Ian,
That was a good description, see the rest below;
*********** REPLY SEPARATOR ***********
On 7/28/06 at 8:29 PM Ian White GM3SEK wrote:
>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.
Exactly.
>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.
Agreed.
>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.
Agreed. They bake both the anode, and the glass itself so they will liberate
all the gas they
can get them to, then evacuate the tube.
>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.)
Agreed.
>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.
Exactly!
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)
I had read earlier that a vacuum could be pulled up to 10 atmospheres. That's
not
saying they do that on the tube, just that they can do it.
>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.
Several types of diodes, thyratrons, and regulators use gas to operate
correctly.
They use the gas to drop the plate resistance, improve regulation, and
efficiency.
This gas is generally an inert gas.
>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.
Agreed.
>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.
Agreed. A getter can also hold just so much gas. Once that limit is reached,
there's no hope for the tube if still gassy. Getters like zirconium liberate
hydrogen gas when heated above 300 deg C, and start working around 700-800
deg C to absorb O2, CO2, CO, etc. The optimum temperature is said to be 1400
deg C. Tantalum though does not have the problem of emitting hydrogen gas
and its optimum temperature is around 1000 deg C if I recall.
>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 should be noted though that if a tube is gassy enough to where a large arc
can
occur, it can be sustained until the filter caps are discharged even though the
power
supplying them is dead (fuse opens or breaker trips). This is a bad arc that
can do
damage. Generally a tube has lost its evacuation, or pretty close to it is this
situation.
In this case, the B+ supply can be damaged because the arc is a dead short or
close
to it. If the arc was to hit the cathode, which can happen, it can destroy the
cathode
bias circuit if any. It could possibly damage a heater transformer also.
>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.
Agreed.
>
>
>
>--
>73 from Ian GM3SEK
>http://www.ifwtech.co.uk/g3sek
>
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Best,
Will
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