Just did some more tests...........
Used another type of core, 3C11

Same problem is still there; tried to damp with 1nF+22 Ohm serie, doesn't
help.
Tried other phasing......didn't work either.

I am using IRFP360, maybe that's the problem? (Rds on 0.2 Ohm)

When I switch TX on , but without voltage on the FET's, I still see the same
but of course much weaker on the output, before the low pass filter.

What I see is only even harmonics, I can not see 136 or the odd harmics.
When measuring at the gates of the FET's I can clearly see 136kHz and all
it's odd
and even products in about same strength.

It looks as if it has something to do with the combination of the 2 FET's
together
with a coil as load.
Maybe I'm running wrong FET's (IRFP360)

I will put it to rest now......hope to have more luck in the coming weeks.

73

Dick, pa4vhf