Class A into Class AB

No. There are also amps that have a sliding class A bias system, where the bias is increased with the signal level. This allows them to idle without much heat. Krell used a system like this.

In tubes, you have class A1, class A2 (where grid current exists during part of the cycle of the waveform being amplified), class A3 (which was patented by Jack Elliano), class AB1 (no grid current) and class AB2 (grid current exists during part of the waveform cycle).

To be clear though:

There are amps that slide from class A to class AB (e.g. Pass XA25, Luxman).

There are amps that can be produced with different amounts of class A before transitioning to class AB (e.g. Coda).

-these are both simply class AB.

Every aspect of sound quality was improved by bias increases, especially at low listening levels. I ended up at 3.3x the factory setting with heat buildup being the limiting factor. Hard to believe that a no-cost adjustment could offer such an improvement.

Usually you have a certain amount of dissipation that occurs in the output section. Something like the Bryston I would expect to not be heavily biased, but quite often when you increase the bias current, its also helpful to reduce the Vcc+ and Vee- voltages so as to prevent the output devices from being damaged. The heatsinks are probably designed to present a certain 'thermal resistance' which in plain parlance means you may overheat the amp in certain circumstances. So be careful!

Would you call that a class A amp or a class AB amp?

@kren0006 By definition, that's a class AB amplifier. The rest is all marketing.

Any way you look at it the amp is considered a class AB amp. All AB means is that at some lower power the amp is class A, transitioning to B operation above that lower power value. 'B' means that one of the output devices is no longer conducting.

So it might make the transition at 1/2 watt or it might do it at 20 watts. The idea with the 'enriched A operation' (paraphrasing of course, but how ever you see that its a marketing term) is that most of the time you're playing the system, the amp is operating in the A region, which should make it sound better. This is probably only true if the amp is zero feedback. If the amp has feedback, the feedback should prevent the amp from having any extra distortion in the B region.

In practice though that isn't always the case since in most A or AB amps that use feedback, its very rare for the feedback to actually be sufficient. But that's a topic for a different thread...