Total posts : 45366
The diode in the MOSFET that I referred to is a parasitic diode that is in shunt between the drain and the source. This causes negative excursions in the output voltage to be clipped, and this results in a loss in efficiency. Such a diode is not present in JFETs (or tubes).
Interelectrode capacitance affects efficiency because any capacitance that is shunted in some way across the antenna capacitance of a short antenna decreases efficiency. Another type of capacitance that reduces antenna efficiency is the interwinding capacitance of a loading coil. Circuit board capacitances are also bad. I would not use a circuit board for an output tube stage for a Part 15 AM transmitter. I’d use an “airboard” instead. Airboards are not mechanically rigid, but they work better electrically.