I've been blessed!

[Penthode]

A brightness drift? I have a few questions.
I recall the old rule of thumb to pinpoint drift how long does it take to occur from switch on? If the drift is within the first few minutes after powering up, I would suspect a vacuum tube. If over 10 to 20 minutes a resistor and after 20 minutes a capacitor. Is there a reliable timeline?

Looking at the schematic, there is limited DC coupling to the input of the 12BY7. So the drift we can be 80% certain is the from the 12BY7 through to the 21AXP22A and it's surrounding components.

You did not report blooming so we can assume it is a bias change.

My first focus would be measuring the voltages and checking was changes from start up to running when the screen goes dark.

So first three questions:

How long a period does the darkening occur?

Have the capacitors been replaced around the 12BY7?

Has the contrast control 12BY7 cathode modification been made?