[AMPS] Rocky Point effects

[Ian White, G3SEK]

I had written:
>>The reaction is chemical. No electrical power is required.
>>

Rich Measures replied:
>A maximal load of codswallop.  The reaction requires heat.  
>
>enough

Oh no, not enough yet...

I respect to your practical experience of HF amps, but I will not defer
to your ignorance.

Many contributors to this list have qualifications we don't know about,
because they are not relevant to the subject of power amplifiers.
However, if the discussion wanders far enough, some of those
qualifications do become relevant.

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.

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.

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.

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.  I don't have access to exact quotes from any
manufacturer's bulletins on this aspect of tube construction and
operation, but would be interested to see any.

To repeat the earlier explanation: gettering action is reasonably fast
but it still has to wait for all the gas atoms to bounce randomly around
inside the tube, hit the activated surface and stick there. This takes
time, so the getter can easily be "overrun" by a sudden release of gas
that has been trapped beneath a metal surface. That can trigger an arc
in a tiny fraction of a second, long before the getter can react; but it
will still catch up later.


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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