> The amount of current drawn form the cathode has nothing to do with the
> emission life. In fact the hot cathode produces a cloud of electrons
> around it and creates a virtual cathode that is about one third the
> distance between the physical one and grid ( going from memory here) and
> the current is actually drawn from this pool or cloud of electrons. The
> life time of the cathode has to do with the migration of materials within
> the cathode, evaporation of materials and contamination.
That's my understanding also, with the exception of MOX cathode
tubes that can be permanently damaged by excessive high voltage
or use of the cathode at emission limits (cathode too cold for the
amount of current drawn).
> I believe the source of this AC vs DC filament life time myth may
> have come from the early work with halogen lamps. There was a difference
> but that was in an high pressure gas device with a very hot filament. No
> cathode current there.
The only problem is with directly heated filament-cathodes and low
bias voltage tubes, the voltage drop across the filament can be a
significant portion of the bias voltage. That would mean the filament
is not supplying equal current over the entire area of the filament.
That shouldn't hurt life, but it certainly could affect IMD and other
parameters.
That effect could be minimized by taking the emission current
return off a point in the dc supply part of the way between the
filament ends.
73, Tom W8JI
w8ji@contesting.com
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