Will Matney wrote:
>
>Thanks as I didn't understand what you meant. I take it when this
>coating is heated, it traps the gas molocules by the reaction.
>Receiving tube getters absorbs the gas during the flash from a chemical
>reaction.
While the barium metal getter is being deposited onto the glass (after
the tube has been sealed), some of the remaining gas molecules are
buried under the metal. In addition the process leaves a chemically
reactive surface that acts like a fly-paper to trap any further gas
molecules that emerge in the longer term.
Transmitting tubes rely only on the fly-paper effect (adsorption).
>That's why the silvery coating turns white when more air is introduced
>inside the envelope.
The white material is barium oxide, so:
>Anytime though you see a receiving tube which the silvering is white,
>you might as well can it then as it's full of air. Generally, you'll
>find a crack in the glass around one of the pins or under some anode
>caps. Some slightly gass
> y tubes, this silvering will discolor around the edges where it's
>faint around the outside and silver in the middle. These generally will
>make the gassy light go off on a tester but I have seen some that
>didn't. I've replaced them anyhow and cured the problem.
>
That's about right... but this is a a leaky tube. A leak is never
recoverable. The occasional gas arcs that we see in transmitting tubes,
which sometimes are recoverable, are NOT due to gross leaks.
--
73 from Ian G/GM3SEK
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