In message <01BDBA6E.EEA98D60@166-93-76-122.rmi.net>, Richard W. Ehrhorn
<w4eto@rmii.com> writes
>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
Marvellous, thanks, Dick; you also answer the (unasked) follow-up
question about running higher anode voltage.
Steve
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|