[Amps] Switching Supplies for High Current Filaments

[John Lyles]

I am using a switcher to power the filament for a TH781 tetrode, as a driver for a TH628 Diacrode final PA. The tetrode needs 9.5 VDC at 325 Amps, and the switcher works very well. 
I was nervous doing this at first due to possibility of conducted RF on the short welding cables from the bottom of the cavity PA to the supply, less than a foot in length. However, testing 
it yesterday proved that there is absolutely no effect on the filament voltage when I key the RF on (150 kW peak) or off. I am also watching for sideband noise on the RF carrier, from the switcher, but its hard to measure
with pulsed RF system. For the final, I have a SCR regulated 20 VDC supply delivering 1000 amps to the filament. We plan to light that tube up this afternoon or tomorrow before the long weekend. Right now am verifying
that the airflow and water flows are all adequate, and applying Tempilaq paint to the bottom of the tube to verify that its lower than 175 deg C when the filament is lit. Then I pull the tube out and 
look at the results, before commencing RF operation. 

Both Thales tubes require a 450 second ramp up of filament voltage from zero, and DC filament power. The negative terminal is the 'heater' and + is ground, the cathode. Similarly they require 450 seconds ramp down on turn off, if possible. 
One ramp per day is all they recommend doing on a regular basis. 

John 
K5PRO