I reference
http://www.r-type.org/articles/art-123c.htm
Which states:
Class AB relates to the case where the grid bias point lies between those which apply for Class A and Class B operation. Class AB working can be used with two valves in an AF push-pull output circuit, and it enables a higher power output to be achieved than would be given by the same valves in Class A.
A Class AB amplifier functions in the same manner as a Class A amplifier for small signal inputs, because the input waveform is still applied to a linear part of the IaVg characteristic. As input signal amplitude increases, the negative parts of the grid waveform become applied to the more non-linear section of the IaVg characteristic, and the distortion offered by each valve increases. However, the ability to cancel out even harmonics which is given by a Class A amplifier tends to apply here also, and this distortion is reduced. At yet higher input signal amplitudes each valve may be cut off during negative input peaks, whereupon one valve only passes anode current during these periods.
Since two valves in Class AB offer a greater power than the same valves in Class A, Class AB operation is attractive for applications where an increased distortion at high output powers is acceptable. With reasonably careful design, Class AB operation can also be used for the output stages of domestic high quality amplifiers.
Class AB stages may be divided into Class AB1 (where no grid current flows) and Class AB2 (where grid current flows during part of the positive input half-cycle). It is normal practice for the Class AB2 grid bias point to be closer to cut-off than occurs in Class AB1, and Class AB2 gives a greater power output. One sometimes encounters the statement that Class AB2 is intermediate between Class AB1 and Class B. For domestic amplifiers, Class AB1 is preferred because the output stage can be preceded by a simple voltage amplifier circuit, instead of by a power amplifier as would be required with Class AB2. Also, the distortion introduced by the output stage will be lower with Class AB1.
Unlike a Class A push-pull output stage, a Class AB push-pull stage draws an increased HT current as input signal amplitude increases. The increase in HT current is not as marked as with Class B operation, and does not preclude the use of cathode bias for Class AB1 working. An external source of grid is, however, required for Class AB2.