After months of psychological counselling following the rampant Marshall bias issues of last year, my heart sank when I see this MusicMan HD130 2575 with the same problem.
This amp has the tube PI in what looks to be a Cathodyne configuration. Oddly, the 10 ohm Cathode resistors are missing, with the cathodes connected directly to ground. As it appears that these are only used to monitor cathode current, I have left the resistors off and monitor the individual currents with my bias probe.
The bias voltage at the junction of the two 330K resistors is relatively stable at around -42 volts. Its what happening on the pin 5's of all the tubes that is wild. Over the course of a few minutes, the voltage on the pin 5's ramps up from -42 to -22, with the cathode current going from 12mA to a staggering 70mA - with a plate voltage of 700V!! Meanwhile the bias supply is relatively stable at -42 to -40. I thought leaky coupling caps so I replaced the 0.047uF caps.
I pulled out the still good USA Sylvania's and tried a fresh set of JJ EL34L's. Same. Same. I'm stumped as to what else it could be. There doesn't seem to be anything else in the circuit that would effect the bias voltage at pin 5 that doesn't effect the actual bias supply. The Bias supply is stable.
Have you seen anything like this before? Any suggestions on where to go from here are much appreciated. Meanwhile, I will try to find that psychologist's phone number. I can recommend him for therapy following Triple Rectifiers, 410C's, SVT's, and more! Only problem for you is that he's in Sydney. :-)
Music Man 2475-130_and_2275-130.pdf
This amp has the tube PI in what looks to be a Cathodyne configuration. Oddly, the 10 ohm Cathode resistors are missing, with the cathodes connected directly to ground. As it appears that these are only used to monitor cathode current, I have left the resistors off and monitor the individual currents with my bias probe.
The bias voltage at the junction of the two 330K resistors is relatively stable at around -42 volts. Its what happening on the pin 5's of all the tubes that is wild. Over the course of a few minutes, the voltage on the pin 5's ramps up from -42 to -22, with the cathode current going from 12mA to a staggering 70mA - with a plate voltage of 700V!! Meanwhile the bias supply is relatively stable at -42 to -40. I thought leaky coupling caps so I replaced the 0.047uF caps.
I pulled out the still good USA Sylvania's and tried a fresh set of JJ EL34L's. Same. Same. I'm stumped as to what else it could be. There doesn't seem to be anything else in the circuit that would effect the bias voltage at pin 5 that doesn't effect the actual bias supply. The Bias supply is stable.
Have you seen anything like this before? Any suggestions on where to go from here are much appreciated. Meanwhile, I will try to find that psychologist's phone number. I can recommend him for therapy following Triple Rectifiers, 410C's, SVT's, and more! Only problem for you is that he's in Sydney. :-)
Music Man 2475-130_and_2275-130.pdf
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