The term bias refers to the amount of voltage held on the grids of the output power tubes. Bias is associated with the “Operating Class” the power amplifier is working in, by design. There are three classes used in hi-fi: Class A, Class A/B1 and A/B2. Class A means that the output tubes are biased so that both tubes are always conducting. In class A/B, the bias is set so that on a signal peak, one of the tubes is off for some part of a signal wave form cycle. In class A/B1, no grid current flows into the grid of the tube, and in class A/B2 some grid current is driven into the grid of the tubes. There are also class B designs where both tubes never conduct current at the same time, only alternately. We at Cary Audio do not make Class B designs. They are frequently used in radio transmission designs, such as amplifiers used for amateur or ‘ham’ radio.
The operating Class of the amplifier is determined by how much bias current is present. If there is a lot of bias voltage, only the tube which is driven by the positive going half wave of the signal at any moment is conducting. Class B sounds distorted because the point where the signal ‘crosses over’ from positive to negative and begins to drive the other tube is not reproduced cleanly, and this creates crossover distortion in the circuit. As the bias voltage is made less negative, crossover distortion diminishes swiftly, and you are in class A/B2; a little less negative, and they both conduct more, and you have class A/B1. If you go further, you get to the point where both tubes always conduct, making the amplifier work in class A, which has the least crossover distortion and best sound of any of these operating conditions. (Cary Audio Class A single ended output tube designs have ‘0’ crossover distortion due to circuit design and their operating parameters).
All power tubes have slightly different DC gains, so the same bias voltage on two different tubes produces two different current levels. ‘Matched pairs’ are two tubes selected to be close together for DC gain. The idea of matching output tubes usually refers to ‘push-pull’ stereo or mono circuit designs.