BIAS: 

Tube Bias refers to the negative voltage used to set the idle current of the power tubes (preamp tubes are self biasing). The bias of the output tubes is usually adjustable to allow the user to set the suggested the idle dissipation (in watts) for the tubes as published by the manufacturer. If possible, contact the manufacturer of the amplifier, they may have specific instructions.

All audio tubes must be biased. Small Signal (i.e. 12AX7) tubes are Cathode Biased. In this circuit a resistor is placed between the cathode and ground. As the high voltage plate draws electrons from the heated Cathode is leaves more and more of a positive charge on the cathode until the tube biases itself; the circuit is sometimes called self-bias. This circuit is inefficient, so other than small (single 6V6 and EL84 amplifiers) almost without exception Power Tubes are Grid Biased.

Grid Bias, Set Bias, Adjustable Bias, Negative Bias, Fixed Bias and any name other than Cathode Bias is a synonym for Grid Bias; there are only 2 ways that audio tubes can be biased.

With Grid Biasing a constant, or “fixed” negative voltage is placed on the control grid, choking off the current through the tube to some extent. A signal into the tube is injected onto the steady negative voltage, manipulating the current flowing through the tube into an amplified version of the signal. 

The Grid Bias voltage is usually adjustable to set the initial state and maintain the best performance throughout the life of the tubes.

With amplifiers that have an adjustable bias always set the bias before testing the tubes.


BIASING INSTRUCTIONS:

Warning: TUBES GET HOT! AMPLIFIERS CONTAIN HIGH VOLTAGES!

  1. With the amplifier off and no tubes in the amp, plug the tester into an empty tube socket. Turn the amplifier on and adjust the bias voltage as far negative as it will go, -100 if possible. Turn the amplifier off.
  2. Insert one tube into the tester, leaving all other tubes out of the amp.
  3. After the tube warms up, take the amplifier off of “Standby” (turn on) and adjust the bias voltage less and less negative until the desired percentage of tube’s wattage is achieved. If you cannot find specific instructions from the manufacturer, amps that do not have an adjustable bias tend to have the tube set at around 75% of its published wattage. (22.5 watts for a 30 watt 6L6GC)
  4. Look at the plate (big grey boxy part) in the dark and check for red glow (Called Red Plating) In this case the bias voltage isn’t negative enough and the tube is pulling too much current. Not only will this burn the tube, it may burn the output transformer, the power transformer or the high voltage circuit of the amplifier.
  5. After the tube has cooled repeat the instructions for each of the other power tubes to verify that they are matching and none of them are operating outside of the safe zone.
  6. After the tubes have cooled unplug the tester and place all of the tubes back into the amp.

You cannot set your bias to a specified voltage; by itself, a voltage tells you nothing. The idle dissipation, in watts, of the tube is what you are adjusting, not how many volts you read on the grids; you are adjusting the negative grid bias voltage to change the plate current in the tube.

Because bias is a negative voltage, “Over biased” is too large a negative voltage, which holds the tubes in a condition of too little current. “Under biased” means that the tubes have too little negative voltage on their grids; the tubes are pulling too much current and can cause serious damage to the tubes and the amplifier. For amps with adjustable bias, always re-bias the amp whenever you change power tubes. Bias levels also drift and should be checked as the tube ages and adjusted accordingly to get the best performance and longest life from your tubes.