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.
The Philips data sheet for the QEL2/275 (= 4CX250B) prescribed a reduction to
5,5 V if the tube was driven to
full class C ratings at 500 MHz, as an amplifier (presumably CCS ratings), but
not as a frequency multiplier.
Curiously enough, I have neither seen any references to filament voltage
reduction in class A or AB services,
nor to how to properly handle intermittent UHF class C operation.
(Presumably, the cathode would be underheated in the transmission pauses, and
then come up to heat
when RF drive is applied by some sort of regenerative action)
I am no expert of the oxide cathode chemistry, and it would be interesting to
hear from
anyone that can go deeper into the fine print of the cathode lifetime vs.
filament voltage business.
73/
Karl-Arne Markstrom
SM0AOM
----- Original Message -----
From: "John T. M. Lyles" <jtml@lanl.gov>
To: <amps@contesting.com>
Sent: Monday, June 23, 2003 6:29 PM
Subject: [Amps] longevity of oxide cathode tube
> A comrade at another national laboratory uses 4CX350A by the dozens.
> They have 4 driving 4 in a class A high level feedback preamp circuit
> at 2-4 MHz. And many installations of this setup. This month we were
> chatting about the short (< 1 year) tube life they are having, and I
> decided to check with CPI/Eimac. I learned that this tube had been
> specified for a certain filament voltage, but it was for certain
> military SSB gear (i.e., Rockwell Collins) and that if run at the
> datasheet nominal voltage, short emission life would result. However,
> if backed off 10% on the fil V., then normal long life would be
> found. With adequate cathode current for peaks.
>
> They sell as different version, the 4CX350S (I think that was it?),
> which runs the filament at the proper temperature at the rated
> datasheet value, for long life.
>
> 73
> John
> K5PRO
>
> >
> >Having managed Telefunken 20 kW HF transmitters at work using
> >parallel 4CX250B's as drivers, the average filament hours we got
> >usually were around 2 years (16000 h) before stage gain decreased and IMD
> >increased to a point where the tubes had to be replaced (the PA
> >servo tuning required a quite high stage gain to
> >work properly). These transmitters were used for voice SSB services,
> >with a VOX relay reducing
> >plate current to about 5 % of the normal idling value. (for a photo,
> >take a look at the end of this page:
> >http://home.t-online.de/home/Hbusch/sdj.htm)
> >
> >On the other hand, the filament hours obtained on 4CX250B's in the
> >Standard Radio ST1610 ship's
> >transmitters usually were less than 8000 when the transmitters were
> >used for automatic radiotelex service.
> >A lot of effort was spent to find the causes, and poor filament
> >voltage regulation together with long intervals
> >of stand-by without plate current was deemed to be the culprit.
> >
> >However, it seems that some oxide cathode transmitting tubes can
> >have exceptional life spans
> >if properly handled. The otherwise infamous Eimac P290A comes to my mind
> >where a few tubes surviving "infant mortality" sometimes would
> >perform according to
> >specifications for more than 70000 h. It appears that filament
> >regulation, proper cooling
> >and flash-over current limiting can be of influence for prolonging
> >the oxide cathode life.
> >
> >I would find it interesting to know the date code on the "retired" 4CX250B
> >
> >73/
> >
> >Karl-Arne Markstrom
> >SM0AOM
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