Mark V 35

[LarBal]

Anyone have a Mark V 35 schematic? Customer uses the DI out or whatevr it's called and ran it without the internal 10r dummy load switched in. Said he smelled smoke, and now it sounds blatty and crappy.
Full volume is there, but something is wrong. It's pretty impossible to tell what's what in there, besides normal voltage tests. All tubes are new. I'm about to call this past 30min a loss and give it back

[LarBal]

This is a fixed bias EL84 x 4 amp.
With all 4 of those pulled, I'm getting -10V on one pair at the grids, and positive 212VDC on the other pair at grids! How is that even possible?

[The Dude]

Leaky/Shorted coupling cap from PI plate to output tube grids.

[LarBal]

Aha yes, I'll start there. Thank you. Wouldn't that DC voltage make it's way to the other pair(assuming it's set up like a normal 220K grid leak fixed bias circuit)?

[Helmholtz]

Wouldn't that DC voltage make it's way to the other pair(assuming it's set up like a normal 220K grid leak fixed bias circuit)?

No, the other side just sees the bias supply voltage, which is low impedance.
But this bias supply voltage might be somewhat more positive (i.e. less negative) now.

[LarBal]

You guys are great. Thanks so much for the speedy responses. Indeed the .1u coupler off the PI was leaking, and yes, was making the other side less negative. Bias voltage steady all the way around at -16V. Luckily I could get to it from the top of the board.

[Helmholtz]

Power tubes having seen positive grid bias are likely flawed now.

[LarBal]

I had the same thought. For now everything is working and the amp sounds great. When his amp went down last week, he ordered ALL new tubes from mesa and inserted them himself.
I imagine the best course of action would be to advise him on the matter and suggest a new set