AM Modulation

[rabruner at aol.com]

Ham rigs with clamp tube modulators, such as the DX-60 and others are not capable of 100% modulation, and attempting to "talk them up" just produces distortion and splatter.  The varrying carrier level also contributes some to distortion as well.  Listening to selective fading on AM shortwave stations demonstrates what that can sound like taken to an extreme.  These transmitters can sound acceptable when correctly adjusted and operated with restraint.
   We shouldn't confuse screen modulated AM broadcast transmitters with these 'controlled carrier' transmitters.  The broadcast transmitters achieve full modulation, good carrier stability and excellent audio fidelity.  Not the same beast at all.

Bob W9TAJ


Grid modulation takes a lot less power than plate modulation, but without 
applying techniques like Doherty, Terman-Woodyard etc, the efficiency is about 
the same as straight linear, about 30% carrier for AM.  Screen modulation was 
(and still is, in some circles) popular, because only the screen voltage needs 
to be modulated (usually half or less of the plate voltage), and the audio power 
required is relatively small.  A modulation transformer can be used, but it is 
usually not necessary, and transformerless coupling of the transmit audio 
permits high quality sound from a simple transmitter.  But despite that, the 
sound was usually pretty raunchy!  There were a number of screen modulated AM 
transmitters on the market back in the day.

Controlled carrier' was popular in many screen modulated designs. The transmit 
signal was reduced when no audio was present.  It made the meters kick up when 
the user would speak (a prized characteristic, it seemed to mean you were loud), 
and the reduced duty cycle was a little easier on the transmit tubes.  But it 
would have sounded better if the carrier had instead been raised during periods 
of low or no transmit audio, to maintain approximately constant peak level, 
because then the receiver agc would have created a noise reduction effect.  In 
fact this is seen in amateur AM linear operation where the linear amplifier has 
fast alc, and it can actually sound good.  But instead we got lots of noise 
between syllables with most of the old controlled-carrier screen modulated rigs, 
not to mention the atrocious distortion that came from very poor design, so the 
received signal sounded a lot noisier than it really was.

   Bob, WA3WDR

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