Traian,
is the effect you're seeing the same as the 'vac ion pump' that some big
tubes have? As I understand it, the ionisation attracts the gas ions to the
electrode and keeps the vacuum up.
I first saw it back in 1966 ( I do feel old at times!) on a Siemens YH1045
travelling wave tube. 10 watts in, 10kW out at 5.4GHz or so. 18kV on the
collector at 3 amps, and 1 amp at 22kV on the helix. The vac ion pump ran
at 2kV: we had to get the current below a few microamps, and it was
interesting with 1960's technolgy sensing the low currents and getting
temperature stability - the few integrated op amps around had too much bias
current. Anyhow, we ran the vac ion pump all the time and it kept the tube
hard. I ssupect Traian is seeing a similar effect that cleans up the
vacuum.
73
Peter G3RZP
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