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. :-)

I solved this by running the 12.6VAC center tap from the power supply
to pin 5 of the regulator tube. Insert a jumper from pin 5 to pin 7
and you're in business. This also allows the jumper to be removed and
a regulator tube can still be used if you'd like.

Barry - N4BUQ

- 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

I have done that in other receivers.

Regulated dc is fine for the filaments. In theory, dc may cause the
filament to wear out quicker than ac, but we are picking at straws. With
dc, the electrons leave one side of the filament, while ac balances out the
electron flow. I have never had a dc filament from such a regulator
conversion fail, though.

12 volts is find for 2x6 volt filaments in series.

I first applied this to a Hallicrafters SX-101. That had a separate
transformer for the filament and was easy. My shack was cold and I had a
120 volt heater in the shack. When it came on and off, the voltage on the
filament of the oscillator changed and there was a slow drift (especially on
10 meters). Although the plate voltage was regulated, I soon realized it
was the change in filament temperature that caused the drift. A secondary
benefit was realized from the conversion because the hum modulation of the
cathode of the 12BY7 was removed and cw signals sounded cleaner.

Colin K7FM

Thank you guys!

Now that I think of it, many mil radios
had heaters fed directly off batteries,
while high voltage came via a dynamotor

I'll try the 12.6 Vac jumper,
and when I'll do the DC mod
I'll post it somewhere.

Thank you guys!

Now that I think of it, many mil radios
had heaters fed directly off batteries,
while high voltage came via a dynamotor

I'll try the 12.6 Vac jumper,
and when I'll do the DC mod
I'll post it somewhere.