[Amps] tube flatulence - gettering

[R.Measures]

On Nov 19, 2004, at 9:42 AM, jsb at digistar.com wrote:

>
> Question:
>
> I was given a homebrew single 3-400Z amplifier which works but I 
> noticed
> with ZSAC the top of the glass envelope has a very faint blue hue when
> looking across the top of the tube envelope.  When looking down at the 
> top
> of the tube I don't see this color, only when looking across the top -
> keying CW does not pulsate the color, the blue hue is the same 
> regardless
> of output power.  I have a couple other old junk clunker 3-400Zs that 
> do
> the same.
>
> Is this normal or is it an indication of gas?
>
Hello, Jason  --  Blue on the glass is not gas.  It is caused by 
high-speed electrons fluorescencing the fluxing agents in the glass 
recipe.  If the blue light is in the region between the anode and the 
grounded grid, that's atmospheric oxygen and nitrogen getting through a 
bad seal.  Normally, glass fluorescence occurs in areas where the 
electron stream from the hot filament misses the anode and hits the 
glass.  In time, the struck area of the glass turns brownish.

> I have some 8163 Amperex 3-400Z in a Drake L4 that do not exhibit this
> coloration, ever.

Amperex 8163s have a solid, machined graphite anode-cooler that has no 
gaps where the filament-glow can be seen from above. OTOH, Eimac 8163s 
have a sheet-metal anode cooler with small gaps between the fins that 
allows one to see the filament-glow.  My guess is that these 
sight-paths to the filament is where the glass is fluorescing.

>  The Eimac 3-400Z tubes do, even after having been used
> to produce enough output RF to turn the anode reddish-orange for 10-15
> seconds.  Or does the getter not getter that quickly...

--  I have not seen any gettering take place in any gassy 8163/3-400Z 
or 3-500Z.  Neither Amperex or Eimac mentions such on their tech. spec 
sheets.  
>
>
>
> thanks
>
> 73 Jason N1SU
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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org