What is class A...?

Class A works in the linear region all of the time. In class A the valve never enters the cut-off region as in class B (and class AB). Since class A Quiescent point is in the linear region current is being drawn all of the time. Class B is very efficient with no current being drawn at idle, with its Quiescent point being at the cut-off point. But since in class B the valves only conduct for half the input cycle, cross-over distortion occurs when the valves leaves the linear region and requires a finite time to switch on. Class AB is a compromise where the amplifier predominantly works in the linear region as in class A, but for larger signals will enter the cut-off region and will cross-over. Since the cross-over for class AB is not at idle cross-over distortion is not noticeable. Subscript  '1'  as in  Class A1  indicates that no grid control current is drawn by the valve, while subscript  '2'  as in  Class A2  indicates that the valve draws grid current during or before saturation at maximum output.