[Amps] 3CX800A7 Spares - cook them periodically?

g8on at btinternet.com g8on at btinternet.com
Wed Jul 16 16:30:59 EDT 2014

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 had 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...


Peter G3RZP

----Original message----
>From : Randy.Randall at UCHealth.com
Date : 16/07/14 - 18:49 (GMTDT)
To : amps at 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 E. Randall AB9GO
Network Architect
UC Health
Randy.Randall at UCHealth.com
Phone 513-585-7146
Fax 513-585-7159

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of John Lyles
Sent: Tuesday, July 15, 2014 1:14 AM
To: amps at 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.

Amps mailing list
Amps at contesting.com


Confidentiality Notice: This e-mail message, including any attachments, is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies of the original message.

Amps mailing list
Amps at contesting.com

More information about the Amps mailing list