Meat Plow wrote:

Not sure, I just trusted the word of my elders that damage could occur.
Maybe some of the deeper thinkers in alt.guitar.amps could explain exactly
what mechanism(s) is/are involved in damaging a AB amp with no load. Now
that you've (sparked)laughs my curiosity, I'm eager to know.

No deep thinking required. The mechanism is inductive kickback in the
primary of the output transformer.

Think of it this way. If there is no load on the map, then the
secondary of the output transformer can't conduct any current. If it
never conducts, then for purposes of mental exercise, it isn't even
there. You could (in your mind) physically remove the secondary without
changing the situation in question.

At that point, you are left with a pair of push-pull output tubes
connected to the ends of an inductor - the output transformer primary.
The inductor has a center tap which is connected to a power supply.
When one of the tubes turns on, current begins to flow in it's half of
the primary. The current builds up at a rate determined by the
inductance of the primary. Once current is flowing, what happens if the
tube suddenly drops out of conduction? (Think of an audio transient like
a drum-snap). The result is that the current can't continue to flow,
but the energy stored in the inductor's magnetic field has nowhere to
go. The magnetic field collapses and produces a very high counter-emf
(i.e., voltage spike).

This is the same mechanism that generates the high-voltage spark in a
car ignition or a television flyback - allow current to flow, then
suddenly cut it off.

By the way, this can happen in ANY amplifier, tube or solid-state, that
uses an output transformer. In most practical cases, though, tube-based
amps use an output transformer because tubes are high-impedance devices
that have to be matched to the low-impedance load (the speakers).
Transistor-based amps have an inherently low output impedance, and so do
not require a matching transformer.

Bill