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From: Rich Measures[SMTP:measures@vc.net]
Sent: Monday, December 08, 1997 11:38 AM
To: Ian White, G3SEK; amps@contesting.com
Subject: Re: [AMPS] parasitic suppressors
>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.
>>
I mentioned this during the parasitics debate.
>
>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.
>
The vacuum in power grid tubes is high.
>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.
According to one chem. data book, the boiling point of gold is 2966
degrees C, at a pressure of one atmosphere. In a high vacuum it says
that slow evaporation is possible at lower temperatures, however, it says
nothing about the boiling point in a high vacuum.
>
>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.
True, although the additional power needed to raise an object in a
black-chamber-vacuum to a specific temperature is a function of T^4. In
1986, an Eimac engineer said that gold-sputtering was due to an
oscillation condition.
>
>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.
Perhaps, if you saw a gold-sputtered 8877 grid assembly through a low
power microscope, you might see things a bit differently, Ian. The
event that causes the damage apparently happens suddenly.
- IMO, overdrive tends to dislodge patches of cathode coating, without
damaging the gold plating on the grid. {photograph in September 1990
article "Parasitics Revisited".}
cheers
Rich.....
IMO overdrive PER SE doesn't damage anything.
It's my understanding that drawing too much average cathode current so that the
electron "cloud" above the cathode becomes depleted - perhaps only above a
relatively less-hot patch of the oxide - allows the high plate-to- cathode E
field to penetrate to the cathode surface and literally "pull" chips or chunks
off the cathode. This might, but doesn't necessarily, result from overdrive. I
think it's one reason for extended cathode warm-up time in oxide cathode tubes
- to achieve relatively uniform heating and avoid damage to slower-heating
areas of the cathode.
My understanding is that severe grid overdissipation, which may result from
overdrive, underloading, lack of adequate HV (all interrelated), can evaporate
- or if you prefer, boil or sputter or whatever - gold off the grid.
A grid surface without gold falls prey to secondary emission which screws up
linearity, Ig, etc. To the extent that errant gold lands on the cathode, it
tends to "poison" the oxide and in extreme cases literally kills emission.
Can anyone confirm, deny, or clarify the above, or otherwise enlighten us all?
Thanks & 73, Dick W0ID
R. L. Measures, 805-386-3734, AG6K
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