This resource was developed by Mesa Engineering. Mesa Boogie amps are some of the best tube guitar amps on the planet.
So what is a Class A tube amp and how does it differ from a Class AB?
The most important thing for a musician to understand is that the different amplifier Classes and Configurations serve different purposes and styles. No one is flat-out better. A player looking for maximum clean headroom will want a Class AB, push-pull amplifier because power is its forte. A player seeking a certain vintage vibe may prefer a Class A amp, but no matter what tube is used, it will definitely be less powerful. Either type can be made to sound warm and lush or brash and biting – mostly as a result of other important factors such as the pre-amp and driver circuitry, transformer and component choices and, of course, speaker options.Class-A-Exposed-and-Explained-by-Randall-Smith
The previous material was written in 2005 by MESA Founder, Randall Smith. Click here to download and read the full article (with illustrations & examples).
If the original source linked above is down, you can also download this copy of Class A vs AB from Mega Distortion.