Last time I looked 50K 5 or 10W pots were available in wirewound, I
think Allied or Mouser had the Ohmite units. I don't think you'll find
this sort of dissipation available in a ceramic pot. I'd recommend
that you go for something more than 4W if you replace it because it
does get warm.
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Unless you want to restore this valve equipment as much as possible to its
original state ,you could of course adjust the screen voltage with a
carbon track potmeter , using a high voltage transistor ,a high voltage
zener diode and a few more passive components.

Frank GM0CSZ / KN6WH