Hornmiester Posted April 20, 2014 Share Posted April 20, 2014 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? Quote Link to comment Share on other sites More sharing options...
mustang guy Posted April 21, 2014 Share Posted April 21, 2014 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? Quote Link to comment Share on other sites More sharing options...
jorjen Posted April 21, 2014 Share Posted April 21, 2014 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. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 21, 2014 Share Posted April 21, 2014 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. 1 Quote Link to comment Share on other sites More sharing options...
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