>AG6K spoke of the primary emission from screen grid of 3CX800A7 due to
>overheated cathode, with subsequent barium deposition on the gold wire.
>This certainly is a concern if, as he said, the filament voltage is grossly
>high in a particular design. We should try for 2-3% of nominal, preferrably
>on the low side, for long life.
>We see this problem sometimes in our medium power 200 MHz tetrodes, that
>supply 150 KW to the finals. Since it's a pulsed RF plant, it manifests
>itself as RF power that changes across the 1 millisecond pulse. It is seen
>on the screen current,
not the same thing.
>which, in a pulsed system can be seen on a current
>transformer with an oscilloscope. The screen current will actually change
>direction as the pulse gets longer, from heating. It is dependent on RF
>power and length of pulse and repetition rate.
>We have seen it due to manufacturers contamination of the screen.
>Interestingly, the cure seemed to be to run the thing with the filament
>hot, and try and burn it off!
Running the heater above the rating in a 3cx800a7 causes the problem.
One can not burn it off of the grid by overheating the cathode because
doing so evaporates more barium.
R. L. Measures, 805-386-3734, AG6K
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