>
>2 wrote:
>
>> Over-current fuses open to limit current, but over-current transistors
>> often short, which does not limit current. The 8877 grid is capable of
>> dissipating 25w in AØ mode. Since typical grid-cathode potential is c
>> 57Vrms, 25w works out to c. 440 grid mA. I have never seen a kaput 8877
>> grid that had uniform gold evaporation - such as would be the case with
>> too much DC grid current. Mr, Murphy said that everything is more
>> complex than it first looks.
>
>
>How much of this gold sputtering could be caused by long-term
>long-duration mild-to-moderate overdrive of the grid?
>
\ Overdrive normally flakes off the cathode's emissive coating. There's
a photograph of a flaked cathode in "Parasitics Revisited" in the
September/October 1990 issue of *QST*. As can be seen in the photograph,
the grid is not damaged. If you do not have this article, I can post the
photo on Web site.
>I ask this, because I know plenty of moron hams with amps that obviously
>do not understand the principles of the amplifier, and
>routinely overdrive them. The number of Extra-Class hams who do so is
>astounding [thanks a lot, Gorden West]!
>
\ Some Hams apparently have more money than common sense.
>On a side note, as a no-code tech, I was having to explain basic antenna
>principles to extras who most obviously didn't know WTF
>they were talking about.
>
>Basically, what I am asking is what is the factor of abuse in these
>gold-sputtering cases caused by what I call "ABCD Hams"?
>
\ Probably none. The way the gold plating boils off the grid suggests
that the energy doing it is quite likely UHF. Thus, the current would
be concentrated near the surface.
Just as current distributes in a 1/4 wave vertical antenna, the heaviest
evaporation (and current) apparently takes place at the base of the grid.
I have seen some cases where the underlying molybdenum could be seen
near the base of the grid.
cheers, Jim
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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