“Just a clarification on the filament depletion process called “de-carburization” where the carbon is slowly cooked out of the thoriated tungsten filament.
As the de-carburization process continues, the hot resistance of the filament goes down causing the filament current to increase for a given filament voltage. For example, a new 4CX20,000 filament would draw about 140A @10VAC while the same filament nearing end of life would perhaps draw as much as 160A @10VAC. This is why the filament voltage had to be lowered when the new tube was installed, to compensate for dynamic impedance of the filament transformer designed in to limit the cold start in-rush current.”

And there you have it!

Regards!