Marshall 2100 JCM 900 Head Hum

[rf7]

This is the same amp that I was working on before that had an oscillation. It has a higher than normal 60Hz hum, that you get when you turn up the Sensitivity control. Is that normal for these amps?




Previous thread on amp:
Marshall 2100 JCM 900 Head Oscillating

[Chuck H]

Not IME. I gigged with one for a couple of years. Memory is scant, but if there were an excessive hum I would have noticed.

[rf7]

It turns out this amp does not like JJ's in the V1 position. I found a NOS tube that was much quieter.

[Chuck H]

I've had a disproportionate percentage of Russian preamp tubes that were hummy right out of the box.