[Milsurplus] Cold Filament Inrush Current

[Jim Whartenby]

Yes Ken, the hot and cold resistances are quite different but I would think that the cold resistance measured by an Ohmmeter would cause the worse case peak current condition and this applies only to a constant voltage source capable of unlimited current.  An over sized battery with very heavy cables would do the trick.
The reason that the transformer ESR is important is that a portion of the filament applied voltage is dropped across this resistance which is due, in part, to both the primary and secondary windings copper losses.  The actual voltage applied to the tube filament is reduced by this ESR at start up.  The applied voltage across the filaments then rises while the peak current decays as the tube warms up finally resulting in a much lower "steady state" heater current.
Obviously there ain't nothing simple about any of this.  There were several papers and book chapters written about this very problem in the 1940s and 50s.  Especially concerning heaters of power tubes, series / parallel connections in military radios and series connected TV tubes.  Most of these problems were reduced with the development of controlled warm up tubes.
This is part of the reason why the pilot light in AC/DC "All American Five" tube radios glows brightly at turn on then slowly fades to normal brightness.  These pilot lights never seem to last very long in my experience.Jim

      From: Kenneth G. Gordon <kgordon2006 at frontier.com>
 To: ARC-5 <arc5 at mailman.qth.net>; milsurplus at mailman.qth.net 
 Sent: Thursday, March 15, 2018 1:07 PM
 Subject: Re: [Milsurplus] Cold Filament Inrush Current
   
I would think that it would be easy to figure cold-filament in-rush current for any particular 
tube: 

1) measure the filament resistance of the cold tube with a good ohm-meter, 
2) use Ohm's law to figure the current at that resistance and the applied voltage, 
3) calculate the hot resistance from the published figures for filament voltage and current.
4) Extrapolate from those figures.

For instance the 12SK7's published figures are 12.6 VAC at 0.15 amp, which figures out to 
84 ohms.

I have not measured the cold resistance of a 12SK7's filament, but that should be easy to 
do.

So....are those two resistance values different?

Other factors, of course, are involved: for instance, is the tube a "controlled warm-up" type? 
Is it a "battery tube"? Etc.

Ken W7EKB

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