My experience with ion pumps is pretty limited, going back almost 50 years. The
Siemens YH1045 TWT (10kW out, about 6GHz) had an electrode which was kept at
about 2kV DC and attracted gas molecules. To 'prove' the vacuum, the current
had to be measured and in those day, MOSFET input op amps were still decades in
the future. For the Marconi P2000 ground satellite station tx, I (helped by
some others) produced a dual MOSFET input stage amplifier and set of Schmitt
triggers. The Ion pump current had to be below 25 microamps to allow the
filament to be turned on: it was then allowed to rise to 100 microamps before
the filament was turned off. Then the current had to be below 2 microamps to
allow the HV to be turned on and if it rose to 25 microamps, the HV had to be
turned off. Trying to do that with the input bias current of bipolar
transistors was pretty well impossible, especially over a temperature range,
but Ferranti came out with a dual MOSFET which enabled us to do it. I h
ad some
help from friends in what was then Marconi's MOS group....I think the ion pump
electrode had a magnet associated with it, but I can't really remember. The TWT
had 22kV at about 1 amp on the helix and 18kV at 3.5 amps on the collector, so
it needed a fair amount of cooling water!
The TWT cost the equivalent of 6 senior engineers salary for a year: the dual
MOSFET cost 0.2% of the engineers yearly pay, so it sure wasn't cheap...
73
Peter G3RZP
----Original message----
>From : Randy.Randall@UCHealth.com
Date : 16/07/14 - 18:49 (GMTDT)
To : amps@contesting.com
Subject : Re: [Amps] 3CX800A7 Spares - cook them periodically?
"Warranty requires that they be stored with external ion pumps energized at
least every 60 days."
What is an external ion pump?
Thank you,
Randy
Randy E. Randall AB9GO
Network Architect
UC Health
Randy.Randall@UCHealth.com
Phone 513-585-7146
Fax 513-585-7159
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of John Lyles
Sent: Tuesday, July 15, 2014 1:14 AM
To: amps@contesting.com
Subject: [Amps] 3CX800A7 Spares - cook them periodically?
The 3CX800A7s shouldn't leak sitting on the shelf. Keep them stored away from
extreme temperatures (freezing and hot) and Rip Van Winkle tubes should be
ready to use after a 30-60 minute wait after turn on once they are put back in
use. This does allow for the getter to attach with stray molecules once the
filament heats up.
> Have some NIB Eimac 3cx800a7's (YC-238) as spares for my Alpha amps.
>
> Question is, should they be put in some kind of fixture and cooked with
> air/filament v applied periodically to keep the boogymen away ? Is
> filament and airflow all that is needed to periodically "condition" them?
> Or is this all hogwash?
A fixture for these tubes is not needed. You can turn them on periodically, as
Carl suggested, and that is the best test.
> If there is any way to preserve them in storage I'm game as they seem
> to last forever if properly used - so a long time in storage is probable.
>
> Do these external anode tubes tend to leak any more than other types ?
Not that I've heard of. Most of the similar sized (kW sized) triodes and
tetrodes from that company seem to have good brazed ceramic to metal seals, in
my experience. What bit of stray molecules remaining seem to be adsorbed by
getters when they are turned on. Its when they get larger, like 200 kW and up,
that the seals become very large diameter and in multiple places, that the
sources of leaks increases. In some of these tubes such as RCA's 7835/4617
triode, warranty requires that they be stored with external ion pumps energized
at least every 60 days. Same is true for a lot of the big CPI klystrons and
those from other companies.
> How does the "getter" work on these suckers (if any exists)? Do they
> need to get equivalent to red hot like the 3-500z's to have the getter work?
Most certainly some type of getter, either a cold one or a heated one, is used.
Some tubes use the plate material as a getter, some use a separate getter
mounted near the filament connections, and some use a small glass capsule
getter that is broken open during sealing. The application of flashed getter
inside the envelope is not done in high power ceramic metal tubes, for obvious
reasons of contamination of the insulation. Manufacturers are tight-lipped as
to what they use in a particular bottle, and usually its when they are broken
open the surprise is learned.
> What has to be applied to keep them healthy ?
> Anyone have a schematic for a burn in fixture?
If it were to be made, it would be a simple socket with blower/fan, airflow
interlock, filament power at a slightly low voltage with the proper current
limiting during turn on. More elaborate would be a tester that applies plate
voltage, and of course bias on the grid or cathode to prevent excessively high
beam current. As this is done, though, the chances of the tube becoming an
oscillator goes up, so 'stoppers' like parasitic chokes and damping resistances
would be considered. Its just not worth the effort if you have the amplifier
that can test them occasionally.
73
John
K5PRO
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