[Amps] preserving tubes

[Roger (K8RI)]

On 10/4/2012 4:16 AM, ian at 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 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)