BK Butler Tube Driver with Bias Knob

The Bias control is very helpful as it adjusts the amount of current applied to the cathode of the tube. Eric Johnson used to spend a lot of time looking for perfect tubes to use in his standard Tube Drivers, but now he can just set the bias easily with the stock tube. Just go up or down a few clicks and he nails what he wants. The Bias control also adds dynamics and allows you to custom match your Tube Driver for any particular pickup/amp combination.

Technically it varies the electrical 'strength' of the tube and that affects gain, texture and dynamics. It does not change the EQ directly but because the way the tube responds can be varied; you will hear EQ differences at various settings.

Most players find a 'sweet spot' about 2:30 - 3 o'clock on the control.

All in all, it is a 'pro tool' type of control that each player can set to their own subjective taste.