Problem with driver stage (ASM 3A)

Posted by stevethemaestro (A) on December 30, 2014 at 17:16:17

I am having a problem that appears to be in the driver stage of my ST-70. About two years ago, I upgraded the driver board from the stock 7199 based board to the ASM-3A. I had Joe build it for me. Installation was a snap. It has worked spectacularly up to a few weeks ago.

There is a very edgy hum coming from one or sometimes both channels at 120 Hz but changing tone over a short time, with added harmonics being heard clearly in the buzz. When I introduce a low level signal (1000 Hz) in that channel the tubes run away and start glowing blue inside the plate. Bias rises rapidly; I then shut off the amp to avoid damaging the output tubes. I can switch the input to the opposite side and get the same result over there by introducing a signal through the input.

I removed the four ECC88 driver tubes and measured the bias; rock steady at 50 millivolts. (I am also using Joe's bias circuit.) The tubes sit there quietly and don't run away. This suggests that the problem must be in the driver stage, correct?

I don't see any obviously burned or damaged parts. The circuit has behaved perfectly for a long time, so I don't know why it should suddenly go haywire. Perhaps the problem is elsewhere? If so, where should I look?

Thanks.

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Topic - Problem with driver stage (ASM 3A) - stevethemaestro 17:16:17 12/30/14 (1)

RE: CAE PC-7A output wiring questions - Joe Curcio 11:04:24 01/20/15 (0)

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