Tubes are rated as to how much power their plates can handle without some kind of catastrophic failure. 811's have less plate dissipation than 572's (been too long, can't remember and am too lazy to look it up).
The output of a tube depends on certain things. The voltage on the plate being one of them. 811's use less voltage than 572's, which means that the 811's voltage on the 572's plate won't give you full output, or even as much as you can get from an 811. So, running those 572's with 811 voltages will always mean less output than the 572's are capable of. It also means that they are just sort of loafing along, and will last longer than an 811 under the same conditions.
Sound like a really good idea? It ain't bad, but there are some 'catches'. Using 572's in those conditions also means that the resulting signal will be a little bit 'wider' than if the 572's were run correctly. Nothing really noteworthy, but still there.
An Ameritron AL-811 can be 'modded' to run 572's at their normal levels (voltages,etc.), but there are definitely 'catches' with that! The simplest 'mod' is to change the primary voltage on the transformer to produce more secondary voltages. The 'catch' there is that it also raises the filament voltage and that ain't good at all. It will also result in more heat production than the stock fan can handle, and push several other components at -least- close to their ratings if not over. There's more to it than appears on the surface, sort of. Best advice is to don't do that, just get a 572 amplifier. Or a 3-500 amplifier, more better. Or a 3CX1200 which is probably the biggest 'forgiving' tube that's common.
- 'Doc
Why do I think I know a little about 811's? Cuz I've got AL-811, serial number '13' sitting on the desk over there. I've cussed that thing since the day I got it. But it worked, and still works after what, 20 years? The later ones are not exactly the same, my wrecking this one so many times is probably one of the causes for that. It's also been replaced by an AL-80A, and now an AL-80B. (That AL-80A was used by a friend to drive a really 'big' amplifier, believe it or not. Don't ask, I have no idea what tube was used in that thing.) None of that's bragging by the way. Well, maybe the #13 thingy, somebody has to be one'a the first idiots to buy something. Right?
It ain't easy being an idiot!