Seymour duncan 84-40 intermittent voltage drop

[frank]

Hi. I've been trying to resurrect an old Seymour Duncan 84-40 that I've had since high school. Over the course of time, it's sat unused in a basement where my young nephews have tinkered with it in unspeakable ways.

I've got it back up and running, and the only mod I've done is to put a standby switch between the power trans and the hv PS. Works great at first, but after it heats up, the voltage on the power tubes drops pretty drastically (30 V+) whenever I hit a chord on my guitar. The output drops considerably for a second, then comes back, though after a while, this will happen even without playing loudly.

It gets (or should be getting, according to the schematic) 550V on the plates, and though my meter maxes out at 500V+, the rest of the voltages check out very accurately to spec, so I'm guessing the 550V is good, too. I've replaced nearly every PS electrolytic and power resistor in the PS. Nothing looks to be burned, but I don't know the subtleties of looking for burned resistors, etc. I've tried various bias settings (including the correct one--600mv at the cathode resistor), but still similar result.

I'm at the limit of my knowledge. Could a bad tube cause the HV in the power section to drop? Or is it something else?

[Chuck H]

Sure a tube could cause that. Have you cleaned the tube sockets yet? Try a set of known good tubes and clean the sockets.

Chuck

[Rob Mercure]

You state that the voltage drops but not whether there is any other symptom that could be a "problem." Now I'm not familiar with the model and what output tubes and what the output transformer primary impedance is but there has to be some voltage drop across the OT primary as the tube's current swings with the guitar signal and with 50 volts isn't out of "ballpark." What are your output tubes, how much current do they draw at idle, what's the control grid bias voltage and, if you know, what is the OT primary impedance?

Rob

[frank]

The amp uses 4 El-84's. The rest of the details will reveal how much I don't know: I believe each tube draws 15ma. Also that the bias should be around 600mV as measured across the cathode resistor <-these two facts from member Enzo in this thread: http://music-electronics-forum.com/showthread.php?t=291

There is 275 volts hitting the screen? or is that the grid? I don't know the OT primary impedance because I just fried my meter (I know enough to be dangerous).

The voltage drops were measured where the leads to the power tubes meet the board. Each voltage-drop would occur for at least a second or more, and like I said before, more frequently the longer the amp was on. Sometimes I didn't even have a guitar plugged in. I could watch my meter swing from 275V+ to about 225V+ and hear a definite "sucking" of the sound from the speaker, similar to when you just shut off an amp and the sound begins to die away.

Man, I've replaced a good deal of this thing's guts by now. I'm running out to the store to buy alcohol to clean the power tube sockets. It's about the only thing I haven't done except get a priest to exercise its demons.

[frank]

found the problem....bad solder joint in the power section once the amp got nice and hot. Now that it's fixed I've learned that the amp sounds like crap. Glad I spent all that time fixing it. I think I'm gonna salvage what parts I can and turn it into a TT or something....