AM Modulation

Bob Bruhns bbruhns at
Wed Feb 17 10:27:14 CST 2010

True, definitely not the same beast.  A number of hams have removed the carrier-control circuitry from the old amateur screen modulated transmitters such as the DX-60, T-60 and others, and with good screen modulator design and proper biasing and tuning, the results can be pretty good - although a 6146 will only produce about ten to fifteen watts of carrier output that way.

   Bob, WA3WDR

----- Original Message ----- 
  From: rabruner at 
  To: tacos at 
  Sent: Tuesday, February 16, 2010 11:54 PM
  Subject: AM Modulation

  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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