Mesa Studio .22 plus loud hum

[vintagekiki]

Post #9 is correct. 'zip' files work.

[Randall]

Sorry guys, I don't know why I can't upload this schem., so please refer to the above zipped file.

I think I am closing in on it now. If I ground the grid of V2A the noise and signal stop. Am I correct in thinking it must be between the grid and input? I've checked what grounds I have access to, and measure ~585 ohms from grid to ground, although it does rise slightly if I keep my probe there, a cap charging?

Another strange thing is when I switch to Lead mode, there is a loud squeal for a second and feels like the gain rises, but not instantly.

[g1]

From V2A grid to ground is 585 ohms, not Kohms?

[Randall]

My mistake, yes it is actually 585K ohms.