Rich, you say:
>4. High grid current caused high EM force on the hot (1800 degree C)
>tungsten filament -- which caused the filament to bend and short
against
>the grid.
Now if the EM force is going to bend the filament, the current has to
be DC grid current, not AC at 130MHz - surely? Well, maybe 'surely', but
it seems unlikely that the reversing magnetic field could overcome the
inertia if it was AC. So the grid current (DC) has to be sufficient to
burn the choke out. But even saturated, the best a Thoriated flament
will do (according to the Reference data for Radio Engineers) is
100mA/watt so 7 Amps is the max emission, and probably lower. How long
would the parasitic have to be there to burn out a 1 amp rated choke?
And to get 7Amps of grid current, how much emission is left to go the
plate? If it even had say 3 Amps of grid current, wouldn't it take
rather longer to go?
It would be interesting to see the grid of the the failed tube: I
wouldn't be surprised to see a bent filament or grid, but I wonder at
the cause.
Puzzled
73
Peter G3RZP
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