FWIW, cooling 750 watt parts is going to be a PITA. All the more so
if you use two of them push-pull on the same amplifier module / board.
Water will help, but only somewhat, and has been mentioned in another
post, carries its own hassles.
The reality is that these SS amps tend to be no better than 50%
efficient, and frequently more like 40-45%. So a real legal limit amp
designed for key down operation (you mentioned RTTY) needs to be able
to dissipate something in the ballpark of 1500-1800 watts of heat.
Now the ARF-1500 claims efficiency of 75% according to the datasheet,
but the 2006 QEX article measured the operating efficiency at ham
frequencies as being in the 60% range best case (and 40% worst case).
With these very high power density devices, that amount of heat is
being generated in a very small physical area (roughly 1" x 2.5" in
the case of a pair of ARF-1500's mounted for push-pull).
That means that (best case, 60% efficiency) you need to plan to
dissipate roughly 1000 watts out of an area of 2.5 square inches.
That is a serious cooling problem, to say the least. The problem
isn't so much getting rid of the heat at the end of the process, it is
getting the heat out of the die and into the heat spreader, and then
through the heat spreader and into the heat sync or water cooling
jacket...
***dan, K6IF
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