"Why not just replace the pot with a fixed resistor?"
you could but it would be hard to find a 489.6 ohm resistor
And they are a bit off any ways. you might be able to find
a set the would work it it should stay
but there is the spot for a pot and this way you can make it dead-on.
" Instead of the pot, why not design a circuit, sampling output to use as feed back, to replace the pot. Remember that filament voltage drops to about half when the pj is turned off. "
The circuit does have feedback it is the feedback part that make the voltage adjustment.
There does not seem to be any thing to drop the voltage when it is off/standby, so it must be dropping the voltage on the input it the LM117 voltage regulator.
The difference in turning the pot all the way back and forth after the mod will only change the voltage 0.21V
before the mod it would be 10.42V . And this is with easy to get parts of common values. And just one cut on the PCB. I am trying to keep it simple and easy to do. and should never have to be messed with again.
if it moves a few percent over time no big deal
a few percent of 0.21V is very small but a few percent of 10.42 and is a different story.