[Boatanchors] Gassy Tubes

[Garey Barrell]

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>

bonddaleena at aol.com wrote:
> Hi Bill, thanks for the reply.... The tubes are marked "Sonar", the maker of the radio.
> In any case, I was just curious 'what' the Grid Leakage part of the tube tester was checking.
> There is not a hard failure in 1/2 of each section, of each tube. The portion of the tubes that 'fails', starts indicating 'leakage' as soon as the tube starts warming up. The leakage indication then fairly quickly goes to full scale (bad). As I mentioned, the problem is with the tubes. I just received some replacements that check perfect in both sections.
>
> If the tubes were 'gassy',  BOTH sections should have checked bad, correct?
>
> So, my conclusion is that the tester is applying a bias to the grid to see if it is capable of blocking the electrons from the plate.This would explain why the 'leakage' increases as the tube come up to temperature. There might (??) be internal damage to the grid structure of the 1/2 of the tube that checks bad.
> Probably due to heat/stress as you mentioned, because these tubes run very hot!
>
> The radio works with the 'bad' tubes, but it is much more sensitive and has much better audio with the new tubes. I am constantly amazed how available these 'odd' tubes are on eBay, and how cheap, too.
>
> I know that some of the R-390A experts will hand pick tubes to find the ones that deliver the ultimate sensitivity. They are looking fore every db possible.
>
> Thanks again,
>
> ron
> N4UE
>
>