Intermittent Red-Plating

[lowell]

I have a jcm900 4001 amp here that was red-plating after being on for ten to twenty minutes. I have only seen it do so once, and therefore have not been able to troubleshoot it. After it red-plated the first time I switched the tubes from the right with the ones from the left to eliminate the tubes or the amp as the source of the problem. Ever since I have not been able to get it red-plate again. I even switched the tubes back. I've poked everywhere and turned it on , left it on, and tried just about everything I can think of. Any insight?

[txstrat]

Have you checked the bias circuit? Solder joints, bias pot etc.?

[Enzo]

Could be that the red plating tube was simply not making good contact in its socket. Removing it and putting it back in refreshed the contact and no more problem. Inspect the socket for loose pin grip.

[Robert M. Martinelli]

+1 on what Matt and Enzo said.

Looks like you temporarily lost the bias voltage; to make yourself sure it won't happen again ( possibly damaging your power tubes ) clean and re-tension the tube sockets; renewing the solder joints on tube sockets could be a good idea and won't do no harm.
If ALL the tubes were redplating check the bias circuit network and renew the solder joints there as well.

HTH

Best regards

Bob

[lowell]

thanks

I checked all of that and no problems, so I think I'm ok.