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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
I love low power tube amps for practice, and I already own a couple. Now I'd like to build my own one, and while I am at it make it smaller than any of the available amps. (Current record holder seems to be the Z.Vex Nano )
1J18b subminiature tubes are cheap to get on eBay, and at 60V 1mA seem to be about right for a tiny bedroom-volume amp. They are pentodes, and it seems impossible to get sub-mini dual triodes, so I figure I would need a lot of them, maybe 8 in a V8 shape? Two for a push-pull output stage, four gain stages and the final two for this phase inverter stuff. Tone controls could be OpAmp driven, I'm not a purist. These tubes draw 25mA fillament current at only 1.2V, so possibly I could power the whole amp off a single rechargable AA battery, with a tiny inverter providing the 60V plate supply. I recon if I aim for 100mW output, the total power consuption will only be about 1W, so I can run for an hour on a single battery. How cool would that be?
I think a few people on the board have constructed tube amps, so maybe I can get some input if this is a good idea.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If they can only pass 1mA of plate current, you may need a lot of them in parallel to get an impedance low enough to match with any output transformer you can buy. If you try making a transformer yourself to match a single pair of tubes, it may need tens of thousands of turns of hair-thin wire.
I wouldn't care to try making a power amp with any tube smaller than a 12AT7 or 12AU7, either of which can flow a few tens of milliamps if they need to. I think two 12AU7s in parallel just about matches the little push-pull transformer that RS used to sell (and maybe still do?) Another cool tube is the ECC88 which is about equal to a double-sized 12AT7.
I used to use a practice amp powered by a single EL84. It was still loud enough to get complaints from my neighbours For a while I dreamed of a bass amp powered by a bank of Sovtek 5881s, with a switched mode power supply to keep the weight down, but it ain't gonna happen The output transformer would have cost over $500, and I found that a lower powered amp works fine.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
OK, good point. I was in fact planning to wind my own OT (mainly because even the smallest I know of, the Hammond A123 is way too big for my application), but not with thousands of turns. I want to match to a nominal impedance of 8Ohm, and the tube has 50kOhms out, thats roughly 1:100 turns ratio.
Yup, either I'll go with the 12au7 output stage, or solid state. Both have drawbacks, the 12au7 might just about run on 60V plate current, but I'd need an additional 12V for the fillament. Solid state is probably going to be too complicated for me, since it usually takes much more components.
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Guitar amp entirely made of 1J18b subminiature tubes?
Two for a push-pull output stage, four gain stages and the final two for this phase inverter stuff. Tone controls could be OpAmp driven, I'm not a purist. 
For a while I dreamed of a bass amp powered by a bank of Sovtek 5881s, with a switched mode power supply to keep the weight down, but it ain't gonna happen