AM modulation

Mike O'Dell mo at
Tue Feb 16 20:36:13 CST 2010

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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