Series and cathode modulation (as defined by Jones) are two different things. One is the equivalent of plate modulation and the other is a combination of grid and plate modulation.
The 1940 Radio Handbook spoke of Cathode Modulation as being with a transformer in the ground circuit of the Final Amplifier Cathode(s) and called the direct tube placed there as "Series Modulation' of the Cathode.
My question - In Cathode Modulation, what is an advantage of using the transformer in the Cathode circuit, in stead of the tube? (of course, there can be no PWM through a transformer)
A possibility that I am considering is that with the transformer it works with less power loss thana with the tube (power coming from the modulator - but only something like 20%). That the additional tube placed in series requires consideration of the dissipation lost in the tube so a much high voltage is required (based on the peak voltage limits of the tube(s) used - a type of Series Resistance form).
[Another paper on "Series Modulation" used the tube(s) as a Plate Modulation approach, not using a transformer (nor Heising), but simply having the tube control the flow of current through it to the Plate of the Final Amplifier (being placed between the Final Plate and the HV Power source - cathode of the Modulating tube going to the Plate of the Final Amplifier) - again - a higher voltage is required to compensate for the extra tube.]
or - am I missing something? - vital? -