how the sound will be impacted? Will I get louder? Or does it allow to get louder? Will I get more bass? Does it related to the power rating in the specification?
The bias sets the point where the tube idles. When you apply a music signal the amount of current increases and decreases. Lets say for max power the current needs to swing plus and minus 40 mA from the bias point. If the bias current is set too low (say 30 mA) then it won't be able to make a full swing down since it wants to go down 40 but will hit zero and clip before it gets there. Even if you don't hit the zero point the tube operates in a non linear manner as it approaches zero so anytime you get near there distortion goes up.
If you set the bias current too high you might hit the limit on the plus 40 swing and clip, but even if you don't the tube will run hotter than it needs to at idle and shorten its life.
So you want to set it where it can make a full swing in both directions without running the tube too hard or swinging down into the nonlinear area near zero.
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