1940's Hammond PA amp

[The Dude]

Good catch, Enzo. I hadn't even noticed. That'd be a good step one. Replace those jacks if they are as drawn.

[bobloblaws]

It was meant to accept a mic level input, so there's lots of gain there. I'm wondering if it's just picking up something in the room while it's opened up. Is there shielding in the head/cabinet that's not there while you're working on it?

On a side note, I noticed there are no cathode bypass caps at all in this amp- likely again because it's designed for a microphone. My guess is that there's plenty of/maybe too much low end. You might try adding a cathode bypass cap to, at least, the first gain stage(s) to tame the lows a bit if that is a problem.

Thanks Dude.

Ok, first of all I figured out the hum issue, which I described as intermittent. The only thing that was intermittent about was whether I had the scope hooKed up! With the scope hooked up the amp was mains grounded via the scope and I haven't replaced the un-grounded AC cord yet. I guess that was a no-brainer eh? Should have done that first. In any event, no effect on the 13K oscillation issue. Hard to say about the source of that oscillation. In any event I'm wondering if it might be worth trying taking the 1st gain stage out of the equation (the 6SL7).

As for bypass caps, doesn't that add bass? Or is that only in the case of large value caps? What values would you suggest experimenting with?

[bobloblaws]

I think the mic input back then was expecting a crystal mic.

Note your input jacks do not have grounding shunts. They sit there acting like antennas.

I misled you. sorry. My drawing wasn't completely accurate. The jacks in fact had ground tabs and I replaced them anyway with new jacks that have them as well.

My first complete schematic so I'll put it down to a rookie blunder :-)

In fact there was something else I had missed which was the grid resistors on V3 that you drew my attention to earlier.