Most auto bais is quite reliable however.

The servo can also require its own adjustment so that it does its job and brings the tubes in line accurately. The disadvantage is that extra circuitry can fail and cause damage to the amp when it does. The advantage of the auto is that it makes the replacement of tubes very easy to just slip in and go.

With auto bias, the amp has a built in circuit( feedback comparator or servo mech.) so that when the tubes are in place and the amp is on.it will automatically set the mA current level of the tube circuit, compensating for tube differences and thus run them at the recommended point. Go with the maufactures recommendation here. Both are not good as they have thier obvious problems, and sonics tradoffs. Manufactures design the circuit for the tube to have certian voltage applied that results in optimum life, sound, and safe operation of the amp.īasically the difference is that with fixed bias, you adjust the bias externally using a mulitmeter( or the amp has meter or led lights) and either an adjustment knob on the amp, or screwdriver slotted to fit an adjustment hole either inside on a board, or outside the amp to adjust the current mA reading that is recommended by the amp manufacturer, so that the tubes run at a desired bias point for long life and good sound.(deep breath ) Fixed bias is only 'fixed' until after YOU adjust it to the right setting or your prefered setting as some do.( bias for more power/ shorter tube life.or less power and longer life.

Bias is the voltage applied to the tubes grid.

Easiest way to bias guitar amp