[Amps] 3CX800A7 Spares - cook them periodically?

John Lyles jtml at losalamos.com
Tue Jul 15 01:13:40 EDT 2014

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 


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