Not all transmitting tube getters are heated. Some of the big
transmitting tubes that we use for particle accelerator (the old RCA
designs) have cold chemical getters in a large glass cartridge which
is broken open during the final seal off of the tube. These work
well, but are sometimes placed in areas which do not have appreciable
heating.
We also have ion pumps, small fist-sized components which stick out
of the tube, called appendige pumps. With these I can monitor the
exact vacuum condition while running the tube with high power RF. The
limit here is the conductance of the attaching tubing, from the core
of the tube out to the pumping element. A 3500 VDC low current supply
is used on these VacIon pumps.
As for the gases involved, we sometimes have carbon monoxide,
nitrogen, water vapor (BAD), hydrogen, and others. These are not
desired of course, but are incidental to the RF operation or the
processing of the tube when it was made. Years ago Eimac had trouble
with large shortwave tubes like the 4CV250,000A. After dozens of
gassy tubes, it was determined that the getter was getting swamped by
outgassing of hydrogen. Where would this come from, you ask? The
plating process for the tube components had changed to meet
California environmental code changes, and they went from pulsed to
DC (or vice versa) for the new power supplies in the plating tanks.
Increased electrolysis and dissolution of water in the baths was
trapping hydrogen underneath the silver plating on the metal seals
next to the ceramic. When the tubes were lit up, over time, hydrogen
ingressed into the vacuum, ruining the tubes. We get some of our
tubes from rebuilders who don't silver plate the anode now, as we are
using the same tubes for DC pulsed switches (plate modulator for
bigger RF tubes). And CPI/Eimac learned a hard lesson about process
changes, and the long term effects which may creep into the final
product from them. Vacuum tubes can be quite sensitive to
manufacturing process steps to the degree that a small change can
spoil the tube, but it isn't obvious in the final test steps. Also,
sometimes it depends on the end-users duty factor, those who don't
push the tubes to their full capacity may get by OK, while a station
(or particle accelerator) using them near their maximum dissipation
limits finds the trouble from gas.
As you say, they are already there freely sharing the space. When
ionization occurs and these residual molecules and atoms become heavy
charged particles, they slam into the cathode or other elements and
cause havoc. Heating of the elements in the tube always raises the
vacuum level (towards atmospheric), especially when a tube hasn't
been in that regime before. We always condition our expensive tubes
(as do most high power RF users), typically 24-150 hours or even
more, before putting them into the final socket and throwing on the
power switch. During this process, the RF duty factor (if pulsed) or
the average power level is slowly increased in steps, while watching
the vacuum level and allowing the getter and ion pumps to remove the
residual gas from the active region in the tube. One technique
practiced by one of the labs in the US is to reduce plate voltage to
about 1/4, and detune the PA tuning, and run the tube drive up, while
monitoring the gas. This heats the elements and outgasses, but the
plate swing is lower so that there is less chance for acceleration of
the ions and collision with the elements in the tube (and arc
over/crowbar).
73
John
K5PRO
>Transmitting tubes are different because the getter is already in place
>when the tubes are being pumped down, flash-heated way above the
>operating temperature to remove as much gas as possible, pumped some
>more and finally sealed. In that case, the getter surface would be
>pretty close to a chemical equilibrium with the residual gas pressure
>*before* the tube is sealed, so afterwards it would tend to *maintain*
>that condition.
>
>We're also aware from a few weeks ago that the getters used in
>transmitting tubes need to be hot in order to work at optimum
>effectiveness, so the vacuum can deteriorate when the tube is left cold
>for long periods.
>
>However, the basic principle is that electron tubes are quite happy to
>share the space with quite large numbers of gas molecules. That is
>because the electron current is usually much larger than the small
>reverse ion current... but if it isn't, things can go badly wrong.
>
>--
>73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
>
>
>So the question that remains for me is: What kind of gas molecules
>remain? Surely they are not an easily ionized gas like helium, neon or
>argon. Surely not oxygen (big no no); nitrogen? I would think that the
>atmosphere used in the manufacturing process just prior to evacuation
>would be kept as free of the big four as possible so that any remaining
>molecules wouldn't be one of them. Does the "getter" material only
>collect oxygen molecules or does it collect others as well? Does anyone
>have more information on this?
>
>Thanks and 73, Tony W4ZT
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