For a long time I have considered the idea of making small, high quality amplifiers that could be used with portable radios and CD players. It wouldn't have to play loud enough for the whole neighbourhood to hear it but it would need to be higher in sound quality than the typical portable or clock radio. In addition to this the question has arisen of what to do with an accumulation of Compactron type television vacuum tubes that will never be used in television service again. I concluded that these two concerns might answer each other and that working on them together might be a good way to go. One of the nicest things about a project like this is that the price is something everyone can afford. I built my amplifier entirely from parts that were scavenged from old television sets. I have a problem with throwing anything away so this leaves me with a good source for parts to be used on projects like this one. I didn't use any fancy parts, only plain old 10%, 1/2 watt carbon comp resistors, used Sprague capacitors, and the tube sockets from the same equipment I got the tubes from.
Considering that most portable equipment is powered by 6 volts, 9 volts or 12 volts, the power output of such equipment will likely be somewhere in the range of 350mW to 1.5 watts, realistically, so an output power of 2 watts would be plenty of power to do what we want. It is also convenient in that the output capabilities of TV audio output tubes and vertical output tubes are along those lines if we want to run directly off the AC line and avoid having to buy an expensive power transformer. There are many types of Duodecar TV tubes which have a pentode and a beam power tube, or a triode and a power pentode in the same envelope. The ratings of the smaller of these power tubes are very similar to the familiar 50B5/50C5 output tube which is commonly used in AC/DC AA5 radios. These are the tubes I have chosen for my cheap (free) amplifier project.
I divided my tubes into groups according to basing. The ones that I have the most of are based "12EZ," perhaps the most common of this type tube. In particular the tubes I have with this base are 12AE10, 13V10 and 17BF11. These seemed like a good place to start. All of these are pentode / beam power tubes and in my circuit can be switched with one another and the only change that needs to be made is the heater voltage. Since a lot of gain is not required, and the idea of a pentode front end doesn't appeal to me, I decided to connect the pentode unit in a triode fashion. This still provides just enough gain for the way I will use the amp. If you should want to use it with a mic or guitar or as an intercom amplifier it would be very easy to make use of the pentode connection and increase the gain significantly. One small problem is that the relatively low plate supply voltage on the first stage results in a maximum output swing of about 8 volts. The power tube requires all of this to reach full output. In other words, both stages enter into distortion about the same time. Cathode bypass capacitors and as high as possible power sensitivity are required.
The ratings of the beam power units indicate that the maximum plate voltage should be 165 and the maximum grid #2 voltage should be 150. This is about what you get with a half wave silicon rectifier if you power it right off the AC line. The plate dissipation rating is about 6.5 or 7 watts, and the specs say you'll get between 1.5 and 2 watts of output with a load impedance of 2,500 to 3,000 ohms.
I'm using a 3k to 8 ohm transformer that I salvaged from something and I have verified 2 watts of output with the 12AE10, the 13V10 and the 17BF11. My output transformer is fairly large. It probably was designed to be a 5 watt transformer but it sounds very good in this amplifier and a small transformer could certainly spoil the performance. It may be possible to use a 70.7 volt line transformer if you are unable to come up with a single end output transformer. A 10 watt one will work fine but you will need to open the core of the transformer and place a piece of paper between the E and the I sections to avoid saturation due to the DC that this transformer wasn't originaly designed to deal with. If you must buy one, a good quality transformer will be worth buying even if it costs a little more. Very cheap ones have a lot of internal leakages that ruin the performance. Connecting to the 2 watt tap on the 70 volt transformer will provide a 2,500 ohm load. I have found the performance to be OK for impedances between 2,000 and 4,000 ohms but straying too far from the ideal 3,000 ohms results in lower output power and less satisfactory performance.
Since tubes do require power to heat the cathode, a small, inexpensive power transformer will be required. Almost any unit that provides the required voltage can be used. The tubes I used required 450mA of heater current which was supplied by another salvaged unit. You can buy an adequate transformer for about 5 dollars.
Here is the schematic for the circuit I ended up with after many hours of testing and listening. The values of the components specified here were chosen by listening to the amplifier in actual use rather than by going with theoretical values. I realise that the anode dissipation under these conditions is slightly above the design center values but I have not found any ill effects whatsoever.
Following is a list of tubes that should work well in this application. It isn't intended to be exhaustive, but it lists a few suggestions.
12EZ Basing (pentode & beam power tube)
12BT Basing (pentode & power pentode)
12DZ Basing (triode & power pentode)
Having used the amp for a while I find that it sounds pretty good but I'm still planning to try some design changes. My test speaker has primarily been a Minimus 7 since they are small and probably representative of what one might use with it. It plays loud enough to enjoy with that speaker but it certainly couldn't be considered "loud." A pair of these in a stereo configuration would probably be perceived a lot louder. The speaker I plan to use at work is larger and more efficient but not as musically accurate.
The amp has fine low frequency response but, with only two watts of power available, you can use up all the power in a hurry if you are listening to bass heavy music on an inefficient speaker. You may find that you will want to add some negative feedback to lower output impedance a little and improve linearity through the output transformer. I didn't use any feedback because I wanted all the available gain while still being able to use the triode connected input stage.
You will be disappointed if you try to compare this amplifier's sound to that of a high fidelity amplifier. It doesn't sound like a 45, a 2A3, a 300B, or even a 6BQ5. It does sound better and is more fun than an LM386 and the like, however. The price, simplicity of design, and enjoyment of construction and use are what make this amp worthwhile. Besides, it provides a use for tubes that would otherwise be considered obsolete.
Horizontal output tubes are capable of much greater output but also need higher voltage on their plates. A voltage doubler circuit could be used for this purpose if you still wanted to run off the AC line with no power transformer. A good example of what can be done with TV sweep tubes can be seen in the Frazier F-106C power amplifiers that I mention on my first page. These amplifiers use 6GT5 horizontal output tubes.
If you decide to build an amplifier of this type because of what you saw on this page or have built a similar amplifier before you can drop me a line . Be sure to remove the two X's in the address.