Dan (Drubin), the main reason for the cautions about not operating tube amps without a load is unrelated to magnetization. The concern is mainly the possibility of damage to the insulation of the windings in the transformer, and possibly also to the output tubes, that may result from what is called "inductive kickback," aka "inductive kick." That is the same effect by which the ignition coil used in cars produces the thousands of volts needed to fire the spark plugs. The voltage across an inductor (a coil) at any instant of time equals its inductance times the rate of change of the current flowing through it (i.e., the change of current per unit time). In the absence of a load on the secondary side of a transformer that would absorb the energy applied on the primary side, the primary acts like an inductor rather than as the primary of a transformer, and an abrupt change of current resulting from fluctuating signals can produce very large and potentially damaging voltage transients.
Obviously that will be a concern mainly if a signal is being processed through the unloaded amp. But IMO (which which some others will differ), although damage is presumably unlikely if no signal is present it would still not be good practice to operate an unloaded tube amp having output transformers even if no signal is present, because conceivably turn-on or turn-off transients could in effect put a brief "signal" into them.
As far as magnetization is concerned, as I indicated I haven't previously seen any reports of that being a problem with any audio amplifiers. But it does seem more likely that it would occur with a SET amp than with a push-pull amp, because with a SET amp the DC bias current, and in fact all of the current that is put through the primary of the transformer, is always flowing in one direction. It's magnitude fluctuates with the signal, but not it's direction. So the magnetic field generated by that current is always exerting effects on the magnetizable material in the core of the transformer in one direction, in contrast to a push-pull design where it alternates. That is a significant consideration that has to be taken into account in the design of transformers that are to be used in SETs.
As I said in my previous post, though, I'm not at all certain that operating a SET amp unloaded could cause or contribute to magnetization of the transformer. And in any event I would feel safe in assuming that Rebbi was knowledgeable enough to not do that. But I mentioned it as a possibility that from a technical standpoint seems conceivable to me, analogously to the mention that was made in the Bill Whitlock paper I referenced to how an input transformer used with a microphone that is "phantom powered" with DC could be magnetized if the mic were connected or disconnected while power is present.
Best regards,
-- Al
