>Rich, you say:
>
>>- I do not believe that an 8877's grid can be gold-sputtered by an
>arc.
>
>I don't know what the 8877 grid looks like after it has had the gold
>boiled. But I would surmise that IF it was boiled by an arc, there would
>be one spot on the grid totally boiled and burnt, where the arc struck,
>and the gold progressively less boiled off as you got further away from
>the burnt spot.
I agree, and this scenario is not what I see. I have never observed an
arc-mark on a gold-sputtered grid.
>If the gold is burnt off more or less uniformly around
>the grid, then I would argue for the problem being grid dissipation,
>i.e. the grid has been heated more or less uniformly by grid current.
>
Through the microscope, I see gold blisters, which tells me that the gold
boiled and evaporated. Gold boils at 2966 degrees C. However,
gold-sputtered grids exhibit no signs of having reached this temperature.
IMO, this indicates that the problem may not be as simple as one might
think. As can be seen in the photograph in "Parasitics Revisited" (9/90
& 10/90 QST), most of the gold boiled away at the grid collet end of the
grid, which is grounded, which should have been cooler due to heat
sinking. These two things lead me to believe that simple grid
dissipation was not the problem. I have seen the same pattern in
numerous, fatally gold-sputtered tubes. Typically, such failures
reportedly occur after the amplifier has been tuned up and the grid
current looks ok. At the instant of failure, an unusual noise is
sometimes heard
- 11 years ago, an Eimac tube specs engineer told me that the 8877's
design team concluded that grid gold-sputtering was caused by an
oscillation condition. Those who doubt that VHF energy was present are
hereby invited to measure the resistance of the VHF parasitic oscillation
suppressor resistor.
Rich---
R. L. Measures, 805-386-3734, AG6K
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