it turned out that a MOSFET was wrongly placed. Once fixed I am able to get (at 12.0 V) about 7 W on 80, 2 W on 40, 1 W on 20 and 60 mW on 15 meter. I have read that there is a mod for the KN-Q10A, replacing the audio transistor 2SD822 with a 2SD1971 for a higher gain at higher frequencies, in order to fix low power at 21 MHz.
The 2SC1971 is a VHF (175 MHz) transistor with 10 dB gain and 6 W output power. I can get locally 2SC2166, which are HF (30 MHz) transistors with a higher gain (13.8 dB) and same power, 6 W.
Is the Ft characteristic something essential for this application?
Maybe somebody has a good advice on this. Any hints appreciated.