[AMPS] parasitic suppressors

[Rich Measures]

>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.....

R. L. Measures, 805-386-3734, AG6K   


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