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Re: [Amps] preserving tubes

To: "Roger (K8RI)" <k8ri@rogerhalstead.com>, <amps@contesting.com>
Subject: Re: [Amps] preserving tubes
From: Bill Fuqua <wlfuqu00@uky.edu>
Date: Fri, 5 Oct 2012 21:07:46 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
How thick are the walls on the cans?
Most ordinary cans will crush with a vacuum inside.
That is a differential pressure of 15 PSI.

73
Bill wa4lav
At 07:49 PM 10/4/2012 -0400, 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 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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