Ian Wrote:
> Surface chemistry was part of my degree course, and while earning my own
> PhD in the kinetics of low-pressure gas reactions, I worked alongside
> people who were studying gas/metal reactions at similar pressures to those
> in so-called "vacuum" tubes. I have many times *witnessed* a vacuum being
> cleaned up by gettering action, and have *seen* how fast the pressure
> reading falls.
Textbooks covering design and manufacture of vacuum tubs
specifically describe an arc as one of the means of gettering a tube.
One doesn't need a PhD obtained in study of low pressure gas
reactions to have that information, he simply needs to read readily
available engineering texts on tube manufacturing.
> All vacuum tubes are heated way above the highest operating temperature
> while being pumped down, to drive off as much as possible of the gas that
> is always attached to metals, glass and ceramics. Most of the gas comes
> off quickly and easily at these temperatures, but there is always a
> residue that takes a long time to diffuse to the surface. Therefore the
> manufacturers provide a "getter" as a kind of passive vacuum pump that
> will continue to operate for years after the tube has been sealed and
> shipped.
Not only that, there is gas ingress through seals. Few, if any, seals are
perfect.
> The getter works like flypaper. Gas molecules that emerge from the
> insides of the tube structure will bounce around until they hit the
> chemically active getter surface. They don't always stick, because some
> molecular-scale 'sites' on the getter surface are more active than others,
> but eventually they will hit an active site and be permanently removed
> from the vacuum space.
Which is why gettering agents intentionally added to a tube must
be operated in specified temperature ranges. The same material
will give up certain gasses at some temperatures while absorbing
others, and can re-absorb the gas it gave up when operated at a
different temperature.
That's why 3-500Z's and similar tubes actually gas up and fail when
operated too "cold" for extended periods.
> This is a purely chemical process. Some getters operate cold, eg the
> silvery-looking barium films used in small receiving tubes. Others
> require heat to activate the metal surface, and thus increase the
> sticking probability.
>
> In medium-sized glass transmitting tubes, the getter is the anode
> structure itself (or the outside part) and this does require heating to
> keep it chemically active. I'm not sure where the getter is in metal-
> ceramic tubes.
Generally on the heater or cathode structure. In many tubes it
serves dual functions of supporting the cathode and gettering the
tube. The reason the gettering agent is in the cathode area is
because the anode in an external anode tube never gets hot
enough to getter the tube.
Poor material or material contamination in the heat dam of the
8877, that also serves as a gettering device, was the root cause of
the many 8877 failures in the 80's.
73, Tom W8JI
w8ji@contesting.com
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