> From: Rich Measures <measures@vc.net>
To: <amps@contesting.com>
> Date: Tue, 2 Sep 97 21:00:45 +0000
> >1.) It has no grid protection circuit, making it possible to easily
> >overdrive and damage the grid.
> Over-drive is most likely to strip the oxide coating from the cathode.
> [see photo in "Parasitics Revisited" (9/90 - 10/90 QST) All of the
> 8873/4/5 grid damage that I have looked at under the microscope has been
> due to the gold plating on the grid boiling away. Gold boils at 2966
> degrees C. The idea of being able to raise the grid to such an extreme
> temperature with over-drive is amusing. ........
The claim it is a "parasitic" is sad, not amusing.
The grid dissipation is FIVE watts. Rated grid dissipation is reached
at about 60 mA of grid current in the SB-230.
Since the grid heating is caused by kinetic heating of the grids
outer layer by electrons, and gold melts at a lower temperature than
the tungsten base material, the soft gold goes first.
Anyone in their right mind would know a 100 watt exciter could easily
cook a five watt grid if the PA is underloaded. Anyone doubting that
should ask Eimac, and not take any strangers word...especially if
that stranger sells "magic cures".
73, Tom W8JI
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