Chris Pedder wrote:
> 12. Apply only enough drive to indicate either 1000 watts of output power,
> or 200ma of grid current.
>
> The figure of 200ma of grid current appears to be at odds with Eimac specs.
> Is there anyone who can comment on this?
Hi Chris,
If grid heating was due to resistive losses, the grid would also have a
current rating.
The grid is heated by conducted and radiated heat from the anode and
filament, as well as direct heating of the grid by the kenetic energy of
electrons smashing into the grid. Of these three things, electron
bombardment is the primary source of heat.
You won't find a current rating for the grid, because the dissipation
depends on the grid to cathode voltage times current integrated over the
entire RF cycle. Grid to catode voltage (and grid current) depends
heavily on PA loading in a grounded grid amp.
Rated grid dissipation (25 watts) is typically reached at about 250-275
mA of grid current in that particular circuit under worse case
conditions. Dissipation at 200 mA is about 10-15 watts under normal
tuning conditions.
The reason the manual sets a limit of 200 mA during tuning is so you
don't overdrive the entire PA, which stresses many components besides
the tube.
Note the manual tells you to tune for maximum output, and reduce drive
whan you are done tuning....to about 400 mA plate current on CW. At that
current grid drive will be 150 mA or less normally.
73 Tom
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