On 10/4/2012 7:49 PM, Roger (K8RI) wrote:
On 10/4/2012 4:16 AM, ian@ifwtech.co.uk wrote:
If a tube has leakage to air, it is doomed anyhow, and storing it in a
vacuum can only delay the inevitable. Leakage will start right in
again when the tube is put into service.
True, but:
The military used to get 4CX150s
That should have been 4X150, not 4CX. Both the 4X150 and PL-172 had
metal to glass seals and tended to get gassy just setting on the shelf.
The 8295A version of the PL172 used a ceramic to metal seal and were not
noted for having the tendency to go gassy.
73
Roger (K8RI)
in vacuum packed cans.
Lets say a tube with metal to glass seals like the 4CX150, or PL172 has
an expected vacuum life of 15 years. If you have a bunch of NOS (in the
cans) that are 30-40 years old when you open the can the tube is
starting its life in the real world with an expected life of 15 years,
while one that was never in a can is most likely gassy without ever
having been used. OTOH that gassy tube is highly unlikely to be at
atmospheric pressure inside, so if you place it in a hard vacuum the
difference between inside and out is not great and the tube is unlikely
to ever return to a high vacuum. At least not within a practical time.
IE the difference between 1 X 10^-5 and 1 X 10 ^-7 Toor or atmospheres
is miniscule compared to 15 psi (atmospheric pressure) and 1 X 10^-5 Toor.
Too bad we didn't vacuum pack all those spare PL172s when they were new.
They'd be worth a small fortune now.
73
Roger (K8RI)
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