[Amps] Regulated filament current

[David Lisney]

Hi, I once worked for a lamp manufacturer in R&D, as you can probably imagine some lamps were specified by current but most by voltage. For our own life testing even the "current specified" lamps were operated at a fixed voltage.
IE 200w halogen lamps rated at 6.6a (airfield landing lights) were actually tested at 30v.
Tungsten filament lamps exhibit a positive resistance change with increasing temperature as we all know, what stunned me was that having plotted a curve of power/voltage the points sat on a straightline over a wide range.  IE not the square law curve a standard resistor would be expected to follow. My concern with RF PA devices would be that particularly in directly heated cathodes the temperature would rise as the anode current and drive rose. In the constant current example this would cause the filament voltage to further increase which is the opposite to some of the manufacturers suggestions. I believe for example that a 4CX250B with a nominally 6v heater should have the voltage reduced to 5.5v if you are giving the device a good "battering" close to it's maximum frequency and dissipation rating. A constant current supply would do quite the opposite. I may be totally wrong, but it seems the best approach would be a current limited constant voltage supply.
Regards David G0FVT
>> My inclination is to adjust the current so that the
>> voltage across the tube is 12.6 volts.  Does this seem like the correct
>> thing to do?
> 
> 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.
> 
> Constant current heat-up time will be longer than constant voltage as well.
> 
> Any it has to be asked, why?  Just simply cold-start the filament in the
> traditional manner.  It's robust and inexpensive, and it doesn't provoke
> the designers into adding another chapter to their do's and don'ts for
>