HELP ME SAVE 74 SUPERLEAD!!

[heckitar66]

Not the greatest pic. Maybe you can see something I'm missing. To answer your question, it's not just one side. I only have 80v at pin 8.

Does it make a difference what size resistor I use to try your suggestion?

[Doug H]

Does it make a difference what size resistor I use to try your suggestion?

absolutely, I'm suggesting you wire up a temporary cathode circuit to rule out the existing one. You can do that without pulling the board or anything drastic which makes it an easy test.

but since both sides of the tube are low I'm not sure what to look for. Do you get any DC on the mix resistors?

[heckitar66]

Nothing at the mixers.

[Doug H]

stumped...

are you actually getting different DC readings on each side of your pre amp plate resistors?

[heckitar66]

I'll check. In the meantime, is it a bad idea to connect the ground buss to the negative side of the preamp filter cap? I remember reading that it's bad to have two grounds and since the buss is attached to the pot casings I'm not sure. I'm just fishing because I'm stumped too.

[Doug H]

As long as they are both connected solidly to the chassis it shouldn't matter in the larger picture.

Might be time to take it back to the tech but this time tell him specifically about the bad voltages.

[heckitar66]

You may be right. I don't know what else to look for.

I have different readings across the 10k plate resistors on v2 although one increases and the other decreases.

On v1 I have 242v on the side of the resistors furthest from the tubes but on the resistor that goes to pin 1 I only have 97v. I get 172v on pin6.

[heckitar66]

Looks like the 100k resistor on pin 1 might be bad.

[heckitar66]

It's reading 222k?

[Doug H]

You may be right. I don't know what else to look for.

I have different readings across the 10k plate resistors on v2 although one increases and the other decreases.

On v1 I have 242v on the side of the resistors furthest from the tubes but on the resistor that goes to pin 1 I only have 97v. I get 172v on pin6.

That's a 100k and a 1k on v2, and they are cathode resistors, the plate on v2 is on the socket, the one you replaced.

Sounds like there is some black belt voodoo gone wrong on that guy. You said the v2 plate resistor looked fried, did you measure it after you took it out? if it wasn't 100k who knows what might have happened to that amp to fry a plate resistor like that. You'd have to be suspicious of anything touching the b+ line.

I get more and more surprised at the state of this amp that two techs said it was fine. Judging from your readings it clearly has big problems You did the right thing taking it in with your level of knowledge. This should have been fixed for you a long time ago. Debugging this really involves some hands on checking of a lot of high voltages while the amps on, and then possible replacement of some important parts.

[heckitar66]

I stand corrected. You are, of course, correct.

I did measure it. No continuity and it looks burnt.

I just removed the 100k resistor they goes to pin 1 of v1. It's still reads 222k. I'm replacing it with another that reads 99.9.

I've always been fascinated by guitar amps and I'd love to learn what you guys know but life gets in the way. I really appreciate that you and others like you take the time to help out guys like me.

Even if I can't fix it, I've made progress and I can take it to a tech and tell him more specific details so he might be able to fix it.

Thanks, man.

[Doug H]

Absolutely.

Something grave must have happened to cook all those resistors. I'm not even sure how that could happen. Those oldpihers are supposed to be pretty tough. All I can think is that it was run with the ground lifted so the b+ voltage was way too high, but still...

You did replace the one on v2 tho and your readings are still off, seems strange. One wouldn't expect the cathode resistors to get fried, but I've never looked at an amp that had plate resistors get cooked so I don't know what to expect.

[heckitar66]

It looks like its had a hard life. The front of the chassis is bent, the front and rear panels are pretty beat, the top side of the chassis is really dirty and it looks like its seen some moisture, and the PT mounting bracket is bent outwards somehow.

I now have better readings on v1. It's sounding a little better too. I'll keep looking for similar things on v2 and I think you were right about the presence cap too so I'll order a .1 for that.

[heckitar66]

Readings on v2 are a hair lower for some reason.

[Doug H]

it looks like its seen some moisture

firing up a wet amp could do untold damage. Your v2 readings baffle me. Even if the 100k cathode on the CF is fried, I don't see how that would lower the plate on the other half of the tube. Again, black belt stuff, there are guys on this board that would know that sort of stuff. Checking resistors as you are doing is probably your best path forward as you have found some bad ones already.