Tim Wescott wrote:
If you want to minimize the drive power you're going to have to use a
tuned grid circuit; having such a power-efficient design is going to make
your circuit susceptible to oscillation, as it'll take less than 1% of
the output power to come back in to the grid circuit and cause problems.
So a 120 (or 170) milliwatt input implies both a tuned grid circuit and
neutralization.

If you want to go broad-band, you need some dissipative elements in the
grid circuit to broaden the response. Something like a plain old
resistor would do quite well. With resistive loading, your grid circuit
will both be broader band _and_ your need for neutralization will be far
less. I'd be tempted to think along the lines of a 1W QRP rig and
resistor in the grid circuit that dissipates 800mW. I'll leave the
calculations to you, but this will broaden your grid circuit response
considerably, ease your bias network design problem, and reduce (or
eliminate) the need for neutralization and/or obsessively high degrees of
isolation in your circuit layout.
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Tim ,If I understand the above correctly you refer to a passive grid
circuit with for example a 200 or 300 or 600 Ohms non-inductive resistor
and a toroid type of auto-transformer .

With the exiter output impedance being 50 Ohms , the auto-transformer
impedance step-up would have to be as follows ;
For the 200 Ohms resistor 1: 4
For the 300 Ohms resistor 1: 6
for the 600 Ohms resistor 1: 12

The resistor to be chosen would be able to swing the grid voltage
sufficiently to drive the tube ,which would have a set negative grid
voltage for whatever Class the tube will operate eg class C , AB or even
A ,all that for the exiter power available.

Such a passive grid scheme can be found on the web site of AG6K.


Frank GM0CSZ / KN6WH