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Hey Scott,

  Here's my take on Sag, Bias, and Bias X.


Power tubes use a large voltage supply (+450ish), called the B+ voltage, to do their work of amplifying the incoming audio signal.  Sag is what happens to that voltage when a large, fast change to the input occurs, causing lots of current to be drawn from the power supply in order to amplify the surge in sound.  So the main voltage to feeding the tube actually drops for a short period of time under the heavy load as the tube consumes the power to amplify the sound.  The end result is that the resulting amplified sound is compressed because the tube is somewhat starving for the voltage it needs to keep up with the big change.  An analogy could be the difference between someone jumping as high as they can from a concrete floor (no sag) to someone trying to jump as high as they can from an air mattress (high sag).  Their height is "compressed" because as they try to jump, the mattress gives way and absorbs some of the energy.


Now Bias.  Power tubes, like cars, need to run at least at idle.  A tube needs a small bias voltage applied to it in order to get it into a mode where it can reliably amplify the incoming signal (called the "linear region").  This voltage is small, on the order of 10s of volts, compared to the 450V above.  The higher the bias setting, the "hotter" the tube is run and to me, the more crispy the distortion sounds.  The lower the bias setting, the "colder" it is run and to me, the distortion starts to weaken but there may be more punchiness to the tone.


Now, Bias X.  Bias X stands for Bias Excursion, and excursion just means how far the bias voltage changes from the base setting.  It occurs under heavy power tube loading.  A setting of 0 means the bias will not vary, whereas a setting of 10 would allow maximum swing under heavy power tube loading.


Here's a blog posting from Sean Halley on these settings and some examples of how they sound:


Hope this helps!  Take care,


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