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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