[ARC5] [Milsurplus] Cold Filament Inrush Current

[Kenneth G. Gordon]

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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