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.
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RI Marshall Super Lead 100 with bias issue
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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.Helping musicians optimize their sound.
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