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Again, I don’t mean to be argumentative but it would be good if you and I both used the same conceptions of terms so we can communicate.
The collector of a bipolar transistor is analogous to the plate of a tube, the base to the grid, and the emitter to the cathode. The reason for this is in a tube the plate current is controlled by the voltage between the grid and cathode and in a transistor the collector current is controlled by the current flowing from the base to the emitter. The common reference for the control signal is the emitter and the cathode, thus the analogy.
The final amplifier Q3 in the AM25 is a field effect transistor where the drain corresponds to the plate, the gate to the grid, and the source to the cathode. Here the analogy is clear because the FET is modeled as a voltage controlled current source just as is a tube. The control voltage is applied between the gate (grid) and the source (cathode) and the output current flows through the drain (plate).
So “plate modulation” in a transistor circuit would be applied to the collector rather than the emitter and can be done by placing the secondary of an audio transformer in series with the collector or drain just as is done in the plate circuit of a tube.
I am aware of cathode (emitter or source) modulation schemes but in the AM25 the final amplifier Q3 source is at ground potential and the modulation is applied by changing the voltage at the drain (plate modulation). Q9 is wired as an emitter follower where the audio and DC output is from the emitter similar to a cathode follower used in tube circuits.
I may have some time on Sunday to try the experiment and, if so, will report here.