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[Amps] Gassy tubes

To: <craxd1@verizon.net>, <amps@contesting.com>
Subject: [Amps] Gassy tubes
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 21 Jul 2006 13:58:36 -0400
List-post: <mailto:amps@contesting.com>
> The way I see it, it's the same, as that connection is in 
> series with any anode fuse or glitch resistor. The way I 
> also see it, if a tube starts to arc, it's gassy and needs 
> to be replaced.

That's absolutely incorrect for large power grid tubes!

Large  power grid tubes often have noticeable seal leakage. 
It comes from the less than perfect bond between the leads 
and the envelope. A perfectly good tube that sits unused, in 
particular if it has been stored unused  in a humid 
environment, can have seal leakage.

Also elements inside the tube can outgas. The most common 
gettering material is zirconium. Zirconium is used on the 
outside of graphite or molybdenum anodes in 3-500Z and other 
tubes. It is the dull gray powdery or grainy texture coating 
you see on the anode surface of 3-500Z, 811A, and 572B 
anodes.
Zirconium getters best at about 1000 degrees C, this is why 
large metal anode transmitting tubes like the 4-400A, 
4-1000A, and 3-500Z must be operated with a dull red to red 
anode color. Zirconium also releases some gasses and absorbs 
other gasses at various temperatures. The varying 
temperature across the length of the anode (and as the anode 
heats and cools) allows the gettering agent to absorb a wide 
variety of gasses.

While coming up to temperature, the zirconium can actually 
outgas and cause a momentary flashover. The arc itself can 
getter the tube!

It would be foolish to throw a tube away because it arcs one 
time or even occasionally. The only time that should be done 
is if the tube repeatedly faults.

If I threw tubes that arced on initial power up out, Eimac 
would have gotten dozens of perfectly good 3CX3000 and 
3CX1200 tubes back for no reason. Repetitious arcs are 
another story.

73 Tom


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