The Damp-Factor...

[Rex]

Class A has no sag...

That's a new one on me. Class A amps can experience power supply sag, too.

 

[Smilzo]

Class A amps can experience power supply sag, too.

If Vox AC30 were class A, then yes.

In class A the tube(s) conduct all the time... dissipation is almost constant, resistance in power supply is constant, so voltage drop in power supply is almost zero (no SAG).
If we shift towards class B (class AB) then dissipation between signal and no signal increase, so voltage drop increase, hence SAG.

 

[Rex]

You're right: average power is constant in a Class A amp, regardless of signal, as long as the amp stays in the linear region. But if it's overdriven, you can get power supply sag.

 

[DJD100]

Right, my bad...

FAS usually ups the Bias a bunch when setting Damp to "0" for Vox, Matchless, Dr.Z etc.

A Damp setting of "0" simulates an amp with no negative feedback in the power amp section; it's not about amplifier class (which is more about bias).

Increasing negative feedback makes the power amp sound and feel tighter; decreasing negative feedback makes it looser and spongier. Which way to go? That's up to your ears.

 

[Rex]

No bad at all. I had to think about it for a bit myself before responding.