[Amps] Outgassing tubes.

[Will Matney]

Jim,

Tubes sure have a shelf life or I always thought they did. The seals to keep air out of the vacuum, I think, can leak even though a very miunute amount. Over time though, this keeps building. The major cause I think is the constant expansion and contraction of the glass or steel the tubes envelope is made of from the heat or cold where they're stored. These seals are made from a different material or have none at all except glass to steel or steel to ceramic. The problem is that one material will expand and contract more than another at the same temperature. This I think, can cause this small amount of leakage. When you put a tube in service where the heat is on and off a lot with a larger variance in temperatures, I think accelerates the process. That's my thoughts on this though as I've never read any engineering information saying this cant happen.

Best,

Will


*********** REPLY SEPARATOR  ***********

On 10/2/05 at 1:31 PM Jim Reid wrote:

>Will wrote,  in part:
>
>> Plus with age, a small amount of atmosphere can seep in, 
>> especially on glass tubes and where the ceramic to steel 
>> meets on a steel tube. Though on any old tube that finally 
>> does start to do this more, one should think about replacing 
>> them sooner or later. They could end up damaging the power 
>> supply.
>
>Perhaps that is what happened to my little AL-811H.  
>My tubes were "only" about 10 or 11 years old, however.
>They were used very little over that time;  maybe they gassed
>up slowly from the atmosphere.  Would that account for
>the low transconductance shown,  see below?
>
>I sent the
>tubes over to KH6CDO in Honolulu.  He measured three of them
>at 800 umoh. Spec is supposed to be 900,  I think he said.  The
>fourth tube could not be tested.  The "glued-on" anode connector
>atop that bottle had come "unstuck" when I removed the plate
>cap connector.  Frank tried to file off some of the glue residue
>to get it conductive from the anode for the test;  he couldn't remove
>any of it;  I wonder if that tube ever participated in the amp,  hi.
>Or,  maybe it was the tube which arced?
>
>Anyway,  as Will suggests above,  it was the power supply
>where failure is now found.  Will see what it is in a few
>days or so.
>
>73,  Jim W6KPI
>
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