Marshall JMP Standby Problem

[AtomicMassUnit]

This amp sounds distorted and crackly when on, but when I shut it down, it suddenly sounds great until the filter caps drain. What would cause that?

[txstrat]

Filter caps?

[Bruce / Mission Amps]

This amp sounds distorted and crackly when on, but when I shut it down, it suddenly sounds great until the filter caps drain. What would cause that?

Under biased power tubes can do something like this when they are very close to thermal runaway,

[Enzo]

And seriously out of whack DC voltage most anywhere in the circuit can. That can be from leaky caps, to leaky JFETs, to who knows.

What amp do you have? "JMP" is about as specific as "General Motors."