[AMPS] heater surge

[Peter Chadwick]

Rich says

>My guess is that the hot/cold 
>resistance ratio in heater type cathodes is much less than that in 
>directly heated/thoriated-tungsten cathode tubes.  Typically, 
>directly-heated cathodes have a hot/cold res. ratio of 8.3 to 1.  

 using the temp coefficient of  tungsten suggests about 3.5 to 4 for a
heater/cathode type tube. The same equation gives about 8.1 for a filament,
so I guess it's more than close enough. Rich, tnx for the info.

So, does limiting the current to say twice nominal help life? I suspect it's
probably not noticeable, while getting the volts right is important. So
using DC on the fils seems best, as advised by GW4FRX. Then it becomes a
case of making the regulator put up with whatever one wants the surge
current to be. But a regulator that can handle about 40 amps of surge is
very different to one that limits it to 10, and  makes it worth limiting the
amps.

73

Peter G3RZP

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