[R-390] Gettering old tubes

[2002tii]

Tisha wrote:

>But I recall seeing in some of those
>1960's vintage tube videos (someone posted a link to them a year or so ago)
>that they would getter a tube after manufacturing by heating it via an
>induction furnace.
>
>There really is not that much to an induction heating device. Maybe someone
>wants to try to make one and see if they can make the getter to reactivate
>on old tubes. There is a limited amount of barium/ zirconium/ sodium (name
>your own metallic gettering materials here). As long as the metal has not
>entirely turned into an oxide it still has some gettering potential and may
>be able to scavenge some gas molecules out of the tube.

Induction heating is how they fire the getter in the first place, not 
how one would rejuvinate a tube.  Once all of the volatile metals 
have boiled off the getter and deposited on the envelope -- and this 
is done to completion during manufacture -- there is no point to 
re-firing the getter.  (The getters in some large transmitting tubes 
are not fired to completion at the time of manufacture, but this does 
not affect the discussion of receiving tubes.)

Gettering is most effective while the vaporized metal is in transit 
from the getter to the envelope -- when it is very hot and very 
reactive.  The residual "gettering" that happens during a tube's life 
is very, very slow because the flashing is in the much less reactive 
solid state and the tube envelope would melt before the flashing 
reached a temperature at which it would be really effective.

To de-gas a tube post-manufacture, you need to heat the getter 
flashing (the deposit on the tube envelope) to the point that it 
reacts with any residual gas molecules in the tube.  This means that 
you need to get the envelope to operating temperature (or preferably, 
well beyond) and keep it there long enough for all of the gas 
molecules to react.  Since (i) the gas density is very low, and (ii) 
each molecule needs to hit the flashing many times before it reacts, 
the time required for the getter flash to clear the gas from 
decades-old tubes (assuming that there is enough flashing present to 
do so) is measured in months or years, not minutes.

Thus, "residual gettering" can keep a tube relatively gas-free if it 
is in regular operation and the gas ingress is slow, but it is not a 
very promising way to react decades of gas ingress from a tube that 
has not been operating.  If you want to try, put the tubes into an 
oven (you want to heat the envelope evenly so as not to produce 
thermal stress), heat them just short of the point where the glass 
collapses, and leave them there for a few months.

Best regards,

Don


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