Strange Noise/Bleed Issue - Marshall 1959RR

[V-man]

I have a Marshall 1959RR. For those unaware of the amp, it's a White tolex 1959 SLP without FX loop but with cascade channel, so CH1 is standard 1959, and Ch2 is both channels together via cascade, thus you can't jumper the amp like you would a 1959 SLP.




So, today I was playing through channel 1 at a "low" volume of 2 (which is the equivalent of my 2550 at 5). I don't know why I did this, but I cranked the volume knob of channel 2. This did nothing to affect the volume, as well it shouldn't, but as soon as I pushed Vol 2 past 5 or 6 I got a loud hum (like you would with a dimed amp with the guitar volume rolled back to 0). Again, playing through this hum caused zero change in signal volume... it merely added that loud hum.

I assume this has something to do with the cascade mod. If it's a "live with it" sort of thing I certainly can, but I'd rather troubleshoot this if possible. Any thoughts?

 

[pdf64]

I think it's probably normal, to be expected, as the cascade mod just mixes the output of Ch2 with the input to Ch1.
So you may be able to use an AB box to switch between them.

There's a possibility that the tube in V1 is wonky and causing excessive hum, so try a 'known good' spare or two in there.

There's an even more remote possibility that the B+ cap for the pre-amp supply is faulty, or there's a grounding issue (eg between the pcb and chassis grounding point - very unlikely to have a safety implication).
How old is the amp?
Pete

 

[V-man]

I think it's probably normal, to be expected, as the cascade mod just mixes the output of Ch2 with the input to Ch1.
So you may be able to use an AB box to switch between them.

There's a possibility that the tube in V1 is wonky and causing excessive hum, so try a 'known good' spare or two in there.

it's an original Mullard 12AX7. I can always try a new one.

There's an even more remote possibility that the B+ cap for the pre-amp supply is faulty, or there's a grounding issue (eg between the pcb and chassis grounding point - very unlikely to have a safety implication).
How old is the amp?
Pete

amp was made in '08.