Hi !!
Well, it's a good question with a not-so-simple answer
The term "plate" modulation is an interesting term - misused, sometimes misunderstood, but nevertheless interesting :-)
"Plate" modulation really means "High Level Modulation" of the RF amplifier stage - the voltage between the Plate and Cathode (in the case of a tube - or the voltage between the Drain and Source for a MOSFET, etc.) is varied at an audio rate. The "plate" voltage is the voltage between the plate and the cathode,
regardless of what method is used to change it if that's what is being moduated [the difference between plate and cathode] it's "plate" modulation.
You could tie the plate circuit to a fixed voltage supply (with respect to ground), and apply modulation to the cathode circuit (assuming you are not varying the grid-cathode voltage), and you have the same thing - high level modulation, or in the case of a tube "plate" modulation. This type of high level "plate" modulation is/was very common in tube broadcast transmitters using high efficiency series modulations. Note: cathode modulation, where the grid-cathode voltage is varied is NOT high level modulation - it is grid modulation, or a combination of both depending on the amount of grid modulation [rat hole alert!].
This High level modulation can be accomplished in a number of ways. A transformer coupled, push-pull (or single ended) modulator is one of many ways to get the job done.
A
series modulator, varying the voltage between the plate and cathode is a high level modulator. If this series modulator happens to use pulse width modulation *internally* as a control function it is STILL a series modulator as far as the RF amplifier is concerned - and a high level modulator at that. Sometimes, one will hear folks talk about a "pulse width modulator", but REALLY, since the pulses are all filtered out and the modulator is generally connected in series with the RF amp, it's just a high efficiency series modulator.
There is no "best" modulator or modulation scheme. It depends on what you want and how you like to run your station. For me, I am very sensitive to wasting electricity - it is very expensive here, and, in any event, I don't like wasting power anyway... so I do everything to avoid doing so, to the point of eliminating all filaments [ so there is no stand-by power loss ] in all equipment (everything is solid state, home brew).
If you're staying in the tube realm, and you want high level modulation, and superior audio quality is what you're after, don't use transformers. You will absolutely be able to produce better audio [all other things being equal] with a direct-coupled, high level modulator.
Of course, low level modulation is also a nice possibility if you don't mind the inefficiency (30 % efficient typical). I used grid modulation on a number of fairly high power transmitters. Quality was outstanding... efficiency was poor.... system was dirt simple to get up and running. That's why I used it. Used 3 4-400A tubes in parallel - grid modulated. Made a nifty heater, too :-) Also had a pair of 813s, grid modulated. Something to think about. You don't hear a lot of big grid modulated transmitters on the air. You hear linears, but not many grid modulators at high power.
Anyway, more info to digest
Talk later and Regards,
Steve