I predict that the damage has been done to that set of tubes. Just changing the line voltage will NOT undo the damage. Might have prevented it, had you done so in the first place.
You can run 120 Volts, so long as you hold the drive down to about half-throttle. If you draw less current from the wall, the voltage drop won't be that bad.
A 20-Amp 120-Volt circuit will be wired with #12 wire. A 15-Amp circuit with #14, usually. The fatter wire will have a smaller voltage drop at the same current draw. The reason the problem is worse on a 120-Volt circuit is that you need TWICE the amperage on a 120-Volt circuit as you need on the 240. This causes twice the voltage drop in the same size wire, for the same output wattage on the linear. Turn the drive down, and the current demand drops with it. So does the voltage drop on the outlet wiring.
If the 811A tubes haven't had this same (mis)treatment yet, try them on a 240-Volt circuit, if you can. Or hold back to about half of full power. Either strategy will work.
The old textbooks describe a procedure to "rejuvenate" the thoriated-tungsten cathode in a tube like that. I have had pretty poor results on the ones I have tried to bring back to life. Mostly it involves turning up the filament voltage on a test rig. More often than not, the filament just burns out.
You DO have to blow air on them when trying this trick. The heat from the filament alone gets out of hand when you turn up the filament voltage, even for 15 or 20 minutes.
If you placed larger wires in the wall, and fed your 120-Volt outlet with #8 or #6 wire, the problem would be partly fixed. Trouble is, most building codes forbid using wire larger than #12 for a 120-Volt circuit. Might void your homeowner's insurance if they discovered it after a house fire.
Besides, wire that large won't fit under the screws on a 120-Volt outlet.
Here's hoping the 811A tubes haven't been "flattened" yet.
73