[AMRadio] RE: negative cycle loading

[Donald Chester]

>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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