[ARC5] [Milsurplus] Cold Filament Inrush Current

[Richard Knoppow]

     As it happens I have a 6K8 in front of me. The tube handbook 
gives filament current as 300ma at 6.3V, = 21 ohms. The filament 
measures 5 ohms cold. That's 1.26 Amps at cold start.
     One place inrush current is very noticeable is in 5 tube 
AC/DC receivers where the pilot lamp is across part of the 
rectifier filament. It usually flashes up very bright on turn-on 
then dims as the filament string begins to heat up.
    You bring up an interesting point: what is the difference 
between plain and controlled warm up heaters? For many tubes the 
A version has controlled heater warm up for constant current 
filament strings. I expect this has to do with the rate the 
filaments heat and change resistance, perhaps some function of 
the mass of the cathode but I don't know for sure.
    Someone mentioned a long lead between battery and filament. 
That could also have enough inductance to delay the current a bit.

On 3/15/2018 11:06 AM, Kenneth G. Gordon wrote:
> 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
>
-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL