AM modulation

bbruhns at bbruhns at
Tue Feb 16 21:25:34 CST 2010

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
Sent from my Verizon Wireless BlackBerry

-----Original Message-----
From: "Mike O'Dell" <mo at>
Date: Tue, 16 Feb 2010 21:36:13 
To: <rabruner at>
Cc: <tacos at>
Subject: Re: AM modulation

thanks for the great brain dump.
i recalled having seen mention of "screen-grid modulated"
AM transmitters but didn't know any of the details.

On 2/16/10 7:08 PM, rabruner at wrote:
> As several have said, high level modulated Class C AM transmitters have
> a greater overall efficiency than linear amplifier transmitters.
> Straight linear amplifiers transmitters are actually usually run in
> class AB2 and have an average plate efficiency in the final of about 30%
> or so. The efficiency off the power line is even worse because they
> usually operate at lower gain than Class C transmitters and have more
> stages, more parts to stock, etc. RCA's Ampliphase improved on this.
> Western electric also had a high efficiency linear amplifier they
> marketed successfully based on the Doherty amplifier. This, like the
> Terman Woodyard, is a carrier tube/ peak tube arrangement with the main
> difference being the Doherty is an amplifier and the Terman Woodyard is
> a modulated stage. W-E transmitters were modulated at very low level,
> about two watts, and used a series of Doherty amplifiers to achieve
> powers up to 50kW. The transmitter arm of W-E became Continental
> Electronics and they continued to make and sell Doherty amplifier
> transmitters. They developed a variant where the Doherty amplifier was
> high level screen modulated which achieved total efficiencies greater
> than Class C amplifiers. The Continental 317C used only 9 tubes, and
> drew only 86 kW off the power main to make a 50,000 watt carrier. The
> two final tubes, 4CX35000s were screen modulated by a pair of 3CX3000s.
> Both final tubes were biased to operate in Class C, giving them good RF
> efficiency and modulation linearity. The two audio tubes, were driven in
> parallel, the plate of one being connected to one final tube, and the
> cathode of the other to the second tube. The transmitter could modulate
> flat to 30 kHz on a dummy load and into the subsonic region. The
> distortion on a dummy load was in the region of a couple of tenths of a
> percent. Many of these are still in service as are the RCA Ampliphase
> transmitters. The tendency, though, is toward PW modulated transmitters
> with even greater efficiencies. One other note. When analyzing AM
> modulation it is incorrect to say that the 'carrier goes away,' or that
> 'the carrier goes to zero.' In a correctly modulated AM transmitter of
> any design, with the exception of clamp tube modulated transmitters, the
> carrier level is invariant. The FCC Rules in fact, require that the
> carrier amplitude not vary by more than 4% under any condition of
> modulation. I realize this conflicts with high school physics books, and
> popular technical magazines, but checking a few references of the
> physics at a high level will bear this out. Collins' book on SSB is a
> good place to start. Even though we have all seen the AM carrier 'go to
> zero' on oscilloscopes, the fact is, it doesn't happen at all. The
> normal presentation on oscilloscope is the resultant of the carrier and
> sidebands --which are out of phase -- adding (and canceling) in the
> scope. Observing AM modulation on a spectrum analyzer will reveal that
> the carrier just sits there doing nothing while the sidebands come and
> go with the modulation.
> Bob Bruner W9TAJ
> transmitters were almost always plate-modulated
>>  why didn't they AM the signal when it was "small"
>>  and easy to do and then just use a linear final
>>  so all the muscle went into the signal instead
>>  of heating the modulation transformer?
> There was a reason.  A linear amplifier is most efficient at maximum power, and
> the efficiency decreases for lower power, reaching
> zero at zero output.
> In the old days, there were a few other alternatives to plate modulation that
> did get reasonably good efficiency.  One was called
> AmpliPhase, it belonged to RCA and it combined two oppositely phase-modulated
> output stages.  Another was the Terman-Woodyard
> amplifier,
>    Bob, WA3WDR
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