Well, after studying the manuals for various THP amplifiers, I have an
answer to my above question about the different number of active devices
(FETs) used in the THP 1.1Kfx amp versus the KPA500 amp which offer rather
similar power outputs.
THP essentially uses the same PA board for their 1.1Kfx (600w); 1.2Kfx
(750w) and 1.5Kfx (1000w) amps by varying the power supplies and the FET
drain voltage - Vd - to achieve different power levels. The PA is called
the "PC1662" but with minor component changes to reflect the different drain
voltages. So, the 4 FET compliment in the 1.1Kfx amp is "overkill" for the
600 watt output while being just right for their 1.5Kfx (1000w) amplifier.
As usual, Elecraft got it right using two of these FET devices in the
KPA500!
73,
Jim Robbins
N1JR
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