Marshall JTM30 bias mod problem & IC3 (LM348) problem

[Tone Meister]

I would measure the Volts ac signal at the output tube grids with the amp maxed.
Use a 100mv/ 1K signal in.

If you see a 30Vac RMS signal & the tubes are known to be good, you may have a bad output transformer.

If there is a feedback resistor, I would lift it for the test.

It will be Friday before I can get back to it, but I will definitely perform this test first thing. From what I can tell there is no negative feedback loop. I will install a known good set of power tubes for testing purposes.

Just trying to wrap my head around this: Given this amp has a problematic bias circuit, are you suggesting the bias voltage could be affected by an OT that is potentially failing? Or are there potentially two separate problems? Thanks JPB.

[g1]

My guess is he is dealing with the lack of volume as a separate issue. That is my take also.
As far as the bias circuit, I've lost track a bit, have C126 and C127 been replaced? R107 and R133 verified correct value and good connection?

[Tone Meister]

UPDATE: Came across this 2015 amp repair in my records and realize I had forgotten to update this thread.

Ended up removing all components in the bias string and realized the previous modder had installed C126 backwards. I had missed that previously. Installed a new 10/100 cap at C126 and replaced all components in the bias string with the 56K R - 50K pot - 56K R at ZD101, ZD102, and ZD103. Was able to adjust bias within the desired range with a pair of 6L6GC, which was the goal all along.

As I had stated a couple times, the issue was so simple I couldn't see it.