Bias Issue

[Hornmiester]

My tube amp has idiot lights use for biasing. There are four tubes which need to be biased. I cannot get one to bias (e.g., the light will not change from red to green with adjustment). I have changed tubes but the problem remains. What is wrong?

[mustang guy]

I don't know, but I am sure the folks here that do are going to ask you what amp you have, etc. Is it the Mac275?

[jorjen]

My tube amp has idiot lights use for biasing. There are four tubes which need to be biased. I cannot get one to bias (e.g., the light will not change from red to green with adjustment). I have changed tubes but the problem remains. What is wrong?

Sometimes(like with my Juicy Music pCats)these types of bias circuits use Quad Comparator IC's(multi legged flat centipede looking thing with numbers on it)that will go bad. They are available at Mouser and are generally fairly easy to swap out. If your circuit uses them, which it probably does, my money would be on you having a bad one.

[NOSValves]

Swap one of the tubes that will bias into the socket that will not bias... If the none biasing socket still won't bias chances are pretty good something is wrong with the amp.