Various means are available to vary the inductance over a limited range -
without resorting to a large rotary inductor. How about this for a
suggestion.
Amplifier is built with small value variable C on the plate side. Then go
though a bandswitch (or vacuum relay) to a choice of output band circuits.
Each band is fitted with a fixed capacitor input (except on 10 and 15
meters, where the small value air variable is sufficient). Then, each band
circuit has a separate L (with limited adjustment similar to a variable -L
link or a spacing change to vary L), then to a fixed output capacitor set
for 50 ohms. Each band would then have a separate adjustment on the front
panel for that particular band. The output of the pi-network would then be
switched with either a relay or a coupled wafer. Since the pi-network would
not have sufficient rejection of harmonics, an additional low pass filter
for each band segment would be switched in to eliminate harmonics.
This setup would have the advantage of bandswitching with only a slight
retuning of one capacitor. The inductor would be pretuned to that band
segment. Parts would be easily home made. Since each band would have a
filter in the output, harmonic output could easily be greater than from the
Pi-L.
One possible problem might be the unused tank circuits. You would not want
to pick up enough voltage to blow the fixed capacitors. Shielding could
solve that problem.
Have not thought it through yet, as it is 4 am and I could not sleep.
Perhaps a bit more sleep and I will abandon the idea. Perhaps it is easier
to spend $5,500 for an autotune amp.
Colin K7FM
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