2. amps-preamps
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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
7 responses Add your response
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noromance's avatar
noromance
9,821 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
6,158 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
1,235 posts
 
Sounds to me that's for both tubes and the bias is measured at the power supply takeoff.
dgaylin
255 posts
 
okay tokay this is all very helpful thanks. I was wondering if it might be times two because of the two power tubes per amp. That makes a lot of sense. 
rodman99999
6,158 posts
 
Happy listening!
atmasphere's avatar
atmasphere
11,841 posts
 
@lonetoe

Technically speaking, 'fixed bias' and 'adjustable bias' are the same thing. 'Fixed' refers to fixing the bias voltage to a particular value.

Normally you set the bias to the manufacturer's spec. I suspect your amp uses the 300bs in parallel so it is as @rodman99999  suggested. For this reason the replacement tubes should be matched.
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