Peter Chadwick wrote:
>
>Just to add to the debate, I haven't seen anyone come up with a boiling
>point temperature for gold in a vacuum - if it's lower than at
>atmospheric pressure, then the arguments about grid temperature take on
>a new dimension.
>
The normal definition of "boiling point" is the temperature at which the
vapor pressure of the liquid or solid equals the surrounding pressure -
which is rather meaningless under high-vacuum conditions.
What matters is the absolute vapour pressure of the gold - or any other
contaminant of a totally "clean" vacuum - and its effect on the
performance of the tube.
I used to know enough physical chemistry to be able to estimate vapour
pressure as a function of temperature, starting out from other data
about the material, but I'm not sure that the approximations would hold
up under vacuum conditions.
One thing I do remember from high-vacuum work is that our normal "feel"
for how much power it takes to heat up an object like a resistor is WAY
off when the object is in a vacuum. When there is no air to provide
convective cooling, temperatures shoot up very quickly.
I could quite believe that in a vacuum tube, only a few seconds of
"normal" overdrive (as opposed to a huge transient) might to heat up the
grid to a temperature high enough to mobilize the gold.
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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