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Re: [Amps] Regulated filament current

To: "" <>
Subject: Re: [Amps] Regulated filament current
From: David Lisney <>
Date: Sun, 6 Sep 2015 17:44:03 +0100
List-post: <">>
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
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