On Jul 12, 2005, at 7:25 AM, Ian White GM3SEK wrote:
> Larry Carman wrote:
>
>> What causes the gas release that produces an internal arc?
>>
> Here's the story (it's a bit long, because I didn't have time to write
> a
> short one)...
>
> A so-called "vacuum" tube is actually a very complex piece of
> chemistry.
>
> For a start, the "vacuum" space is far from empty - typically it
> contains something between a million and a billion free gas atoms
> (depending on the quality of the vacuum and the size of the tube). This
> seems like a lot of atoms, but it's actually close enough to a true
> vacuum to ensure that the main current carriers are the negative
> electrons coming from the heated cathode.
agreed
>
> That can change dramatically if more gas atoms are injected into the
> "vacuum" space. Then the main current carriers will be positive ions
> which travel the opposite way to the electrons, and have a lot more
> mass and energy. This is a so-called "vacuum arc".
So electrons don't carry the current in such an arc?
> All the metal, glass and ceramic materials from which tubes are made
> contain small quantities of trapped gases, mostly atmospheric nitrogen
> and oxygen. Even though the manufacturers try to select high-purity
> materials, and de-gas the tubes during manufacture by heating them way
> above normal operating temperatures while still pumping on them,
> de-gassing can never be complete. At any time later in the tube's
> operational life, small quantities of gas can diffuse to an inside
> surface and be released into the "vacuum" space. This applies
> particularly during the early part of the tube's operational life.
>
okay, but the quantities of trapped gas in all but graphite are quite
small.
> When B+ is present, a sudden gas release may cause an arc, especially
> if
> the metal surface has grown "whiskers" that increase the potential
> gradient and encourage the arc to strike. If an arc begins, it can also
> tear off metal ions that will then sustain the arc... until you do
> something to stop it.
In a vacuum capacitor, the Cu whiskers burn off with no fireworks and
barely any noise.
>
> There are examples of arc marks on a GS35B triode at:
> www.ifwtech.co.uk/g3sek/misc/gs35a.jpg (anode)
> www.ifwtech.co.uk/g3sek/misc/gs35g.jpg (grid/cathode)
> This particular tube had suffered multiple severe arcs because the
> owner
> had tried to run it at too high a voltage, about 4.5-5kV.
Running a heater-type tube at excessive anode potentials can dislodge
bits of the barium-strontium emissive surface, which is conductive and
can cause an anode arc.
> (The photos are by PA3CSG, who had supplied the tube to its
> over-ambitious user.)
>
> A minor gas release problem will usually clear itself automatically,
> because the gas atoms are very quickly collected by the "getter".
We went through this during the Grate Parasitics Debate. When I tested
kaput 3-500Z tubes that had produced a big bang arc, I found reduced
filament-grid breakdown potential - or even a fil-grid short - and a
vacuum that tested under 10uA of leakage at 8000v. Mr. Tom Rauch
argued that the gas atoms which caused the arc had been gettered by the
coating on the anode even though the kaput tube had not been re-tried
in the amplifier.
> This is a deposit of chemically active metal,... ... In larger
> transmitting tubes the
> getter is usually something like zirconium, which needs a higher
> temperature to function correctly.
Does zirconium gettering take place at room temperature? Do 3-500Zs
utilize a zirconium getter?
> ...
Rich Measures, 805.386.3734, AG6K, www.somis.org
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