How did you flatten out 572b tubes in an 811h? Something wrong there.
Powering it from a 120-Volt outlet with excessive voltage sag under load.
So long as your 120-Volt outlet is no more than 8 or 10 feet from the breaker box, wired with number 12 wire you're probably okay. Any farther than that, or if the outlet is fed with #14 wire and you'll get voltage sag under load. The biggest wire it's legal to use for a 120-Volt outlet is number 12.
This reduces the temperature of the filament wires. Unless the temperature is maintained properly, the chemistry on the surface of the tungsten cathode wires is disrupted, and the emission will suffer. The tube just gets "weak".
Most famous example of this was the old Black Cat JB2000. Too many folks would run it from 120 Volts, because it was all they had available. In six months or less the RF power would fall to about half what it was to start. The cathodes in the 3-500Z tube are made from the same thoriated-tungsten alloy as the 811A and 572B. Same hazard from running them with low filament voltage.
Simple way to check for this hazard is to peer inside so you can see the yellow-white light from the filaments. The color of a piece of hot metal tells you its temperature.
If you see the color of that filament light change when you transmit at full power, that's a bad sign. Tells you the temperature of the filaments is falling because of power-line voltage drop. How soon that temperature drop under load will weaken the tubes' emission is hard to predict, but seeing that temperature drop is a bad sign no matter what.
Using a 240-Volt source improves the picture two ways. The current draw is half what it was on 120. This reduces the voltage drop in half, even if the wires in the wall are the same size and length as the 120-Volt outlet used.
The second advantage has to do with the filament transformer's step-down ratio. The filament voltage will fall only half as much per Volt of power-line sag, since it's being stepped down by a factor twice what it was at 120 Volts.
I call this one-two punch the "double whammy". All other things being equal, the filament voltage will sag by four times as much running from 120 Volts as it would running from 240 Volts.
But that's my educated guess.
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