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!
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.
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