[R-390] Gettering old tubes

[Robert Moses]

I have this gut feeling that this is not something that you would likely 
do more than once! <wink> The high current surges that would be produced 
would tend to melt the screens / grids of the tubes unless the tube had 
a metal envelope. If the envelope was glass then the spot of getter on 
the inside of the tube would tend to get very hot and create thermal 
stresses in the glass envelope that could cause cracking.

Lloyd Godsey wrote:

>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/
>
>--------------------------------------------------
>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
>
>  
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>>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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