> Rich's calculations are interesting. With these levels of grid
> current, how much heating of the grid is there because of the effects
> of skin resistance? I admit I haven't done the calculation, but
> intuitively, there must be some heating from this effect, and surely
> that has an effect on the allowable dissipation? 73
There certainly is an effect, but it is small in the 8877. Virtually
all of the dissipation in the control grid is caused by the kinetic
energy of electrons striking the grid, just as occurs in the anode.
We can not directly use the total anode to grid capacitance for
obvious reasons. First, in the 8877 only a small amount of the total
capacitance is due to anode to grid frame capacitance. The large
metal cone supporting the grid is the major item involved in setting
the anode-to-grid capacitance. The displacement current that does
flow through the grid wires divides partially to the cathode, and the
maximum displacement current that does flow down through the grid
structure is maximum only at the bottom and nearly zero at the top.
The bottom of the structure is actually able to best handle this
current, because the grid mounting cone heatsinks the grid.
Very large grid tubes, like the 3CX1200Z7, are another matter. The
grid is huge, and has considerably more displacement current. Even
so, I suspect the resistive heating is small.
The largest problem at VHF is often dielectric heating of the
envelope. I'm not sure how the 8877 fairs in the regard.73, Tom W8JI
W8JI@contesting.com
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