5W Acoustic Amp

[Cliff Schecht]

Just built this for a guy at work. We're doing a horse trade, the amp for a good 5 boxes worth of old TI mil-spec resistors, caps and other random stuff (including an HP 200CD function generator).

The design is a basic Champ style circuit with a few adjustments here and there. The only controls are a volume and tone knob (after the first stage). Since most all acoustics have built in EQ's, I went with a relatively flat/wide frequency response and, for now, have forgone bypassing the cathodes or doing anything else to muck with the frequency response. I will probably add a large value cap to the 6V6 to make the output stage sound like it's fixed bias. Unfortunately it's too late to test this tonight so I'll fire it up tomorrow morning before I head out to the studio.

I'm very happy with this build, it was yet another scrap pile build and turned out quite clean inside. Here's some pics:

[Structo]

Cool!

I have somebody that may like something like that.

The acoustic in question has a piezo right?

[M Fowler]

The pot shafts are way to short need extensions.

Nice looking amp.

[Cliff Schecht]

Cool!

I have somebody that may like something like that.

The acoustic in question has a piezo right?

Yeah piezo I believe. I just threw the acoustic on a scope and measured the output. It's about 500mV to 1V nominal and can get up to 4 V with all of the knobs maxed. I'm going to test it out today and tomorrow with a variety of guitars and see if I spoofed the voicing or anything else.

Mods I'm considering include adding some switchable negative feedback on the 8 Ohm tap and some sort of master volume control so that there is a "pre" and "post" control. Since this amp is sorta supposed to be multi-purpose, it would benefit from a few extra switches and such. I may add another input as well, this guys son is learning to play and they may want to jam together..

[Cliff Schecht]

Plugged in the amp this morning and noticed that the pilot light stayed on even though the power switch was off. DOH! Turned out to be a goofy wiring mistake that put the pilot light before the switch. I installed a new pilot light, rewired that boo-boo, measured a few voltages open circuit, shoved in the tubes and fired it up. Sounds pretty neat, definitely puts out a respectable amount of power for its size. The clean sound is how I pictured it, loud and punchy with a WIDE OPEN sound. I think the large cathode bypass cap did the trick. Feels like a blank slate that just needs the right guitar to make it sound great (twas too bright with the Tele as I expected).

One thing that annoyed me was the extra-high heater voltages. The 6V tap measured at 7.4V which is something that seems to plague me wherever I go. The thing that bugs me is that the transformer set was severed from an Epi Valve Jr which I used to run past full tilt all of the time. I dread to think that I was cooking my valve heaters so hard when I was younger! The 110-115V primary winding plague has once again proved to be problem, at least here in Texas, where the wall voltage always seems to be high (above 120V). Since this is going to a friend and I'm moving in about 2 weeks, I need to make this amp last. Some series resistors on the heater taps will probably be needed, maybe on a switch because I have so much room left in this chassis .