Black Face Twin - Fixing Stuff

Hi all.

First - let me say I am a lucky guy. (And in my case, lucky trumps smart ...)

Anyway, in 1977 or so, I sold a Silver Face Dual Showman Reverb and Marshall 4x12 Bass cabinet and went looking for a Twin Reverb. I ended up with a choice: a shiny silver-face and next to it a beat-up black-face that had been gigged and had the names of three defunct bands stenciled on the sides. And the black-face had these big honkin' EV SRO speakers. (I think it dates from 1967 or so.) Same price as the silver-face. The black-face was fugly - but I chose it. (I think this was long before the black-face vs silver-face thing became a "thing".) But man - was it heavy! (93 lbs.) So, that's me being lucky.

Twenty years ago or so the amp had a whole interesting sound of it's own. Pops, squeals, hisses, etc. It sounded like aliens were reaching out to me! So I opened it up and replaced filter caps and about 30 or so other caps and resisters that were WY out of spec. (Like the big resisters across the 6L6 sockets that turned to dust when touched.) It sounded so sweet !!!

But go back a decade or so (1982?) and I had this brilliant idea to retro-fir a master volume based on an article I saw somewhere. It meant replacing the vibrato intensity control with a dual-ganged pot and adding a couple of capacitors. It worked - but was never really that good.

So now I'm going to undo that.

Before I get to that, I have a question about what is obviously a mod that predates my ham-fisted stuff. Inside there is a small pot wired across the two green leads that connect to the pilot light (and therefore all the 6v heaters.) The center sweeper connects to ground. I have a photo but I guess I can't attach that. But the description is pretty clear I think. Any idea what this could possibly be for?

Thanks in advance!

The only possible purpose for that mod I can surmise is that it's to reduce the heater voltage to compensate for the higher wall-outlet voltages now common. Remember, the amp was originally intended to operate on 117 VAC but modern voltages often measure close to 125 VAC. If your amp is otherwise working fine it might be best to 'let sleeping dogs lie' and leave the mod in situ.

Silverface Twins called that a "hum balance", turn the volume up and adjust for lowest noise!

Hi.

Thanks for the replies!

Sorry - I 1didn't see or respond for a few days. Kinda under-the-weather with Covid. Arg!

Bob