[Amps] Regulated filament current

[John Lyles]

After reading several replies and my email from yesterday about this,
I realized that a clarification is due along with further topics.

When we talk about regulated filament current or voltage, we may be 
referring to using DC power. This complicates things, and the proper 
polarity of the filament power should comply with the manufacturers 
requirement. This can affect the apparent bias from grid to filament, 
along the length (height) of the filament basket.

It is possible to also have AC heating regulated, using servo-driven 
variacs or rheostats, saturable reactors (one of my favorites), or AC 
frequency generators. A common method in broadcast rigs is a 
ferroresonant (Sola) transformer, with a rheostat on the output to give 
a way to adjust the filament voltage. This is a simple constant voltage 
requlator. The output then drives the primary of the filament transformer.

As Steve Wright alluded, there are other potential factors in 
filament/cathode heating in RF power tubes. These are secondary effects, 
but in large tubes they may play important roles in getting the best 
life from a tube.

Foremost is RF backheating. This is essentially RF heating which causes 
the filament to rise in temperature beyond what the AC or DC heating 
power is producing. It is evident if you can watch both voltage and 
current, and notice a drop in current as RF power is increased. Once in 
this regime, it is advantageous to reduce the AC or DC filament voltage, 
to compensate for this additional source of heat. I haven't noted this 
in ham-sized tubes, but over 100 kW it seems to be more common. Maybe 
others can comment on whether this happens in kW level amplifier tubes, 
esp cathode driven tubes.

Second affect is filament heating from radiation from the grids and 
plate. Since these surfaces are adjacent and close, this can be another 
secondary source of heat. Various coatings can be applied to these 
elements to make them less of a blackbody radiator or re-radiator. This 
becomes a significant backheating source in high power tubes.

John
K5PRO

> Message: 1
> Date: Sun, 6 Sep 2015 15:58:40 +1200
> From: Steve Wright <stevewrightnz at gmail.com>
>...
>
> I was looking at similar using a programmable switching supply.  Once
> the tube warms up fully, the current will normally drop off - except in
> your case it won't be doing that.  Also, if "other" undefined RF
> conditions cause the heat in the tube to drop then I think you might
> discover some uncharted territory the tube designers never imagined.
>
> ------------------------------