Mostly for sake of my peace of mind, I'd like to substitute the series
current regulator tube
in my '390 with something less expensive, easier to find, and
potentially longer lived.

I could do as follows:

- rectify the AC with a diode in series with a generous drop resistor

- filter it with an electrolytic capacitor

- regulate the resulting DC with some voltage regulator of appropriate
power,
which wouldn't have to dissipate that much, as I can let the resistor
take care of much of the voltage drop

- feed the tube heaters in series just as they are in AC.

One could do all that with easily available parts.
This would also cut the inrush heater current at startup, which can't
hurt.
However, I didn't read anything so far on such a DC solution.


QUESTIONS:

- Is there any reason to avoid DC?

- What would be an appropriate DC voltage?
Is a standard 12V reg OK or should I shoot for 12.6V or what else?



It would be nice to get rid of that expensive regulator in such a
simple way. :-)