[Boatanchors] Gassy Tubes

[Bill Cromwell]

Hi Garey,

Thanks for that explanation. I knew it wasn't just air but other 
contamination and that 'other' was really nebulous for me. Sometimes I 
have run tubes that display this 'defect' for a little while and it goes 
away - permanently. Other times, not. I had a 6C4 that did that when I 
pulled it out of a terribly drifty oscillator. That Hammarlund receiver 
was virtually useless. The "leakage" wasn't very much..just a slight 
movement upscale. I replaced it with one with "no leakage" on the tester 
and the drift went away.

73,

Bill  KU8H

On 02/26/2014 04:39 PM, Garey Barrell wrote:
> Ron -
>
> I usually skip over the messages with a Subject of 'Digest', but saw 
> this one.
>
> I think what you are seeing is what is called 'Grid Emission', which 
> is also called 'Gassy', because the symptoms are similar. Grid 
> Emission, as I understand it, is caused by particles flaking off the 
> of the Cathode and getting stuck to the Grid.  As the tube heats up, 
> these particles begin to emit electrons and cause the Grid to develop 
> a more positive voltage.  Typically, in a 5 - 20 minute time the Grid 
> being more positive causes the tube to draw more current.  The 12BA6 
> seems to be especially prone to this effect in Drake receiver IFs, 
> causing the famous 'S-Meter drift' as the radio warms up for 10- 20 
> minutes or so.
>
> Obviously, if a tube has two sections, one Grid may become 
> contaminated without affecting the other, displaying the results you 
> see.  If a tube is really 'gassy', also called 'leaky', it indicates 
> that some molecules of air have entered the vacuum, usually via the 
> base seals.
>
> 73, Garey - K4OAH
> Glen Allen, VA
>
> Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
> and TR-4/C Service Supplement CDs
> <www.k4oah.com>
>