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First a little theory; you can represent an AM signal (which is composed of a carrier and upper and lower sideband) using phasors (not Star Trek type) or vectors as follows:  Draw the carrier vector as a reference voltage arrow pointing straight up.  Then add a single tone upper sideband vector connected at the tail of the carrier vector pointing to the right but make its length half as long as the carrier vector.  Now add a single tone lower sideband vector pointing to the left.  The tail end of these three vectors all start at the same point.  Now imagine the upper sideband vector rotating around clockwise while the lower sideband vector rotates around counterclockwise, with both sideband vectors rotating at the same rate.  That rate would be, for example, one kilohertz if the single tone modulation frequency was one kilohertz.  Now you'll see the advantage of this vectoral representation.  Can you see how, as they rotate in each direction, there will be a moment when they meet, lined up vertically with the carrier vector.  That will make the carrier voltage value double at that moment.  In fact that's exactly what occurs.  The entire composite signal voltage doubles.  And when you consider that power is 20log of voltage that means that the power will be 4 times the original carrier power at that moment since the composite voltage has doubled.  That's a vectoral explanation of why the PEP power of an AM signal is 4 times the unmodulated carrier power.  Now let the two sideband vectors keep rotating at 1 kHz until they are pointing directly downward.  At that moment the carrier vector is totally nulled by the two sideband vectors.  That corresponds to that pinch off point you see on an oscilloscope.  The moment where the sideband vectors added with the carrier vector corresponds to the peak envelope width you see on the scope.  Ain't phasors great?! Mr Spock didn't have a clue about this.  OK enough for AM theory, except I wanted to emphasize that this vector representation exists for AM signals, whether they were generated using low level modulation or using plate modulation in the final amp stage. 

Regarding 100% modulation, that's when the sideband vectors are indeed each 1/2 the voltage value of the carrier vector.

Practically speaking, when I use my HT-37 or Johnson Viking Ranger I can very readily achieve 100% modulation.  No amount of post amplification will change the modulation percentage.  Only the overall power will be increased.

With many of the newer solid state ham rigs including my TS-590SG, what I have observed is that the ALC is designed mainly with SSB operation in mind.  When you try to run AM on these "100 Watt PEP" rigs, you run into issues with the AM peak being held back due to the presence of the carrier.  Without going inside the radio and modifying the ALC loop components (although there's a trick using an externally supplied fixed ALC voltage) you can get very close to a nice 100% AM envelope by simply running the rig at perhaps a 15 Watt carrier and 60 Watts PEP.  Don't worry about not quite getting 100 Watts PEP AM out of it since you can now run that signal through your favorite amp such as an SB-220 with about 10 dB of gain giving you a 150 Watt unmodulated carrier and a clean 600 Watts PEP AM signal.  Or put a smaller amp in front of the SB-220 so you end up running the 220 with a 375 Watt unmodulated carrier and 1500 Watts PEP AM output.