Richard W. Ehrhorn wrote:
>
>Pardon, Jon, but sound waves don't propagate in the absence of some medium,
>such as air or water. A big "bang" in a vacuum is essentially inaudible;
>however the multi-hundred-amp current surge, or the voltage drop it creates
>across a plate choke, for example, can and often does result in a very
>audible BANG as the choke explodes.
>
>-----Original Message-----
>From: Jon Ogden [SMTP:jono@enteract.com]
>
>What you likely had happen was an internal arc-over inside the tube.
>Some of the gas molecules inside the plates worked their way to the
>surface of the plates and were released into the envelope. The bang
>sound you heard was likely these gases burning up.
>
The gases do not "burn up" - you're thinking atmospheric chemistry, not
vacuum chemistry.
All the metal and ceramic materials used to make the tube are processed
to be almost gas-free, but this can never be perfect. Most of the gas is
removed at the factory by baking the tube while still pumping the
vacuum. However, it is much harder to remove the gas trapped inside the
bulk metal (remember, we're NOT talking about big bubbles here, just a
few atoms here and there caught up in the crystal lattice of the metal).
Therefore there is a continued risk of gas release in operation, because
some parts of the tube are going to get hot, which encourages migration
of the trapped gas atoms. That's why the manufacturers chemically
activate some of the metal surfaces, to make them act as a passive "fly-
paper". Stray gas molecules that collide with these metal surfaces have
a high probability of forming a chemical bond and being trapped there.
This can carry on throughout the lifetime of the tube, as small
concentrations of gas diffuse from inside the metal to the surface. So
long as the gas is released very slowly and gradually, the "fly-paper
effect" can keep up with it, and it has no effect on tube operation.
This is happening all the time in a so-called "vacuum" tube, especially
when it's hot, and is completely normal.
However, it is statistically possible to have a sudden larger release of
gas - still only a few atoms - and for a very short time this may be too
much for the chemical reaction to keep up. The result will be a brief
"blip" of pressure until the chemical reaction has time to mop it up. If
the pressure peaks into the arcing range for only a microsecond, you'll
get an arc.
Remember also that the gas release doesn't even have to be enough to
carry an arc - it only needs to be enough to trigger it. After that, the
arc is maintained by metal ions ripped off the surfaces. The voltage
across the arc will probably drop to only a few tens of volts, and the
current can head for several hundred amps if there's nothing in the
external circuit to stop it.
When the arc does stop, the "fly-paper" effect can clean up everything
in a few milliseconds, leaving a good vacuum once again.
(If this is hard to understand, it's probably because you're thinking of
pressures many orders of magnitude higher than exist inside a good
vacuum tube. Arcs in "rarefied atmospheres" correspond to tubes that
have an envelope leak; while interesting, this is NOT relevant to what
I've just been talking about.)
As Dick says, there is little or nothing to hear from a vacuum arc
inside the tube. Nothing at all from the arc itself, but possibly
something from the physical recoil of the affected parts. Any loud bang
would have to come from the circuit outside - but there are plenty of
opportunities for that!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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