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Re: [Amps] Arc distance

To: amps@contesting.com
Subject: Re: [Amps] Arc distance
From: Ian White GM3SEK <gm3sek@ifwtech.co.uk>
Reply-to: Ian White GM3SEK <gm3sek@ifwtech.co.uk>
Date: Fri, 28 Jul 2006 20:29:37 +0100
List-post: <mailto:amps@contesting.com>
Tom W8JI wrote:
>> Another thought. Could impurities introduced during
>> manufacturing of the
>> different metals used in tube construction be associated
>> with tube arcing?
>
>There are multiple causes of flashovers. The most common are
>unwanted sharp points, gas, and/or debris. Eimac accepts the
>fact even perfectly good new tubes do that on occasion.

There are two long-term sources of gas in tubes. One is a leak to 
atmosphere, and the other is "outgassing" of structural materials.

Leaks can be large or small, but they are always a one-way trip, with 
only one end. Outgassing is more complex, and not necessarily fatal.

Impurities in metals are the main source of outgassing. When the metals 
are refined from ores, they always have built-in impurities. Materials 
for use in vacuum tubes are extensively refined, by a combination of 
chemical processing and heating in a vacuum furnace... but still some 
impurities remain in the atomic lattice structures.

There are also gas molecules chemically bound onto the exposed surfaces 
of metals, ceramic and glass. These are largely removed in the later 
stages of vacuum pumping, by heating the whole tube far above its normal 
operating temperature while continuing to pump. The better the 
materials, and the longer the time for which the tube is pumped, the 
better the vacuum will be... but they can't pump and bake forever, so 
eventually the tube has to be sealed off.

At that point, the getter is activated. The getter is a chemically 
active material that has been placed inside the tube to act as a kind of 
'fly paper' for stray gas molecules that might appear in the future. The 
getter in receiving tubes (and small glass transmitting tubes like the 
807) is typically barium metal, which had been left inside the tube in a 
little tray. On the production line, an induction coil heats up the 
tray, evaporating the barium onto the glass as a slivery-looking film 
with a highly reactive surface. This will continue to mop up stray gas 
molecules for the life of the tube. (If that film has turned white, it 
means there has been a gross air leak - the barium metal has turned to 
oxide, and the tube is done for.)

Transmitting tubes are different because they run much hotter and 
operate at higher voltages, and a volatile metal like barium would 
evaporate from where it had been deposited, and then condense in all the 
wrong places. Instead, the getter materials are typically zirconium or 
tantalum, which are non-volatile but *need* to run hot in order to 
operate effectively. That is why the main getter in a glass tube like a 
3-500Z is located on the metal anode (the grey surface finish is the 
zirconium getter) and in a metal/ceramic tube it is located on the 
heater (the next hottest location). Most transmitting tubes actually 
have multiple getters to mop up the various kinds of gas molecules, 
using different materials in locations at different temperatures.

Immediately after manufacture, the vacuum will probably be about 10^-8 
mmHg, which is really quite good for a routine production-line process, 
but no great shakes by the standards of a vacuum lab. At this standard 
of vacuum, a typical tube may contain anything between a million and a 
billion gas molecules! (PV=nRT... work it out)

Most of the time, a "vacuum" tube operates perfectly well in spite of 
sharing the space with very large numbers of gas molecules. But 
sometimes you need to remember that the tube is also a reaction vessel 
for some complex low-pressure chemistry.

Coming back to impurities... immediately after manufacture, the vacuum 
is probably pretty good because all the surface impurities were flashed 
off. However, impurities that were trapped deeper inside the metal can 
continue to diffuse to the surface over the lifetime of the tube, and 
can be released as gas into the "vacuum" space causing a small increase 
in pressure.

If the getter is active, it will mop up the impurities within typically 
a few seconds (determined by the time it takes for the molecules to 
bounce around until they strike the getter surface, and by the 
probability that a molecule will hit a chemically active spot that can 
form a strong enough bond to make it stick). But a few seconds is far 
too slow to prevent an arc, which can strike within microseconds if all 
the other conditions are right.

This explains why tubes can arc for no apparent reason, but if you try 
again a short time later, the tube goes back to normal as if nothing had 
happened. (Obviously this requires an amplifier with good HV surge 
limiting and fast shutdown protection. If the arc is allowed to persist 
at high current, it will damage both the tube and the amp.)

It also explains why transmitting tubes generally need to be pre-heated 
after a long period out of use. The process of slow diffusion to the 
surface of the materials means that gas will probably have accumulated, 
and the getter needs some time at a high temperature in order to do its 
job.



-- 
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek

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