Replacing those 811's with 572's will work, but it will not work well without some modifications to the amplifier. Those modifications, plus those 572's, would finance several sets of 811's, so it's just not exactly the 'best' thing to do. The problem is that the 572's would not be running very efficiently at all, wrong plate voltage, for one thing. If you 'bump' that plate voltage to where it would be most efficient, you will also 'bump' the filament voltage to where it definitely should not be resulting in much shorter tube 'life'. If you can 'live' with the lesser output using those 'under volted' 572's they will certainly last much longer than the 811's will.
There are a few other thingys that will result with using those 572's. The characteristics of them being different from 811's means that the usual end points of the load and plate controls will change (dial settings). And, the resulting signal quality will decrease, sound sort of 'mushy'. It's just a lot better to run the 811's as they should be. (Been there, done that, learned the hard way.) The 'proof' being that Ameritron's AL-572 is NOT the same amplifier as the AL-811(any version).
- 'Doc
(That 'buck-boost' transformer used in the '811' series of amplifiers is a very handy thingy to have for it's intended purpose, but it is not a very good idea to use it as a 'booster' for output power. The results may be nice for a while, but there are consequences by doing it that way. The biggest draw-back is a pronounced increase in heat, which will eventually destroy the amplifier. I have a friend who found that by bumping the voltages with the 'buck-boost' transformer that the output of an AL-811H did increase. The problem was that the additional heat softened the tapped coil's bracing and it sort of 'bowed', almost touching other parts before he caught it. The fireworks display would have been spectacular!! So would have been the replacement cost.) ... wish he had taken pictures...