Minor debugging needed for 1987 Plexi

[mort]

I wouldn't mind tearing it out or going through alot of work to sort it out. I'm curious about what a top end bleeder does though.

[Chuck H]

There are many versions. They all amount to literally bleeding top end to ground. If done well, and with a little luck, you just bleed off the oscillation and never dip into audible range.

[mort]

OK well I'll try shortening and/or rerouting my tube leads and see what that gets me. I'm not against bleeding it but I'll try the usual first. Thanks for the help.

[Chuck H]

Take special care to see that the OT leads aren't excessively long, are twisted together and aren't cramped up to any preamp circuits or leads. Make sure the preamp grounds are all on one node, including the preamp filter caps. No daisy chained grounds. run all grounds individually to avoid any potential for in phase coupling. And the power tubes, their filters, the presence and bias circuit, are on a different ground node. You can probably ground the power supply here too. I usually have three nodes.

[mort]

Ok I've just gotten back into this today. I started by shortening the leads that tie to p2 and p3 of V2 [associated with the bright channel] and the fluttering is gone. And the amp is pretty quiet all the way up, so it's almost complete!

But I do have a new issue that's popped up. If I crank both channels to max and start playing, after a few seconds the 1K screen resistor on V5 smokes. They are 1W resistors. Should I just up the value or look for another problem?

[Chuck H]

Measure the voltage across that resistor and compare it the other tubes. You may have a bad tube. Post the voltage you read so we know. You may have excessive screen dissipation and it just happens to be V5 that smoked first.

[g1]

Agree with Chuck, but 1W may be just too low. I don't recall seeing less than 2W screen resistors. Swap tubes around and you will probably find the problem follows the tube, could be faulty tube, or just normal variation.
Marshall typically uses 5W screen resistors.

[mort]

I'm getting 4.5v across the V5 resistor and 4.4v across V4 resistor. Both are being fed 476v and passing 472v w/ respect to ground.

Just go to a 5W rating?

The resistor only starts to overheat at full volume AND with a guitar being played. At full volume idle it does not smoke.

[Chuck H]

That seems fine. That's about 2W screen dissipation at idle. Actually, it doesn't tell us what the tube may be doing when it's conducting more than idle current, but it's probably ok. Bump the rating then. 5W is fine.

[mort]

Thank you for your help, gents.