Hi Steve and all...
It's my understanding that an "electron cloud" forms around the cathode
whenever the current drawn from it is less than its emissive capability.
This negatively-charged cloud effectively protects the rather delicate
cathode oxide coating from the potentially strong anode-cathode electric
field.
When very short (from usecs to a very few msecs), high-amplitude pulses of
cathode current are drawn, the electrons can be drawn from the cloud - up
to a point. The shorter the pulse duration, the higher the current that can
be drawn, and conversely.
If high-enough cathode current is drawn for long enough to fully deplete
the electron cloud, the electric field at the cathode surface may and does
reach an intensity that can literally pull chunks (flakes?) of oxide off
the cathode, not only damaging it & reducing its emissivity but potentially
contaminating the grid sufficiently to destroy the tube's usefulness.
I think this scenario pretty much answers your questions, Steve. But my
info dates back many years, and if anyone on the reflector can confirm,
refine, correct, and/or contradict it on the basis of currently-credible
sources, I'd very much like to see such info.
73, Dick W0ID
>>>> The above is in response to Steve's recent inquiry as follows:
I'm puzzled by some bits of the data. There is a lot of emphasis on the
600mA max. anode current, but under pulse conditions 1A for 200ms is
allowed, 2A for 10ms and 8A max. on short pulses. If it has this much
cathode, why the restriction at 600mA?
Similarly, up to 3500V at the increased currents is permitted in pulse
use, subject to keeping the AVERAGE currents and dissipations within the
same limits as for cw/SSB use. Why does the recommended operating
condition place it so far inside current/voltage/dissipation capability?
Thanks, Steve
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