My 100 W is based on the design by David Bowman G0MRF [2] and I can recommend the approach used by David.  A copy of the circuit diagram can be found at
http://www.g0mrf.freeserve.co.uk/lf.htm

The power amplifier (PA) has been fitted with a low pass filter to prevent the radiation of harmonics.

I had trouble finding the 2SK413 MOSFETs as originally specified by David, so I ended up using low voltage 2SK400 devices.  I also had trouble finding a suitable transformer for the 30 volt power supply.  During testing, I inadvertently destroyed two of the 2SK400s and David suggested using high voltage IRFP450 devices.   These more rugged devices have performed very well and appear to be indestructable.   The limiting factor continued to be my power supply which restricted power output to 100 W.

Nevertheless the G0MRF PA provided good, reliable service and enabled me to make the first Wales-Scotland QSO to Dave GM3YXM/P (395 km).  For many months, this contact remained my best DX on 136 kHz.  It also enabled me to provide the first GW contact with over 20 stations.