[BC] Re: AM Plate Modulation Concept

[Al Wolfe]

> Tom Dimeo clip:
>>He says that the negative half cycle does not cut off
>>the carrier but he is able to get upward of two hundred
>>percent modulation in the positive direction.

    Tom, you are stirring up the cobwebs again. Two classic ways of doing 
this.

    One was the "exalted carrier" or "controlled carrier" approach where the 
carrier level transmitted was controlled by some sort of audio envelope 
detector. The envelope detector would control bias on one of the grids of 
the final RF stage. Often a portion of the audio from the modulation 
transformer was rectified and simply added to the screen supply.


Another method, sometimes known as "Taylor Modulation" (if memory is still 
working from 50 years ago), was to use two tubes in the final modulated 
stage, but with one hooked up backwards. That is, with the cathode of the 
second tube tied to the plate of the first and its plate tied to ground. 
This "Negative Tube", as it was known, would conduct only when the output of 
the modulation transformer drove the main tube's plate negative. Thus, RF 
was generated in proportion to the audio as opposed to no RF being generated 
by overmodulating a conventional RF stage on negative peaks.

    Construction of this circuitry was challenging from the standpoint of 
isolated grid and filament supplies. Actual double sideband suppressed 
carrier transmitters were built using this method by simply grounding the 
normal B+ input to the output of the modulation transformer or modulation 
reactor. Bruce Taylor of Heathkit gets credit for this scheme in the middle 
1950's.


Al  (retired, mostly)
aka K9SI