Dick wrote:

I am using a large heatsink 12x15x5cm.
On this heatsink I mounted a 1.5mm thick copperplate 75x150mm in order to
transport heat quick away from the FET's.
Maybe it's some 'overkill' , but I just want to set up a thing that can run
for hours
on full power; just a bit of reserve...........

Why don't you use one of those heatsinks with blower meant to be applied on 
Pentium or Athlon CPUs ?

They can be found for almost nothing at ham fairs, or salvaged from old PCs at 
the local city dump.

73  Alberto  I2PHD