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.

True. This transmitter only has a single HV winding, so all the B+'s
for the non-final stage go through big power dropping resistors, and
then there's all the screen bias dropping resistors too.

I haven't added it all up, but this transmitter probably dissipates
close to 70W when not key-down in dropping resistors just to avoid a
second winding on the power transformer. If I were to re-do anything
I'd start by adding another power transformer for screen bias and
non-final B+'s.

Tim.