>> 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.
>
* True, but the grid also receives radiant heat from the 55w cathode.
>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.
* Anyone who has done an autopsy on an 8877 knows that the anode and the
grid collet cone are separated by c. 8mm at the closest point. My guess
is that this accounts for no more than 20% of the anode-grid C of 10pF.
Perhaps Mr. Rauch should open the requisite tube himself before he opens
something else?
> 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.
* According Eimac's spec sheet for its 3cx1200Z7/YU-181, the anode -
grid C is 9.7pF. How is 9.7pF more huge than 10pF?
>Evenso, 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.
* Probably fairly well if it can operate full-throttle AØ at 250MHz.
>73, Tom W8JI
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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