[Milsurplus] Tube storage?

[J. Forster]

Scott Johnson wrote:

> Tubes don't generally become gassy do to in diffusion of gases, that become
> gassy because the elements within the tube, through either thermal abuse, or
> poor manufacturing, liberate gases.  The poor manufacturing some of you may
> be able to relate to with regards to some Chinese tubes.  If the gas species
> are reactive, the getter may clean them up a bit, but if not, the tube will
> be gassy.

I'm unconvinced. There are several gettering mechanisms:

Chemical Combination...  with a reactive alkaline metal (Calcium or Barium).
This will take out Oxygen.

Sputtering:.. is essentially the plastering down of inert gas molecules my
depositing metal film over them. This happens in Magnetrons and similar tubes.

Ionization then Chemical Combination as above. This happens with Nitrogen and
similar gases, I think. Here's why:

A couple of years ago, I bought a number of spare valve kits for my WS 19. I
then tested all the tubes and some were gassy. Having little to loose, I applied
filament (with a tube tester) to some of them, figuring to clean them up by
gettering. Nothing happened, so I did some reading about getters.

It them occurred to me that perhaps the flow of electrons from cathode to plate
would ionize the gas and make it reactive and hence the getter might scavenge
it. So I ran a couple of tubes for several days with rated plate current and
occasionally checked the leakage. It worked nicely, BUT the emission went way
down. I guessed that the cathodes had become poisoned by the positive ions.
Cathodes regenerate themselves from within but it's slow, so I tried another
gassy tube, but ran it with full HV but biased so the plate current was about
20% of the rating.

Complete success. I did the same to over 100 tubes, and almost all were degassed
and did not loose emission.

>  In diffusion of Helium is most pronounced in ceramic/metal tubes,
> because the Helium has a much easier time making it through the alumina that
> it does flint glass or borosilicate glass.  At any rate, the diffusion rate
> for Helium, although impressive, doesn't affect the total pressure much in a
> common glass receiving tube, unless a glass to metal seal has been damaged,
> or there was an improper tip-off.

I've seen it destroy glass thermoses overnight and a PMT in a Helium atmosphere
in days. Some newer glasses (such as are used for HeNe lasers) have a very low
diffusion rate, but certainly not all.

-John

> Scott