A journeyman musician of my acquaintance left a Music Man guitar amplifier here for repairs and I finally got around to digging into it last weekend. After some routine maintenance type repairs that any 30 year old amplifier would need, I figured I couldn't avoid troubleshooting any more.
The procedure for setting the power tube bias voltage level is to measure the voltage developed across the two resistors in the picture and adjust the level with the potentiometer that is in the tip of the picture.
Doing this revealed that there was no voltage being developed on one tube. Releasing the circuit board from captivity revealed that one of the bias transistors had a broken lead.
At least I had the common sense to order up some integrated circuits and bias transistors while I was dithering over what to do.
It works great.
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