For original description of the modification go to IZ4BEH . The page is in Italian but using Google Translator is easy to understand main ideas and way to go. I found it very easy to do. One should be, of course,  very carefully not to damage components around the modified places. With 28V supply it is almost 300 watts output with 200mW drive but I  need only 100 watts, have a restriction at antenna.

 

Step 1 – driver before

Step 1 – driver after

Step 2 – amplifier before

Step 2 – amplifier after

Step 3 – bias for driver

Step 4 – bias for amplifier