Bill L. Fuqua wrote:
The bias due to the âgrid leak resistorâ only exist during
times drive is applied that is why class C amplifiers in many ham
transmitters have âClamp tubesâ on the screen grids. They drop the
screen voltage to limit plate current when drive is not applied
because the grid bias is non-existent with out RF drive (key up).
This reminds me of my first amplifier, c. 1963. I used an 813 with a
6Y6G clamp tube in series with a VR150 gas-discharge voltage regulator
tube, per the ARRL handbook. When the clamp drew the screen voltage
below 150 volts, the VR tube extinguished, dropping the screen voltage
to zero and almost entirely cutting off the tube.
This system had one great feature for CW: either because the bias rose
along with the drive, or because of the VR tube's time constant (or
both), the class-C amplifier did not sharpen the keying waveshape the
way a fixed bias amp would. I've often thought about the possibility of
designing a modern version of a CW-only class-C amplifier that would
work something like this. Class-C efficiency would allow, for example,
1500 watts output from a tube or tubes with only 500 watts plate
dissipation.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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