2 wrote:
>
>
>>2 wrote:
>>>>High enough to initiate a plate supply short through the tube?
>>>
>>>If the anode-grid path shorted, there would be an arc-mark on the grid.
>>>I have not seen one in a grid-fil shorted tube -- nor have I found a
>>>shorted tube that was gassy. I doubt that Mr. Rauch's disappearing gas
>>>theory is possible without direct intervention from the Fairy Godmother.
>>>
>>Since you persistently refuse to understand how a getter works, or to
>>accept that arcs can happen in tubes that appear perfectly good, you're
>>unlikely to find much evidence to change your mind.
>>
>Ian -- Please explain how a gassy, shorted 3-500Z is gettered between its
>removal from an amplifier and its being tested for gas with a high-pot a
>minute or so later?
We keep going around this argument in cycles of a few months; and every
time you act as if nobody had ever explained all this before. I am only
explaining it this time for the sake of any new arrivals.
The materials of which tubes are made - especially the metals - contain
trace amounts of trapped gases. When the tube is manufactured, it is
induction-heated to a very high temperature (way above the normal
operating temperature) to drive out as much as possible of these gases
while the tube is still connected to the vacuum pump. When the tube
cools down, it is sealed off.
But more gas continues to slowly evolve into the "vacuum" space. This is
a perfectly normal process, even in a tube with perfect vacuum seals
(leakage is a totally separate problem). To maintain the quality of the
vacuum throughout the life of the tube, the manufacturer creates a
specially activated metal surface inside, called a "getter". The getter
will react chemically with any gas atoms that strike it, and will keep
them trapped on the surface. It's a kind of passive, maintenance-free
vacuum pump.
There are two types of getter. In receiving tubes and small glass
transmitting tubes the getter is the silvery film of barium metal that
you can see through the glass. However, barium can only operate at low
temperatures - at high temperatures, it would evaporate and become part
of the gas problem.
In transmitting tubes, which have top operate at high temperatures, the
getter is some other chemically active metal that is less volatile, but
need to be at a high temperature in order to operate at its best. In
ceramic-metal tubes, the getter is generally mounted at the top of the
cathode pillar, which is about the hottest point inside the tube. In
glass-metal tubes like the 3-500Z, the getter is the dull grey zirconium
metal on the outside of the anode, and it operates best when the tube is
running hot.
When the tube is hot, there are two competing processes going on. On the
one hand, very small amounts of gas are still being evolved. On the
other hand, the getter is mopping it up... but that can't happen until
those gas atoms have bounced around inside the tube until they actually
strike the getter. Not every impact on the getter surface will hit a
chemically active site that will react with the gas atom and trap it, so
the trapping process takes time.
Also, the evolution of gas out of a piece of metal is not a steady
process. The gas tends to come out in pulses of several atoms at a time.
Small pulses are common; larger pulses are rarer; and very large pulses
are rarer still.
If one of these very large pulses of gas reaches the surface and enters
the space inside the anode, then as I said, it takes a little time to
diffuse around to where the getter can mop it up. In the meantime, there
is a temporary higher pressure inside the tube - and it only takes
microseconds (or less) for the tube to arc.
Arcs in high-voltage transmitting tubes are a very well-known
phenomenon, almost as old as radio itself. In Eimac's words [1], "An arc
is a self-sustained discharge of electricity, between electrodes in a
vacuum environment... The arc supports large currents by providing its
own mechanism of electron emission..."
Arcs can happen at any time in the life of the tube, but notably in its
early life while gas is till being evolved, and after the tube has been
stored for a long time (cold, and therefore with very little getter
function). You may never encounter one; but neither should you be
surprised if you do.
When an arc happens, the current through the tube is limited mostly by
the external power supply... until some other circuit component stops
it. Hopefully this will be a fuse or some other protective device, but
unless the current is limited by a "glitch resistor" the surge can do a
lot of damage.
Therefore Eimac recommends that precautions are taken to limit the
amount of energy dumped into the tube, and to limit the current to maybe
40 amps [1]. If these precautions are taken, the tube itself may not be
damaged.
If the arc is extinguished quickly and not too much energy is dumped
into the tube, the tube can recover completely. There may not be much
visible evidence that the arc ever occurred (depending on the tube
construction). If the tube was hot, the getter can collect the gas
within a few seconds.
So it's all a matter of time-scales. An arc can happen faster than the
getter can handle the gas release - but the getter can do its job faster
than anyone can possibly pull the tube out of the amplifier to test it.
[1] FAULT PROTECTION. Varian, EIMAC Division, Application Bulletin #17,
January 1987 (see www.ifwtech.co.uk/g3sek/misc/bull17.pdf)
Having written far more than I'd expected, in order to tell the whole
story, I am never going to write all this again. Let's take comments and
corrections this time around, and I'll park it on my web site ready for
the next time.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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