Small Valve Power Amplifier
This was built just for fun - to see what would happen. I had read quite a bit about valve power amplifiers and realised that push-pull configured triode valves could give some of the best audio available. The push-pull configuration is free of the second-harmonic distortion which is the hallmark of single-ended designs. Also power supply hum is greatly reduced. Generally valves are run in class AB when used in push-pull. This reduces the third-harmonic distortion found in common transistorised class B designs. Triodes are fairly easy to work with, but are remarkably inefficient! Most triode designs use large, power tubes but I didn't have the cash to purchase these desirable devices. On looking through the junk box I realised that I had several old (ex-TV!) twin triodes type ECC82. Well, I thought. If they glow red I'm putting too much through them - but up to that point I don't mind taking a risk!
How to get enough current through the valves to get a usable output power? Just run both halves in parallel. I had seen this done in an early radio design where the designer had used a twin triode instead of the common ECL80 triode/pentode output stage. Well, I thought, if it will work in a single-ended design just use it in push-pull.
Luckily for me, triodes are fairly casual about the load that they accept. I didn't have spare cash to get output transformers wound and I didn't have any to hand. The RS Components catalogue only listed one valve output transformer and that was a single-ended design. However, they did show a diagram of the transformer windings - which were multi-tapped to match various small valves to differing speakers. I didn't consider the 4W power rating as much of a problem - I wasn't expecting much output!
The tapping diagram at the top left indicates that a centre-tapped winding is available using terminals 1,2 and 4. This would give 15k using an 8ohm speaker on secondary terminals B and D. However, even running two triodes in parallel this would seem rather low so I connected the speaker to C and D instead. This would, in theory, increase the impedance seen by the valve(s) to give a closer match. An early experiment at using three triodes in parallel didn't seem to give any improvement. I suppose that there is a law of diminishing returns at work here...
The circuit is fairly straightforward - and fairly typical of small valve amplifier designs. One of my constraints was the power supply. I only had one valve-type mains transformer so I wanted to use that! Thus I only had a maximum of about 40mA of HT for each channel - and a rather restricting 2.0A for the heaters.
You may notice from the above picture that safety was not one of my major concerns when building this! It was never really intended to leave the workshop and I have never got round to fitting a bottom plate... Please don't copy this design! (I wouldn't recommend copying my wiring anyway. It works, but only just.) Oh yes, the routing around the plinth (visible in the top picture) is intended to improve the sound - well, it works on the Linn LP12 doesn't it? ;-)
I originally built the amp without a volume control but added it later so that I could drive it directly from a cassette deck. (Stand up you at the back who said "passive preamp"!). OK, I know that there should be a separate grid leak and input capacitor. I just couldn't be bothered!
At the end of the day, how does it sound? Well, I like it. There is a fluidity and transparancy to the sound which is both relaxing and invigorating. It makes music very easy to listen to. I had expected problems at the bass end due to the very small output transformers - and this may be so with good speakers - but in my small listening room this is not a problem. Hum and noise are just about inaudible with one ear up to the speaker. As for output power, yes, it is rather restricted. But as my room is not soundproof, and I have to share the house with the rest of the family, it is adequate for my needs. I still get asked to turn it down! I hav never actually measured the output power - the numbers are only of passing interest. I would rather listen to the music... All in all, seeing that I must have spent a total of about 23 UK pounds (for output transformers, phono sockets, B9A valve sockets and volume control - all the rest came from the junk box; even the chassis/plinth was scrap materials) on this amp I am very pleased with it!