71 Marshall Red Plating, blowing mains fuse.

[Chuck H]

I only skimmed, but there are some good, but less likely considerations that can be explored in the event that the usual procedure doesn't correct the problem. Mostly there's a lot of rough advice. Like the user being told the amp needs to be set to the 4ohm tap and played into a 16ohm load. This sort of thing MIGHT stop the problem in some cases, but will result in a blown OT in more cases. Not recommended. Also, the poster never did get a solution to their problem after six pages and the thread continues for another two pages with no further response from the OP.

Stay here. Best shot at a fix. Then again... This seems to be a pretty common problem. That is, an inherent design issue. It's possible many of these amps do this and it's only occasionally noticed or problematic enough to cause a failure. Just the same I'm pretty sure some answers will be found on this thread before it's over.

[Leo_Gnardo]

So today I decided to take the amp back to the original tech that did the initial work to correct problem. I felt that since the initial problem wasn't fixed in the first place, that he should have first crack at correcting it. Let's see what he finds. I'll report back in a few days once he gets back to me.

I hope he'll find out whether the problem moves with the tube. First things first, that one's elementary Doctor Watson.

[cluster]

Thanks so much again. Let's see what the amp tech says. I'm sure we'll get to the bottom of this issue.

j.

[cluster]

got an update from the tech on the amp. he seems very sure that it was one faulty power tube that was causing both tubes to red plate on one side. I just ordered a quad of JJ EL-34L for him to put in. Now here is a question: this tech tests his amps using a load (not a cab). He tells me that he turns it up under a load to see how the amp/tubes respond. My question is, is this a valid way to test an amp? I always thought that the best way to test an amp is to dime it through a cab for 15 minutes. Thoughts??

[Chuck H]

Testing an amp by cranking through a cab for fifteen minutes is loads of fun, but not technically any more valid

I don't get how one of the power tubes is causing both tubes to red plate on that side UNLESS something about that tube is dragging the bias positive on that side. And I haven't seen that. That doesn't mean it doesn't happen. Just that "I" haven't seen it.

[Leo_Gnardo]

Testing an amp by cranking through a cab for fifteen minutes is loads of fun, but not technically any more valid

Running an amp into load resistors, especially if it's a big amp, is a lot easier on the tech's ears. And the neighbors'. Prevents unnecessary police intervention. You know how edgy they are lately - best to save their firepower for real emergencies.

I don't get how one of the power tubes is causing both tubes to red plate on that side UNLESS something about that tube is dragging the bias positive on that side. And I haven't seen that. That doesn't mean it doesn't happen. Just that "I" haven't seen it.

Exactly that, in amps with paralleled output tubes a failing tube can cause its control grid to be pulled positive. With only a pair of stopper resistors to shield a good tube from its failing mate, say 3K to 11K in many cases, the bad tube drags its mate(s) into perdition. Just like real life. Seen it happen way too many times in amps with 4 or 6 (or more!) output tubes. Do enough repairs, you'll see it eventually.

[cluster]

Thanks everybody. Lets see if this works. New tubes on the way.