One of the key factors which affects the tone of your amplifier and its long-term health is something called the bias setting.  Unfortunately, there is a lot of misunderstanding among players and technicians as to how "bias" works and what "setting" it really does.  Let's correct that situation here and now!
 
Basically, your amplifier contains analog circuits which work best only over a narrow range of conditions, or "operating points."  Most amplifiers have a hidden screwdriver adjustment (bias pot to you tech types), which sets the operating point to the correct value.  It does this by adjusting a DC voltage applied to the tubes.  Tube action translates the DC voltage into tube current draw, which is how the operating point is expressed.
 
Please understand that we are talking about biasing power tubes (6L6, 7027, 6CA7/EL-34, 6V6 6550, KT-88, etc.) here.  Your small-signal types (12AX7, 12AT7, 12AU7, 6EU7, 5879, etc.) are self-biased for optimum performance, and no adjustments are needed.
 
So what happens at different bias pot settings?  Consider this:
 
If the bias control is set too high, current through the output tubes will be high.  As a result, the tubes will run very hot, and need frequent replacement.  The sound will probably be clean at first, and then turn distorted.  If the metal structure (plate) inside the tubes starts to glow cherry red, turn the amplifier off immediately and take it in for repair!  You can cause extensive damage to the tubes and other amplifier parts if you let this condition continue. 
 
If the bias control is set just right, current through the tubes will be at a proper value.  As a result, the sound will be good, there will be lots of power, and tube life will be long.  To the amplifier manufacturer, proper biasing is a compromise between good sound and reasonable tube life.
 
If the bias control is set too low, current through the tubes will be low.  As a result, the sound will be distorted on loud notes.  The output tubes will run cool.  Tube life will be longer under this condition, but other parts (capacitors) in your amplifier may be stressed by the higher voltages which appear on them.
 
So how is the bias set?  Technicians monitor the tube current draw with a special adapter unit, while adjusting the bias pot for a certain current value on the adapter's meter.  The volume control(s) must be set to zero for this adjustment.  The correct current value is found in the manufacturer's service literature, or under "Typical Operating Conditions" in a tube manual.
 
Let me leave you with these thoughts:  If you buy a set of tubes and choose to install them without setting the bias, you are gambling the original adjustment is close enough.  For best results after replacing the output tubes, always have the bias adjusted by a competent technician.  Due to tube aging, it is also recommended that you have the bias checked and adjusted, if necessary, after about 100 and 500 hours of playing.  This simple step maintains sound quality and extends tube life.
 
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