Manfred has said most of what I was going to say. I'd add a couple
more comments.
RF power fets don't have quite the uniformity of characteristics
that switching fets have. In particular, threshold voltage can
vary, but if you have separate bias adjustment for each transistor
then transistors with the same batch code can normally be mixed
without any problems.
If you're seeing a big imbalance between the boards it's worth
characterising the amplifier sections in isolation and checking
that the splitter and combiner are working properly. When you come
to drive it all up, you need to stop when the highest gain part
starts saturating (or hits dissipation or temperature limit) which
might mean you can't get max. possible output.
As Manfred said, heat is the most common problem. At 1kW out you
could easily have 1kW of heat to shift. As well as overall
heatsink function, the mounting of each transistor is important.
Steve
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