>> From: Rich Measures <measures@vc.net>
>
>> Mr. Erhorn: Did you disassemble any of the out-of-warranty 8877s that
>> failed in the W1AW amplifier prior to June, 1990? . .
>> thanks
>> Rich---
>>
>
>Hi Rich,
>
>I don't know what Dick did, but I can tell you I opened and inspected
>tubes...as did Eimac.
>
>There were two modes of failure.
>
I have observed 7 modes of failure. Loss of vacuum, gold sputtering,
open heater, defective spotwelds, cathode-heater short, heat-dam related
grid short, and stripped-cathode related grid short.
>One was the heat dam problems Dick pointed out, and another was
>poor gold adhesion.
>
I typically see blisters where the gold plating on the grid boiled and
sputtered. If there was a 'gold adhesion' problem, one should see flakes
of gold through the microscope. I see gold melt-balls. I have never
seen gold flakes in a kaput 8877.
- Roughly two out of three kaput 8877s that I decapitate have enough
gold melt-balls to create internal arcs between the anode and the
grounded grid. How does one determine this? By measuring breakdown
voltage between the anode and the grid. Loose gold is indicated when the
positive (anode) leakage current is substantially higher than the
negative leakage current.
The 8877 I selected for opening this morning appears to be an example of
the problem Mr. Erhorn described. There was only 2 uA of leakage at 9kV.
I removed the anode cooler. As expected, there were no gold blisters
on the grid. The cathode coating was pristine. The fatal cathode/grid
short was pretty obviously caused by a manufacturing defect. . . In
the good old days when 2 amateur radio operators named Eitel and
McCullough owned Eimac, this tube would have been replaced even though it
was out of warranty. . .
- . Bigger is not always better. -
Rich---
R. L. Measures, 805-386-3734, AG6K
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