It is a good idea to
provide a load for the modulation circuit after the final has reached
below the zero plate voltage point on the negative peaks of the audio
cycle. This can be done with a single HV diode in series with a
resistor of the same resistance as Ep/Ip. This series network is place
from the plate modulation point to ground so that the diode is reverse
biased except when the voltage at the plate goes negative. This will do
nothing for the ratio of positive to negative peaks of RF but it will
help provide a load for the modulator during this period and keep the
modulation XFMR from acting like a spark coil.
It will also double as a nice overmodulation indicator. For years I used
one with a 866A as the diode. I mounted the tube inside a black box with a
little window on the side. Whenever I overmodulated in the negative
direction, I could see a nice blue flash in the 866. The resistor doesn't
have to be too large. A 50w wirewound will serve nicely at power levels
beyond a kilowatt, since the duty cycle is extremely low, assuming you are
not grossly overmodulating at every cycle of the audio waveform.
-k4kyv
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