Plexi Reissue Bias Problem

[Sparky]

I have a Marshall 1987 reissue on the bench with bias issues. The amp exhibits low output and distortion from both channels and though it will bias up correctly to 38ma, as an example, then climbs off the meter when you hit a chord. I have addressed most of the usual fixes and it does go away but just when you get ready to put it back into the cab you hear a faint pop and it comes back in again. The owner drives the front pretty hard with a DOD 250. I had it up and running fine and the owner put his pedal in front and played for a few minutes and it occured again. I have tested evrything I can think of so far and am still chasing it.
Any helpful hints would be most appreciated.
Sparky

[John Phillips]

I hope I'm not insulting your intelligence, but you have tried changing the power tubes, haven't you? :)

One partially intermittently shorted tube could do that - both the low power/distortion (you'll be getting half a waveform) and the bias runaway.

If it's not that, metering the bias supply voltage at a few places as the current rises should pin it down... is it a general failure or on one side only?

Welcome to the Gear Page!

[Sparky]

John, thanks for the welcome. Here's a list of what I have done so far:

Swapped out power tubes more than once
Swapped out all preamp tubes
Replaced the 220k bias splitter resistors and caps in bias supply
Replaced the output coupling caps
Monitored voltage into PT
Monitored voltage out of PT
Checked and monitored bias voltage at all points
Checked screen resistors and grid resistors
Checked the 47pf cap
Checked all filter caps
Checked for proper lead dress

What I haven't done:

Subbed PT
Subbed OT
Replaced filter caps
Stripped board and dropped in one of my handwired boards,
This only a last resort

I'm sure there's something I may have missed. Maybe upping the grid resistors.? What do you think?

Thanks,
Sparky