Rich said:
>Amplifiers need to be tuned up at 100% of the drive that will actually be
>applied.
Conventional amps should be tuned at 100%, as Rich says. However, this doesn't
necessarily apply if the amp has circuitry that allows correct tuning and
loading to be achieved at lower power levels. Some do - not all. The auto tune
amps tend to. If you have detectors on the input electrode and the plate so the
ratio of the RF volts on these two electrodes can be monitored, you can set the
loading correctly at low power. There was a 3-1000Z GG amp in the 1970 ARRL
Handbook which did this. In a grounded cathode amp, if you monitor the relative
phase of anode and grid volts and maintain the TUNE control for them to be 180
degrees out of phase, you can tune the tank without a plate current dip. It's a
bit more difficult in GG because of fedthrough power.
Most HF tube amps will be about 50% efficient, give or take, when tuned up. If
you reduce the drive, the efficiency will drop off. In the limit, even with no
RF input, you still have the standing current. At very low output powers, the
efficiency in terms of DC in to RF out is extremely poor, increasing as the
drive level is increased.
73
Peter G3RZP
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