RI Marshall Super Lead 100 with bias issue

[WholeToneMusic]

I wanted to get some thoughts on a Marshall RI Superlead MKII that has a creeping bias. It will keep going lower until it hits 0v from -31vdc and then one of the power tubes red plates. (I swapped out Power tubes and the same thing happens.) The amp starts slowly motorboating also. I am suspecting bad filter caps, I though the grid stop resistors could be going into thermal break down so I put a heat gun on them and they went up 50 ohms within 30 seconds. I monitored the Filter cap voltages and they slowly go down also. With the current limiter hooked up the amp runs fine, no bias creep.

[Enzo]

SO what happens when no power tubes are installed?

[Gtr_tech]

I'd disconnect the PI coupling caps and see what happens as well.

[WholeToneMusic]

Hmm Veddy Veddy interesting. I pulled the power tubes and let it go full voltage for 15 minutes and the bias stayed at -37.

[Gtr_tech]

Sounds like a failed output tube then.

[WholeToneMusic]

Heres the kicker, The customer and I have already tried two sets of output tubes. Also we ran it with two tubes pulled. Thinking *He was using a hot plate with it, could be possible he toasted the OT a bit.*

[WholeToneMusic]

The first set of tubes I put into it were some chinese tubes (not the best tubes for HT) I decided to put some JJ's in their and ran it for 30 minutes cranked and it stable. I did hear one thud when I was not playing that concerned me. I am going to bias them at 40mA.