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homebrew bias question.


 
5/5/2000 7:45 PM
john hermsen
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homebrew bias question.
Hi there,  
 
I just finished my first "big" amp. I have an soldano atomic pre-amp and an marshall output stage. When I measured the bias, using the 1 ohm resister, it was 39ma and 49ma whith the 220k bias resiters. Can i change the 220k on just one side or do they have to be the same value? Btw it sounds great, but on high notes it's almost as if there was some light chorus effect on it.  
 
thank you john.
 
5/5/2000 8:00 PM
Jim S.
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The values of the bias feed resistors have virtually no effect on the bias voltage on the control grids of the power tubes. They should have the same value, however.  
 
Chances are, your power tubes are not well matched. To make sure this is the case, you should measure plate, screen, and control grid voltages on all of the tubes (at idle) and make sure they are the same from tube to tube. If so, then it's definitely the tubes and not your circuit and I would suggest either: (1) installing a set of output tubes that are better matched, or (2) installing two bias adjust pots, one for each push-pull side (feeding separate, non-connected bias feed resistors). By the way, (2) is the only proper way to compensate for mismatched output tubes.
 
5/6/2000 9:30 PM
john hermsen
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hi,  
 
I installed a second bias pot (100k's) and set the current for both tubes at 40ma/500v. I still hear a chorus (light) sound specialy at higher tones. As the notes get higher the speed of the chorus goes up to. Could this be because these tubes are more than 10ma mismatched?  
 
 
thanks john.
 
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