Repair Details:
Replaced second preamp tube.
Replaced low voltage supply zener diodes and master volume switching opamp.
Replaced factory installed (in 1990) 56K bias resistor with 47K bias resistor as per 1994 schematic to lower increase the negative bias voltage from -50V to -38V to properly bias the EL34 tube configuration.
Resoldered misc components and organized wire layout to help with noise further.
Rtv’d misc components.
Test ok
I've had a few Russian 12AX7's that somehow had a cathode-heater short, and would hum like mad. I don't know how these slip by QC, but they do. In fact, other than a filter going 'open' (not likely, but it can happen), tubes are the culprit 99% of the time. The zener bit isn't something I'd know how it can affect 'hum'. Unless it looks bloated and cooked like a marshmallow a campfire, why would you change a zener?
The bias makes the most sense, too. That's the big reason a lot of tube amps come back to the store for warranty repair. Well, you'd complain the amp just doesn't sound right, they'd do a 10-second repair job adjusting the bias, and away you go. I once picked up a dead mint 1969 Super Reverb after it had been gone through by the in-store 'tech', but a line up of 912 guitar players who thought they wanted it walked away 'cause the tone sucked bad. I took it, and at home measured the bias. Stone cold, and a quick turn of the screwdriver opened up the tone like you wouldn't believe. I still have that amp, and the store knocked hundreds off the price because it was now an 'as is' amp with no warranty. Big deal.
Going from -50VDC to -38VDC seems right for EL34's, but -52VDC is about right for 6L6's, so it depends on what tubes the amp came with. Marshall is usually EL34, so the bias mod makes sense. The tech can figure this out pretty easily it seems.
From an old thread of mine when my JCM900 was giving me static...Nov 2009
Tech checked the tubes on a tube tester and they all seemed okay. But tapping on the tube created a fluctuation and I guess the static. Replaced an EL34 tube and did a thorough cleaning and checkup of all the sockets, jacks and switches. I fired it up for 1/2 hour and seems okay - no static and tone is good. Seems a little louder and more sustain.
He also mentioned that he turned down the voltage from 42 to 35 as 42 is too hot and will burn out tubes faster. Maximum is like 43 or something and 42 is the setting recommended in the specs but that's if you buy Marshall original type tubes - the kind that came with the unit originally - which are long gone so no idea what brand/style. Doesn't seem to have any bad affects reducing the voltage. $204.......
I think you mean idle current was reduced from 42mA to 35mA. That's OK, and makes the tubes last longer, but you give up a little tone. Actually, you need to know the Plate voltage, but 42mA is hot unless you have a 50-watter with a low B+. Some players will sacrifice tube life for that tone, and it
is a great tone. But your wallet will feel lighter more often.
