This one is a bit different. I picked up a PC power supply from a trash pile at work, thinking that it
might be good for some digital project, but it turned out it was dead and the fan was seized. While
removing the fan, my coworker David Wegmuller got the idea that
it would be a neat box for an amp if you could find a speaker that fit the fan hole. The next thing you
know, David was getting an old, dead PC subwoofer from his cubicle and we were taking it apart. As you
can see on the right, the speaker was exactly the right size!
Sometimes you have to do something just because you know you can.
I wanted to get the whole circuit inside the box (for that PC power supply look) so I kept it simple. Two 6021 dual triodes, a volume control and a speaker out jack. I'm pleased to say that I only had to drill three new holes in the box.
The story of the prototyping phase is on another page (see above). Once again, David came through with a nice piece of brass (see also the Pentotron) to overcome the problem of mounting the input jack in the hole where the power supply's wires used to come out.
I figured with a little luck it could turn out to be a decent 1 watt practice amp, and I wasn't really disappointed. The "subwoofer" produces sounds recognizable as electric guitar and bass, but it's not pretty. I don't expect much from a 4 inch speaker, but I'll be keeping my eyes open for alternatives and playing with filters until it sounds better. Please let me know if you've got any suggestions.
The really good news is that it sounds great for guitar driving an open-back 1x12 or 1x15, and for bass through my 2x8 closed cab. In fact, I'm thinking the PowerMan plus the 2x8 is going to be perfect for acoustic jams …
A few technical notes:
I misoverestimated the current draw of the circuit by a fair amount, so I got a much higher B+ in the build than I simulated or used from the bench supply in the prototye. I'm not afraid of a little excess voltage on my tubes, but now the idle plate dissipation on the output tube is a full 1W per triode – only 40% over the absolute max rating! The plate voltage is about 10% over max, too. I'm liking it the way it is, though, so I may just see how long it lasts.
The 6021s have a 5% tolerance spec for heater voltage, so it seemed advisable to drop the 7+ VAC that the power transformer wanted to give them. The 0.68 ohm resistors (R12 & R13) take care of that problem.
From raw materials to fully assembled:
