[R-390] Gettering old tubes

[Lloyd Godsey]

Ever tried putting one in a microwave? (GRINNNNNN)
Thanx
Lloyd Godsey  KK7IZ
kk7iz at cox.net
480-620-7145
www.kk7iz-parts.com
www.lloydsdipsydumpster.com/

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From: "2002tii" <bmw2002tii at nerdshack.com>
Sent: Wednesday, March 02, 2011 12:40 PM
To: <R-390 at mailman.qth.net>
Subject: Re: [R-390] Gettering old tubes

> 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
>
>
> Copyright 2011.  Not for redistribution.
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