taken new pictures, and tried them in the 5 tube superhet breadboarded radio. The results along with a comparison of various IF transformers and some scope pictures are given after the construction instructions. Soon I will try the 3 gang variable capacitor I got from OSE. Stay tuned.
The similar looking trimmer capacitors available from OSE (Ocean State Electronics) are too large to fit in the boxes as the transformer is laid out. Here is the new parts list.
- Trimmer capacitor, AES# CVT250, 0 to 250 pf, $4.95.
- Enclosure, AES# P-H1590LB, 1.99 x 1.99 x 1.06, $7.95.
- Enclosure, OSE# 1590LB, 1.98 x 1.98 x 1.07, $7.20.
You pays your money and you takes your choice.
- RF Choke OSE# 70F103AI 1 millihenry $2.95
For input transformer.
- RF Choke OSE# 70F684AI 680 microhenry $2.85
For output transformer.
- Fixed capacitor 56 pf 100 volt or higher. Silver Mica or Monolithic recommended.
Ceramic disk third choice. Available from any parts supplier.
Construction.You will note that there are two different values of coils given, one for the input transformer (grid of the IF amplifier) and another for the output transformer (plate of the IF amplifier. The diode detector places a somewhat heavier load on the output transformer, as compared to the grid of a tube, so a lower impedance transformer is required. Don't forget to install the fixed capacitors in the output transformer.
Here are drilling templates for the box and the lid. There are two so you can make two transformers with one printout. Scale them to the size of the box, print them out, and use double stick tape to stick them to the bottom and top cover of the box. The lower portion is the box itself. The circles and crosses in the corners are the holes for mounting the top cover. They are already drilled. They are provided for alignment only. Tape the part containing the outlines of the trimmer capacitors to the bottom of the box. Smooth the part with two holes over the rounded edge to the side of the box and tape it down. The upper part, with two marked holes, excluding those in the corners, should be cut from the other part and taped to the top cover.
Mark the holes with a center punch and drill them.
The 2 holes for the capacitors are 3/16, the four holes for wires are 1/8. And the 2 holes in the side are 7/64. The two holes in the cover, for adjusting the capacitors, are ¼ inch.
The two holes in the side of the box are for mounting it to the chassis and should be tapped using a 6-32 tap. Any machinists reading this will no doubt cringe saying, that's not the right hole for a 6-32 tap. True but most ordinary people don't have a numbered drill set. The screws won't have to hold up a thousand pounds or even a hundred. I actually tried to strip out the holes and couldn't with the usual small screwdrivers owned by electronics tinkerers.
Now that I have prevented irate e-mails from expert mechanics lets get back to the electronics.
Form the leads of one of the RF chokes as shown in the photos above and below.
This picture shows the 1 millihenry coil. Examine the choke and determine which lead goes to the outside of the winding and which goes to the inside. For both trimmers I connected the outside lead facing away from the screw in the trimmer. The other choice you have is which terminal of the trimmer to connect the outside lead to. It doesn't matter which terminal you connect it to, but the second one MUST go to the opposite one. Push the leads through the holes in the trimmer capacitor lugs and bend them around for a mechanically secure connection. Do the same with another RF choke and trimmer capacitor except as indicated above, connect the choke leads on the second one the opposite way.
The meanings of the wire colors are as follows.
Green - Grid.
Yellow - AGC connection.
Red - B+.
Blue - Plate.
Set the two capacitor - choke combinations on your work bench so the two outside - of - the - coil connections are next to each other. If you want the grid coil on the left connect lengths of colored insulated wire as follows. Green, Yellow, Red, and Blue. If you want the plate coil on the left wire them the opposite way. The yellow and red leads should always be connected to the outside of the coil and be next to each other. This is to minimize capacitive coupling between the two coils. We want only magnetic coupling. I failed to observe this in version 1.0. When I took the nuts off the capacitors and took them out of the box I found that Murphy had failed to intervene and the outside of the coils were to the yellow wire on one and the red wire on the other, exactly as they should be.
Physically align the axis of each coil parallel to the edge of the capacitor and centered over the lug which is closest to the other capacitor. Solder the connections.
Bend the insulated wires slightly forward so they will pass easily through the holes and permit the capacitor mounting bushings to line up with the holes in the aluminum box. Place a washer on the capacitor mounting bushing, inside the box, to provide a little space between the capacitor and bottom of the box. There are rivets on the capacitor's underside which might short out. There is a step on the bushing but it doesn't provide much room for safety. Use a 10-32 nut on the outside of the box. This nut and washer are not supplied with the capacitors.
The spacing between the coils should be as shown in the photo near the top of this page. This is pretty close to critical coupling. Moving the coils closer together will give a wider bandwidth but as they get closer a dip will develop in the middle of the response. For a detailed discussion see Phase Relationships in IF Transformers. Also see the oscilloscope pictures below.
The inductance I am using for the output transformer is about what the gentleman in Canada recommended. A very ambitious gentleman I have been exchanging emails with went so far as to disassemble two IF transformers from a junked radio and measure the inductance of the coils. He found that the input transformer used approximately 1.2 millihenry coils while the output IF transformer used approximately 600 microhenry coils.
Performance.I began by measuring the Q of the original coils using a Heathkit Q meter. The advantage of a Q meter over an inductance bridge is that the Q can be measured at the operating frequency rather than 1 kc. The Q read about 75. The new 1 mH and 680 uH coils read 70 and 65 respectively. I could have wished for more but, as the song says, if wishes were horses everyone would ride.
I decided to use my sweep generator to see just what the bandwidth of these home made transformers is. I connected the generator to the primary through a 470 k ohm resistor to simulate the plate circuit of a pentode and prevent the low output impedance of the generator from killing the Q of the primary.
A Commercially Made IF Transformer.I began with a NIB NOS IF transformer that I have several of on hand. It was made by Meissner and the part number is either 16-5740 or 276841-26. Personally I'm betting on the shorter number. The box is clearly marked input transformer. A photograph of the resultant sweep is shown below. 455 kc is at the center of the screen and the calibration is 10 kc per div.
The frequency sweep is quite slow about 4 per second. The sweep voltage is a triangular wave eliminating the need to blank the retrace. The same voltage that sweeps the oscillator is fed to the X axis of the scope. The darker lines running vertically through the pattern are probably aliasing of the RF signal with the sampling rate of the scope. I don't know how the scope sets the sampling rate when operated in XY mode.
As you can see the bandwidth is the narrowest of all the transformers tested. In IF response the -6 dB points are used rather than the -3 dB points as in audio work. Two transformers should add together so for a single transformer the -3 dB points should theoretically be used. Since the detector is seriously loading the transformer in the plate circuit of the IF amplifier the one in the grid circuit is probably providing most of the selectivity.
I decided to drop a couple into my 5 tube superhet. Narrow bandwidth, higher Q, and higher gain all go together. The amplifier oscillated. I had to increase the cathode resistor to 1 k ohm to tame it. A well laid out and properly shielded IF amplifier would probably not need this gain reducing feedback. The resistor is not bypassed.
The tuning was quite narrow as you might expect. These would be excellent for DXing on the AM band or short wave listening. The Meissner company was known for making ham equipment so these transformers may have been designed for use in communications receivers rather than broadcast radios.
The one thing I noticed was excessive distortion on extremely weak signals. This may be due to the fact that I was using an input transformer to drive the diode detector. An impedance buffer might help. An infinite impedance detector might also be used but I don't know how to derive AGC from such a detector. The bandwidth is too narrow to make a high-fi tuner with these transformers.
Transformers Made with The Original 1.2 mH Pie Wound Chokes.The first picture below is the one I was using for the input transformer and the second was the output. I thought I had swept them out but perhaps the flicker on the old oscilloscope made it hard to see what was going on. As you can see I had them seriously over coupled. I wonder what they would do if critical coupling were used.
I didn't make any changes except to slightly readjust the trimmers to account for the difference in capacitance between the radio and the test circuit. When used in the radio, rejection of adjacent channels was quite good. Weak signals were not distorted and the amount of noise did not seem excessive. Although I did not connect it to a high fi system, its frequency response must have been as good as the station was transmitting.
The New Coils From OSE.Below are the sweeps of the OSE coils. The first one is the input transformer and the second one the output. Channel 2 of the scope is the Y axis and you can see the progressively lower Q as the input has to be set to a more sensitive range to keep the display the same height. The output of the sweep generator was not changed throughout these tests.
Adjacent channel rejection was quite good and there was no audible distortion on weak signals. One of the things I would like to do is build an AM broadcast receiver with delayed, amplified AGC so I could tune the dial without having the other hand on the volume control. I also want to use the PLL VFO from OSE to make a digital local oscillator so I could know exactly what channel I am listening to. So much to do, so little time.
Meantime remember, life is short so eat your dessert first and always listen to tube radios.
This page last updated February 16, 2012.