[Amps] 3CX800A7 Spares - cook them periodically?
Randy.Randall at UCHealth.com
Wed Jul 16 13:49:43 EDT 2014
"Warranty requires that they be stored with external ion pumps energized at least every 60 days."
What is an external ion pump?
Randy E. Randall AB9GO
Randy.Randall at UCHealth.com
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.
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