I bought this as a non working amp and as one of two things...a future host for a new amp, or more importantly as something to learn more troubleshooting on. Its a Mesa Son of the Original Mesa/Boogie. It’s making a horrendous noise that cuts in and out. I don’t think I am getting a guitar signal either. There are not too many of these out there from what I understand, yet still not uncommon to find for sale. Mesa has no documentation for them. But I found a schematic I cannot verify the source of. It’s 2x 6L6. This doesn’t seem to have a presence knob though. Maybe it was an option. Is this “motor boating” or just some form of self oscillation?