ZL2AL writes:
> I am currently building an HF amp using a GS35B. It would
> appear that the input impedance is around 50 ohms + or - and
> most circuitry that I have seen does not use a tuned input
> circuit. I have plenty of drive from the FT1000D available.
A grounded grid amplifier does not represent a constant load
to the driving stage, it will vary from very high when the
tube is cut off to quite low (a few ohms) at maximum anode
current. Although the average driving impedance may be close
to 50 Ohms, the driving stage sees the instantaneous swings.
Those swings cause the operating conditions of the driver
to change constantly ... as the instantaneous SWR goes high
at both ends of the range, the exciter tries to cut back,
etc. This constant change results in greatly increased IMD
from the driver and a relatively "dirty" output.
You would do well to add a good input network as close to
the tube as possible. A tuned input with a Q of 2 or more
will provide a bypass for the harmonics generated in the
tube's input and provide a "flywheel" to smooth out the
impedance variations seen by the driving stage.
73,
... Joe, K4IK
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