I'm not an electronics expert, but I have a fundamental understanding of how guitar amps work (which is almost a necessity when you use a Mesa Boogie Mark IV). I wonder if you could briefly explain or direct me to an explaination of the basic difference between class A and class B power stages. As you know, my amp uses both.

>> Technically its the way the output tubes are biased. The output valves (being analog and not digital) vary between fully on (almost like a short circuit), and fully off (like an open circuit).

For the sake of this explanation, consider a power amp with just 2 output valves. In Class B, each valve handles one half of the waveform (one handles the positive half, the other, the negative half). The advantage of this is that with no signal, both valves are off. Here's a diagram showing how each valve handles a sinewave:

In Class A, the valve(s) are already half-on at rest. When you play, each valve cycles between off and on; here's a diagram of Class A:

True Class B causes a lot of crossover distortion (roughness during the changeover from one valve to the other), so most amps use class AB, which is nearly Class B, but with both valves turned slightly on at rest. Doing this, and including electronic negative feedback smooths out this crossover distortion.

Here are some differences:

• With Class AB, valves run "cooler" at rest, and therefore last longer.
• Class A has no changeover between valves, so no crossover distortion, and sounds smoother.
• Class A doesn't need as much (if any) electronic feedback, giving a softer, more organic sound.
  

High power Fenders and Marshalls use Class AB. Fenders traditionally use 6L6 power valves for a gutsy, clean sound. Marshalls generally use EL34s which give a smoother overdrive sound.

Vox AC30s, and many boutique amps use EL84 valves in Class A for classy clean and overdrive sounds, at lower power levels. Boogies and a few other amps give you choices to reconfigure the output stages, although bear in mind that choices can be offered anywhere along the path from Class A to Class AB. Boogies (as you know) also offer flexible preamp overdrive, before the signal even gets to the output stage.

I think you would be most likely to hear the difference in sound using a clean sound, played at a level where the power amp is just starting to break up. Chances are that the Class A setting will have a wider "sweet spot", but less volume.