Shortly after my original post on this topic I received two PMs, both pointing at a design by W7IUV for 400 Watts output, with a hint more is possible, and that its robustness is proven in practice.. This was for use at 475kHz but its design is broadband, so by choice of ferrite could easily span 137 and 475kHz. I won't repeat the URL here as both responders chose to reply individually with it, but if you Google (*) W7IUV something will show.
One reason this design appeals to me is the use of degenerative feedback formed from multiple source resistors. In any amplifier design, using devices substantially below their maximum frequency - which is the case with switching FETs at LF - some sort of feedback should, really, always be used. It will help with bias stabilisation, as well as keeping the overall design more stable. MOSFET amps for HF nearly always apply NFB, often via a dedicated feedback transformer.
The G7IUV design shown doesn't give details of bias circuitry or PSU but these are pretty standard for MOSFETs, although th euse of multiple parallel FETs does ideally require that FETs be all from teh same batch to ensure moderate matching.
Andy G4JNT
(*) Other search engines are available (allegedly)
