I need some quick help for a humming JCM800

[Pearly Gator]

I have a Marshall JCM800 (2204) on the bench right now. It has new tubes and I did a recap. It had an awful hum with the master volume turned up, even before the service. Turned down, it is dead quiet. I lifted the C.T. from ground on the filament circuit and installed a pair of 100 ohm resistors across the filament supply to ground. The hum went WAY down. So, the amp has grounding issues. =/

What is the consensus on grounding a JCM800? Is is time to rewire the whole amp to a "star" ground config? I'd prefer some other fix before going down that path.

Thanks!

Gary

[VacuumVoodoo]

What is the consensus on grounding a JCM800? Is is time to rewire the whole amp to a "star" ground config? I'd prefer some other fix before going down that path.

Thanks!

Gary

That's what I usually do on these including lifting heater CT to about +70V.
It's pretty satisfying to hear the owner ask "is it on?" after you turn the master volume up and the amp is dead quiet.

[Pearly Gator]

Thanks, Voodoo. I guess part of the JCM800's mojo is that it rides the thin edge between instability and tone.

[phsyconoodler]

I have not noticed that with JCM800's unless something is wrong.You might want to check the tube sockets for arcing and the heater voltages.Have there been some repairs done inside that may have been done a little sloppy?