2A3 Projector Amp?

[Justin Thomas]

Hi All,

One of my numerous projects lying around is an old projector chassis with PP 2A3s & a cool old field coil speaker (which I got to check out and learned that just because the OT is on the frame doesn't mean it's necessarily field coil). I know not to expect "megapower" or fidelity or such, so I just wanna make something funky like what Charlie Christian mighta played on. I've got three controls, so Volume, Tone, & ??? Maybe T&B... There's a few chokes in it too, & I'll probably not use that pentode for the first stage cuz it has a plate cap. But everything else I'll give a try. I've got an unused 8V winding so I might use a 7AU7 or whatever I might have in my stash of "potentially useful oddballs."

Aside from some of the electrolytic capacitor & resistor construction, it was interesting to see just how little has changed in power supply design...

Any generic considerations before I pass Go & FAFO? I'll also upload my idea for the mains supply when I get it drawn up, just to make sure it's safe.

Thanks,
Jusrin

[Mick Bailey]

What type is the 1st stage pentode, and what's the gain of that stage? A plate cap wouldn't be an issue so long as its safe, though I frequently see just a metal clip connection just waiting to be accidentally touched when servicing. These can be replaced with a fully insulated type.

I like these non-guitar amps - sometimes they're really surprising; I had a standalone ex-school amp (looked like 1940s) housed in a light oak polished cabinet. All it had was tone, volume and terminals for input/output. It made a superb guitar amp, maybe 12 watts. The school was throwing about 8 of these out. I didn't get the matching speaker cabinet, though. After I realised how good it was I went back but they'd all gone to landfill. Another amp that turned out to be one of the best I've owned was from a jukebox. I picked it up in '78 and believe its still in use. I made a case and control panel and wish I'd never parted with it.

Watch those 2A3s don't roll off the bench and smash, though.........

[loudthud]

That first tube is probably the photo tube that reads the audio off the film.

[Justin Thomas]

First tube is a 57 pentode; Mr. Thud, yes that's it. Second tube is a 56 triode. The phase inverter is a transformer. IIRC the 56 has a mu of not much but I can go back & look.

Jusrin