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adjustable "fixed" bias amps

So when you are replacing tubes on an adjustable fixed bias amp do you go with the manufacturer's recommended setting for the amp, even if it is higher than what the tubes will supposedly tolerate?  I have custom push pull 300b monoblock amps where the amp's recommended setting of 100-120 mA is higher than what the tubes allow (70 mA).  I don't want to fry the new, expensive tubes.    I have reached out to the dealer, but with Covid, it may take awhile to hear back from them.  Thanks for your help!
lonetoe
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noromance's avatar
noromance
10,143 posts
 
Turn the lights off. Is the anode glowing red? Even a little? If so back off the bias until the red is just gone. That's the top of the bias range. Optionally, you can adjust for SQ from there down.
rodman99999
6,206 posts
 
If your amp is push-pull, there must be two 300b tubes per channel (at least).       If those two 300b tubes share the same bias adjustment, they’re splitting the measured current (ie: 50-60mA each).
gs5556
1,243 posts
 
Sounds to me that's for both tubes and the bias is measured at the power supply takeoff.
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