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Over and Under Biasing as means for tone control

What is the word about using bias as tone control?

First what is the risk to Tubes and equipment by doing that?
Second what is the actual effect of doing that as far as tone?

I know in the Guitar world it is not uncommon to do, but Guitar Amps are going for shades of distortion, unlike HiFi listening amps.

I would be interested to hear what people think.
Thanks!
128x128dumbeat
erik_squires
15,662 posts
 
Well in consumer-land, tone control refers to bass and treble balance controls. I'm not sure bias will affect  this in that sense.

Under-biasing risks excessive distortion, over bias risks excessive heat and shortened (possibly VERY short) tube life.
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johnsonwu
467 posts
 
Harmonics control may I say?
if you are talking about power tubes I highly discourage overbiasing to warm up
the sound.  It just gives you supernova and runaway tubes easily. 
Gain and driver tubes are a different story.  You can get pretty good returns from say biasing a 6SN7 from say 2mA to 4mA depending on B+ and plate resistor.  However since a lot of modern designs are direct coupled.  Increasing bias at gain stage by tweaking grid voltage or cathode resistor usually means decreasing bias at driver stage.  Not for the faint of heart.  
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