> when you use the amplifier at powers above the design
> parameters,
> the plate load impedance decreases. Lower plate load
> impedance,
> will necessitate larger capacitors ... if you are pushing
> and amp
> designed for 1200 W PEP output to 1500 W PEP, you have
> decreased
> the plate load impedance by 20%. A 20% decrease in plate
> load
> impedance will require a 15 to 20% increase in the values
> of the
> tune and load capacitor (and similar decrease in the
> coil).
Joe and Tom,
While the tuning cap meshes very slightly more, the loading
cap decreases noticeably in required size as drive power is
increased.
For example with a effective network Q of 12 and 3000 ohms
Rp on 1.8 MHz, the values are:
352pF / 24.21uH / 2091pF
If I increased drive and Rp became 2700 ohms:
353.3pF / 24.21uH / 1916pF. The effective Q becomes 10.8,
which is still more than 1 plus sqrt of Rp/RL. This means
the network will tune fine and efficiency be even better
than with a Q of 12.
The network only starts to be a problem when Q gets down
around 8 or 9 and phase shift approaches 90 degrees.
As drive is increased, we need less loading capacitance.
Plate capacitance barely changes. Q is also reduced.
As drive is decreased, we need more loading capacitance. Q
is increased. Plate capacitance barely changes.
The real problem with the 922 is it barely made 160 at all,
and is short on loading capacitance on the low end of 80.
73 Tom
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