Class AB design is most used for guitar amps. It takes the best of class A and class B so to speak.
The bias voltage on the grid is negative and set in such a way that only a small stream of current is flowing through the tube (instead of the 100% a class A will have). That means that when you are not playing there is still a small amount of current flowing through the tube, which makes it very responsive.
One pair of tubes are working together like in a class B amp. One tube amplifies the positive part of the signal wave, the other the negative part. That way it is more efficient, less power is needed and of course the tubes last longer.