JCM800 2412 loud hum

[CrackerJack70]

Hi gang, I'm brand new to the forum and have a question about my 87 JCM 800 2412. About a year or so ago I bought it replaced the filter caps and re-tubed the amp (tubes were bought off a friend as NOS so not sure how much life they actually have in them) and also put in Vintage 30's. Yesterday I had the amp cranked and noticed a pretty aggresive loud hum. Any ideas on what it could be? Thanks guys...

 

[dtier]

If it came on suddenly and is affecting tone, it could be a power tube failing which can be dangerous to the OT. Make sure it is fused correctly. You might see a power tube plate glowing (red plating) if this is whats happening. If it is more just an annoying hum when not playing perhaps a preamp tube. Did the bias supply caps get changed too?

 

[LyseFar]

Clean the impedance selector switch with deoxit and the plugs for the reverb tank also. Clean tube sockets and input jack … yes a good clean on all the items that corrodes.
Of course with the power off and drained filter caps.
(I assume you mean model 4212) ?