What is meant by Class A, Class B etc?

[FAQ Team]

This refers to the portion of the signal waveform that is amplified by a valve. In Class A, the entire signal waveform is amplified. Quiescent current is set at a level such that anode current continues to flow even on the most negative excursion of the control grid voltage, so in Class A, the valve never goes into cut-off. In Class B operation, a valve has no quiescent current and only conducts during the positive half-cycle of the signal.

This is only of practical audio use when used in push-pull configuration. Each valve in a push-pull pair amplifies half of the signal waveform, one valve amplifying the positive-going half-cycle and the other valve the negative-going half-cycle. Each valve hands over to the other at the zero crossing point. This handover gives rises to crossover distortion emerging from the fact those tiny differences between components make it impossible to synchronise the changeover perfectly. The payback for Class B operation is that the circuit is much more efficient, producing more power with less waste heat. Class AB, unsurprisingly, refers to an intermediate situation where each valve goes into cut-off for less than half the signal cycle. Quiescent current is set between Class A and Class B limits.

This mode allows the valves of a push-pull pair to operate in Class A up to a certain output level, moving toward Class B as output increases. This is a compromise often used in audio amplifiers, since crossover distortion is more objectionable at low levels of output. You may see references to Class AB1 and Class AB2. The suffixed numbers simply tell us whether the control grid of the valve is allowed to swing positive and hence conduct grid current (AB2) or not (AB1). Since both the valve itself and the driver circuitry has to be carefully designed to permit grid current to flow, you will not see Class AB2 operation often. Class C operation occurs where anode current flows through the valve for less than a half-cycle. This is an efficient operating mode but gives rise to high distortion.

It is used mainly in RF circuits where the original waveform can be regenerated by suitable circuitry. Classes of operation beyond C denote digital amplification and are beyond the scope of this answer.