Cut to the chase. The only thing the IRF5xx series has going for it
is it's dirt cheap. The problem is that coaxing them to deliver is
more pain than better devices. Good devices in reasonable circuits
have one advantage, repeatability of performance.

As to MW am service, I'd use the heavier devices and run them class E
in the several hundred watts to KW range. Even the IRF510 in class E
is good for more than 50W for a pair and should run reasonably cool.
Even the really big E amps only need a few watts as excitation.
This works because they are being used as switches, what they were
designed to do. A class E amp can be "plate modulated" in the usual
way but a PWM modulator will be far more efficient. There are a
number of people running class E Am phone on 160, 75 and 40m and
they sound very good.

For decent RF designs that withstand impossible swrs a good RF rated
device is hard to beat. The real problem is finding a source that
doesn't mark them up 500%. However there are some devices out
there that are like the venerable 6146 they do the job and can be had
at less than 0.40$/watt. For 6m the MRF492 for example rated
power out at 12V of 70W and most deliver 80-90. For the price it's
cheap.

For 2m I'd just grab a MRF247, easy 75watts at 2m class C or B and
a pair with Wilkerson splitter/combiners deliver a heafty 145watts on
the Bird. They work well for 2m SSB service with decent IMD. the
last pair(matched) I bought as for a commercial amp repair only cost
70$. For that kind of power that's cheap.


Allison
KB1GMX