What does Class A or Class AB mean in tube amplifiers?

Tube amplifiers require a defined operating point on their characteristic curve for clean reproduction – this is called the idle current or bias current. If this current is very low, this is referred to as Class B operation in a push-pull amplifier: higher output, cooler operation, but also higher distortion.

Higher idle current: Class AB operation, which can also move gradually between Class B and Class A.

If the idle current is high, sometimes up to the load limit of the tube, this is referred to as Class A operation. This has very low distortion and the sound quality is very clean. However, wear is higher and heat generation is greater.

Single-ended power amplifiers operate almost exclusively in Class A operation.