HELP: UL master volume pro reverb Bias problem

I have 435v plate voltage, and around -24 bias. The thing is amp has tube balance pot only, ie no biasing pot. Anyone can help me as how to increase the bias , the amp needs it. Thanks!

Hi homesick,

Which circuit, exactly? Photos would be helpful.

Not a hard mod, but we'll need to know the exact circuit before guessing about values, etc.

If you have "around -24 bias" then my gut tells me something else is weird. I can't imagine the amp not red-plating with only -24vdc bias. You sure your meter is set correctly? You sure you are measuring the right place? Right tubes, etc?

CBG

Hi homesick,

Which circuit, exactly? Photos would be helpful.

Not a hard mod, but we'll need to know the exact circuit before guessing about values, etc.

If you have "around -24 bias" then my gut tells me something else is weird. I can't imagine the amp not red-plating with only -24vdc bias. You sure your meter is set correctly? You sure you are measuring the right place? Right tubes, etc?

CBG


Hi Cool Blue. This is a 78 master volume pull-boost 70 watts. I read 435 volts, & after some tube swapping, I could reach -28Ma as bias. the amp sounds very good. I guess I have no problem with cold-biasing. The amp has a tube balance pot on the back, but no pot to increase or decrease bias:confused:. This is common to post 75-76 master volume fenders. My question is if there a way (resistor change??) to bring bias back within range??... Also, another question. Does the PI tube influence bias values whatsoever? thanks