OK, amp gurus...only one of the two EL84's recently red-plated while playing...it's had a set of 7189's in it a while, but I've tried a few spares I had lying around, and all do it after a few minutes, just the one tube socket though. I took the amp out and metered everything connected to that socket...can't find anything out of spec, but that is with the power off. I can change out resistors and such if a component needs replaced, but can't find anything that doesn't read right, loose, etc. Any suggestions?
If you have the chassis out then please post a photo.
The first to check would be the grid resistor for that tube. It could be an issue with the tube socket, but more likely a resistor.
If you know how to test the filament wires then you should check to see that the connection is good and then test the AC voltage of the filament wires to verify that your are getting 6.2 V ac or close.
It's a spooky test if you haven't done it before and you are doing it hot, you could clip your leads on to make it safer and avoid having both hands in the chassis while energized.
I had a similar issue where my amp burned a grid resistor when the power tube shorted.
Attach the leads with the amp on standby. Then when you are sure they are secure and not also touching where they should not, you can take it off standby. My hands are not as steady as they used to be so I have gone to doing it like this. Also before I power up it gives me a second chance to make sure the leads are in the correct jacks of the meter and the meter is turned to the the proper position.
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