Interesting product! That's a good accomplishment at 20 meters, for sure.
As a note of interest, if I figure an equiv. AM power level for this amplifier, using the designs presented in this forum and elsewhere - designs that will do a minimum of 150% positive modulation - it comes out to 80 watts carrier output, on AM == 500 watts PEP (AM) modulated to 150 % positive.
Carrier:
40 volts * 2.3 amperes = 92 watts input = (about) 80 watts output.
Using the usual rule of thumb for 150% positive modulation, the modulator output would be 100 volts at the full positive peak for our 40 volts of carrier voltage. In this example, the DC resistance of the RF amplifier is about 17.39 ohms (40 / 2.3), so for the full 100 volt output (150% modulation) of the modulator, the current would be about 5.75 amperes and the power will be 575 watts input, or about 500 watts output.
The documentation for the mini linear suggests, an RF rated, high voltage MOSFET is used. There are a number of these on the market; IXYS and APT make these, among others. If one chose the right MOSFET and the right driver, it COULD be done with one device. Whether they did it that way is unknown. I would probably use two devices.
Someone needs to buy one and take it apart
Regards,
Steve