If I remember correctly, this was the normal procedure for
some cavity magnetrons, as soon as RF pulses were detected at the output
a relay turned off the filament current.
----- Original Message -----
From: "2" <2@dock.net>
To: "Karl-Arne Markstr?m" <sm0aom@telia.com>; "AMPS" <amps@contesting.com>;
"John T. M. Lyles" <jtml@lanl.gov>
Sent: Wednesday, June 25, 2003 5:14 PM
Subject: Re: [Amps] longevity of oxide cathode tube
>
>
> >Interesting,
> >this description fits in on the 4CX350AC, which is marketed by Svetlana
> >and others
> >as a lower temperature cathode and consequently longer life version of the
> >4CX350A.
> >
> >I have encountered the 4CX250BC version of the 4CX250B, which should have
> >similar
> >cathode properties. As far as I recall, the 4CX250BC was tried as one
> >possible cure for the
> >tube life problems in the SRT ST1610 transmitters, but only with moderate
> >success.
> >
> >The 4CX350AC was considered for our Collins 208U-10's, but the solid-state
> >driver chain
> >modifications were done instead.
> >
> >It appears that tube manufacturer application engineers caution you
> >against running oxide cathodes both undervoltage
> >and overvoltage, overvoltage seems also to speed up the malignant type of
> >grid contamination that causes grid emission and thermal
> >runaway besides long term loss of emission,
> >but I cannot recall seeing any firm statements on the ill effects on
> >undervoltage, except low initial emission.
> >
> >In the "old days" of UHF Class C amplifiers it was common to derate the
> >oxide cathode filament voltage to compensate
> >for RF losses and electron back-bombardment of the cathode and a
> >consequent increase in surface temperature.
>
> *** I've heard of cases in the 2GHz range where it was found that
> back-bombardment was such that the heater potential could be removed
> shortly after nominal RF output was achieved -- i.e., the cathode
> temperature was being maintained by the RF.
> >
> >...
If I remember correctly, this was normal procedure for
a lot of cavity magnetrons, as soon as RF pulses were detected at the output
a relay was expected to reduce or turn off the filament current.
73/
K-A
SM0AOM
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