[Cobaltblue]

I have concerns about using a variable DC supply since if the pass transistor goes SC or a fault occurs elsewhere you could end up with the full raw DC on your filament. You could argue that you could incorrectly set the fil volts on any tester but still.
Accidentally connecting the HT to the filament supply I would want to make sure the regulator was bullet proof against such accidental connection.

Cheers

Mike T