Please Help - red plating, hum, vol reduce - Blackheart 100w

[erigm]

Hey everyone,

PLEASE HELP! Hoping I can get some ideas and troubleshooting advice. I have a Blackheart BH100H that's giving me some problems. Here are the symptoms:
* One pair of power tubes is red-plating. It is the "push" pair, and it seems like it might be volume dependent ... meaning the problem doesn't manifest itself unless playing loud (I think).
* Bad hum. The hum only appears when the power tubes are red-plating, but not every time the tubes red-plate. This symptom is preceded by a popping noise.
* Large volume reduction when tubes are red-plating, but this doesn't happen all the time when the tubes red-plate ... but it only occurs IF the tubes are red-plating. This symptom is preceded by a popping noise too.
* I am also hear some crackling and popping during startup/warmup, but way before any of the other above symptoms occurs.
(all symptom descriptions are to the best of my recollection ... meaning I could be wrong and nothing is definite).

I also want to say that I don't just let the amp sit there with red-plating tubes. As soon as I notice the condition happening I turn the amp off. ALSO, it's not the tubes because I've swapped them and the condition stays consistent with the tube socket positions, not the tubes themselves.

All of the symptoms above seem to be less apparent when the amp is in "triode" mode, or "class A" mode (cathode biased with a reduction in plate voltage using a lower voltage tap).

So far my theory is it must be cold solder joint or faulty component around the bias area of the "push" side. This would lead me to believe that it's either the bias resistor or coupling cap preceding it (one or both opening under heavier load conditions).

My other second guess is it has something to do with the screen grids, but I would think that would only affect one tube, since they have separate screen resistors and I don't think both would go bad at exactly the same time. In that case it would have to do with the wire that leads from the screen grids on the "push" side to the pentode/triode switch.

I played for over 30 minutes last night at low volumes with no issues. This was after noticing the problem earlier that day. This morning I played it at high volumes and it crackled during startup (for no more than 10 seconds), then was ok for a bit (5-10 minutes), then started red-plating ... but there wasn't the hum or the consistent volume drop like before.

Ok, I think that's it for now. Thanks for listening everyone! Any guesses?

Eric

P.S. I'm sorry, I can't include a schematic ... but it's basically a 100watt marshall with a fixed/cathode bias switch, and a triode/pentode switch.

[Racing]

When u state that it is either cathode or fixed what does that entail? Does it mean that when not in cathode bias mode that the cathode resistor is bypassed completely? Or does it render negative voltage assist only?

Some current prod cathode bias amps are IMO very optimistic in their cathode resistor needs and i presume this is done on the allmighty altar of power.
Checking bias for a cathode setup is rather easy. Just measure voltage drop across and reformulate that to milliamperes. Subtract screen amperage and then multiply with B+ to get actual wattage. As different tubes are rated differently it isn´t unheard of that tubes that are on the jagged edge are set to run anyways and as the tube-s in case get older..bias requirement drifts and brings all sorts of havoc.
Also check actual resistor for reference and do so cold and warm component BOTH. Also suspect the cathode resistor DC decoupling cap. Lift one of its leg and retry to check for results.

For a fixed bias setup,one running voltage negative vs ground,if you lose out in controlling the powertubes something IS amiss within the bias circuit. Period.
For a fixed setup always measure actual bias voltage sans the powertubes first.

[erigm]

Racing,

Thanks for the suggestions.

I replaced the coupling cap on the "push" side between the PI and the grid block resistors of the power tubes ... but when I powered the amp on to test it, it crackled and popped much more than before. I then turned the MV all the way down to make sure I wasn't hearing any preamp noise. Then I noticed a buzz (hum), but it went away ... but then it came back, and I saw the two "push" tubes glow blue inside at the same time as the buzz ... then it went away (both buzz and blue glow) ... then it came back. This went on as the frequency of the buzz increased ... not in pitch, but how often it was doing this. Then it got so fast I couldn't hear the beats .. then the tubes started red plating again so I shut it down quick.

I don't know if this symptom is definitive of anything, but I guess I'm thinking OT more now.

Eric

[Reeltarded]

I'm not! It's something about the grid feed to the redplating side.

I have a 50 that lost a feed from being knocked over (full stack) and that happened. Electron storm of 200% magnitude in good side, no signal and redplate bad side.

My mule pulled that on me about a month ago after twiddling with the PI couplers too. It was simply a bad connect at the tube socket. Huge electron storm flashing, and other tube going black and red at those times.

Huge hum and popping with the failure.

[erigm]

I'm not! It's something about the grid feed to the redplating side.

I have a 50 that lost a feed from being knocked over (full stack) and that happened. Electron storm of 200% magnitude in good side, no signal and redplate bad side.

My mule pulled that on me about a month ago after twiddling with the PI couplers too. It was simply a bad connect at the tube socket. Huge electron storm flashing, and other tube going black and red at those times.

Huge hum and popping with the failure.

I've reflowed the solder on all the power tube sockets, bias resistor, coupling cap, grid block resistor, and I replaced the bias resistor and coupling cap. That's why I've moved on to the OT. I'm going to have shop look at it next week.

[Reeltarded]

Ahh I see.

[erigm]

Hey guys,

Just wanted to let you know I returned the amp and ordered another one. I decided I just didn't want to deal with it, so hopefully it was just a lemon and the one coming will be good to go. Thanks for all your help.

Eric