Peavey VTM 120 Repair Troubles

[Jynk_O]

Hello, first post!

I’ve got a Peavey VTM 120 given to me that I’ve been using to learn my way around valve amp repair (experience has mostly been SS) that’s been giving me trouble in the home stretch. Came in awful condition. It’s working now, but only for a bit. Broken traces were jumpered back together, all new tubes, new power tube sockets, new caps, even replaced the grid resistors, and all traces reflowed. Molex connectors replaced with thermax wiring.

After about 30 minutes, and seemingly quicker when using the high gain input, it redplates. This occurs only on one side of the push/pull output at a time. From what I can catch, the b+ and subsequent plate voltages are unstable.

This seems prevalent in the first 6L6, which has a plate voltage that fluctuates between 494-497v, occasionally reaching 500v. This is when you can hear the amp rustle a bit. The b+ reflects the same voltages, but the center tap itself seems to remain about 494v. The two 6L6s on the end of the output remain constant at 493v.

I feel like I’ve tried everything, but I’m not sure what the cause could be and why it’s intermittent, and only occurs after it has been warmed a decent amount. Any advice or assistance is greatly appreciated!

[The Dude]

B+ will fluctuate slightly with tube conduction. I don't think that's telling you anything. I'd first look at bias voltage and make sure it's stable and stays so. Check bias current in general. Make sure you are not overbiased. Also check control grid voltage on output tubes. You could have a leaky coupling cap from the PI.

[Enzo]

You are not trying to read plate voltages while playing are you? Take those readings at idle.

[Jynk_O]

Thank you both for the quick responses!

Had more of chance to sit down with a better meter and let it warm up better. At idle, all plates sit roughly around 504v, but the rumbling (followed soon by redplating) occurs after a quick drop to about ~487v. Control grids sit comfortably at -50.4v when operating normally

[The Dude]

That sounds like it could be motorboating. Check solder connections on all of your filter caps. It could be that one has cracked loose and is intermittent. Be sure to discharge them first so as not to kill yourself.

[Jynk_O]

I’ve gone over them ‘til they’re nice and shinny but the issue remains: originally had hoped putting the new caps was the fix, but while it reduced noise, the problem persists.

It seems like when I changed the grid resistors, it helped stabilize it somewhat, making it take a little longer to display problems

[Jynk_O]

Went all the way through again, nothing standing out but the issue persists- I’m about ready give up on it, in all honesty. Probing around with a chopstick doesn’t seem to have influence on recreating the problem.

Edit: V1 seems to make quite a bit of noise when probed and turned up.

[Jynk_O]

I remembered that I didn’t replace C1 and C6 on the preamp- could this bleed into the issue of the power amp?

[g1]

Control grids sit comfortably at -50.4v when operating normally

What about when it is not operating normally and the fault is occurring?
And sorry if I missed it, but you've tried other power tubes?

[Jynk_O]

What about when it is not operating normally and the fault is occurring?
And sorry if I missed it, but you've tried other power tubes?

Haven’t been able to catch the grids during the fault from fear of letting it go too much, but will try! Tubes are all new JJ 6L6GCs, and tried moving them around/trading some for other new ones to no positive outcome /:

[Helmholtz]

I would consider mounting 1R cathode resistors at 6L6s for easy control of idle currents.

[g1]

Tubes are all new JJ 6L6GCs, and tried moving them around/trading some for other new ones to no positive outcome /:

So just to be clear, it is always the same socket that has the redplating no matter which tube is in there?

[Jynk_O]

So just to be clear, it is always the same socket that has the redplating no matter which tube is in there?

It’s moves back and forth between the two pairs on each side of the push/pull so not entirely consistent, but it’s usually v3 and v4 that redplate.

All tubes are rated with a current of 31, however, the v3 and v4 do have transconductance of 4130 and v1 and v2 have a transconductance of 4110. Would it be safe and worth mixing them to have one of each on each side of the configuration?

[The Dude]

............What about when it is not operating normally and the fault is occurring?............

^^^Yes that. Sure, grid voltage is normal when the amp works. You need to check it when the amp is not working.

[Jynk_O]

Got it! When screen grids (based on measurement taken on v3) sit around 488, but when rumbling is audible, it falls to about 425vdc quickly and then back up