Cathode follower without elevated heater voltage

[Dark Mavis]

Lemme see if I can find it

[eschertron]

On the bright side, if you've swapped the preamp tubes around and did not affect the problem, then it's not cathode-heater voltage that's the issue. Sorry, that's all I've got...

As has been said, replace the filter caps first to see what that gets you.

[catalin gramada]

I keeped to test an 12at7 for a week with 150V into cathode hoping something to happen as time it have 90v max specification. Very disapointed, nothing was going on. The tube is intact and work perfectly in my amp. I elevated the heaters just for confidence, not sure if need it

[nickb]

I've tested several Sovtek 12AX7LPS up to 500V and seen no evidence of heater to cathode breakdown. For that reason I doubt that this is ever much of an issue in the average cathode follower stage at 100V or so.

[g1]

In spite of everything else going on in there, that's not a lot of tubes to check basic DC voltages. That could go along way to troubleshooting a simple issue like a cathode or plate resistor failure (which would be a lot easier than total gut & rebuild ).
Idle DC voltages at plate and cathode for all preamp and power tubes.

[nosaj]

Lemme see if I can find it

Got the power supply section?

nosaj

[Dark Mavis]

Got the power supply section?

nosaj

I'm trying to get hold of it. If I can't, I'll just draw it out. I got a whole bunch of papers off the guy. There was a manual...which seemed to be just a bunch of drawings of the circuit board, with different wires labelled in each one, but I think there were separate pages for things like bias supply, power supply, output tubes, etc. I think there were datasheets for the output trans, showing all the windings and stuff.

Edit: yup. Got them. There's nothing in great detail about the power supply. And the cap values in the amp itself are different, but I'll post it anyways. I also just realised I have to order some replacement fuses cus last time it was powered on, I got a short reading the plate voltage and the fuse and filament resistors went up in smoke. Guess I should buy a probe that's fit for purpose, too instead of poking around with those stabby ones. The empty eyelets next to the last filter cap, are confusing me. I don't remember if they were always empty, or if I took something out of there. There IS one other filter cap, but it's all the way over by the input jack...for some reason. And I definitely have a 22uf 350v cap in another amp that I'm sure came from this one. I just can't be sure, since it's been so long. It's not part of the bias supply, cus that one is over by the driver. It's entirely possible that I'm just convincing myself that I took a cap out of this amp almost 20 years ago

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That's copied verbatim from the manual/instructions. That's not what's in the amp, though. I THINK the first stage was 2X100uf caps, I dunno. I subbed them for some 250uf caps just to try to rule them out. The other three are all 47uf. Dunno what the resistors are. I'll just draw out the power supply that's actually in the amp. The drawing from the manual also doesn't show the resistor between the first and second stages, or the cap that's o0ff on it's own by the input jack

[Dark Mavis]

In spite of everything else going on in there, that's not a lot of tubes to check basic DC voltages. That could go along way to troubleshooting a simple issue like a cathode or plate resistor failure (which would be a lot easier than total gut & rebuild ).
Idle DC voltages at plate and cathode for all preamp and power tubes.

Will do. I'm just waiting on a replacement fuse

[Dark Mavis]

Okay, I got a replacement fuse. In fact, I got a bunch, but hopefully my hand doesn't slip again. I'll measure all the plate and cathode voltages, the bias voltage, the voltage at each filter cap and either side of each tone cap and coupling cap, and write them on the schematic then post it here

[Chuck H]

Please measure DC voltages at grid pins as well.

[Dark Mavis]

Hmmm...I just powered the amp up for the first time in like 15 years. Just for a smoke test, really. After, I went to discharge the filter caps - Which I normally just do by reading their voltage until they discharge themselves through the meter to a level where I'm happy to just short them to the chassis through a screwdriver - but they were reading like 10 volts, even though the amp had only just been turned off. I GUESS they've deteriorated that much, just sitting there...?
On the plus side...no smoke, nothing went pop, and there was sound from the speakers..though also the occasional crackle as everything warmed up.
Guess I'll put the new filter caps in and get measuring

[Chuck H]

That seems typical to me. I see that more often than high voltage hanging on caps after shut down. There are bleed resistors in the circuit (the balance resistors) and if you shut down power with the standby switch in the play position the tubes will draw some current before they cool, and therefore some voltage off the caps since there's no longer an AC wall connection.

That doesn't mean the caps aren't bad. But it does mean it's hard to tell with that criteria.

The cracks and pops are also expected. The old caps and perhaps other components, tubes included, need to re acclimate to having voltage on them. There may also be oxide buildup on unsoldered connections or any cold solder joints.

[Dark Mavis]

That seems typical to me. I see that more often than high voltage hanging on caps after shut down. There are bleed resistors in the circuit (the balance resistors) and if you shut down power with the standby switch in the play position the tubes will draw some current before they cool, and therefore some voltage off the caps since there's no longer an AC wall connection.

That doesn't mean the caps aren't bad. But it does mean it's hard to tell with that criteria.

The cracks and pops are also expected. The old caps and perhaps other components, tubes included, need to re acclimate to having voltage on them. There may also be oxide buildup on unsoldered connections or any cold solder joints.

I definitely remember that they would take far longer to drain before. I guess it doesn't matter anyway, I'm gonna replace them anyways. But I think I'll measure all those voltages with the old caps first Maybe it would help to have both.
Edit: I noticed that one of the 100k plate resistors in the drawing is 220k in the amp, so I think I'll also check all the values of all the resistors and caps and update the schematic to also include what's actually in the amp.

[Chuck H]

Using higher plate loads for typical guitar amp stages to raise gain was one of Dan's favorite things to do. That is, the different value was intentional. Again, since the amp "basically worked" once upon a time I would probably leave that value in place for now.

[Dark Mavis]

Yeah, I Googled Dan Torres. Almost wish I hadn't. I'll leave the parts in place, but, since the amp has never worked correctly, and is a home-assembly job, I'm to go over the whole thing right now and make sure there are no obvious mistakes. Then, this afternoon, I'll measure all those voltages