[R-390] Tube heaters on DC

[Flowertime01 at wmconnect.com]

Fellows, kc0lwn at yahoo.com,

Once upon a time (1971) I seen some R390's in Korea that were running on the
24 volt DC truck generators. The Trucks were late 50 vintage and still had 
generators. We also had some stand alone 24 volt DC generators that when with 
the "Radio Trucks" so you did not have to keep the big truck engines running to 
power the equipment vans mounted on the truck chassis.

The R390  120/240 AC power supplies were replaced with a different module 
that had a vibrator type B+ transformer that ran off the 24 Volts. The power 
supply modules were pretty well filtered and there was not a lot of trouble with 
vibrator hash. The generators were well filtered by the power bus in the "radio 
van" there were very good filters that trapped every thing in the power 
source for the vans.

Any way 24 volts is like close to 13.8 x 2 = 27.6 volts. We never tried to 
adjust the voltage for overrun.

AC volts is RMS not peak to peak. So DC volts is OK.

Is DC volts more quite than AC volts.

Maybe or maybe not. Depends on your source.

The tubes in an R390 or A are indirectly heated cathodes.
So as long as the AC or DC is not coupling lots of RF into the tube it makes 
no difference from that point of view.

At 60 cycle or DC the tubes mostly have plenty of filtering for that noise.

Now back to the thermal noise of the tube. The heater heats the cathode.
The cathode emits electrons.
Electron emission is a very noisy process.
This process cares not if it is AC or DC powered. 
Heat is heat and that's noise.

So the tubes will have the same thermal noise level from either a DC or AC 
filament voltage.

Once upon a time all tubes were DC filaments.
IF AC filaments made tubes more noisy, someone would have
retained DC filaments in the RF from end of at least some types of receivers.

Aggravation of DC filaments is like to exceed your return on investment.

If you were home building a receiver, you mite consider a DC filament for the 
RF stage. But after that, there is no advantage realized from the filament 
noise aspect.

Roger KC6TRU