2SK408; 2SK409 FETs

Those are old Hitachi (now Renesas) 180V 2A n-channel MOSFETs meant for use in HF and VHF rf power amplifiers. The '408 had one package pinout, and the '409 another, likely for push-pull circuits. Both had their source terminal on the center TAB pin.

That's correct - the same parts with different pinouts to lay them one against another. The amp I need to re-build has 5 of them in each part of push-pull circuit.

What are the specs (or perhaps I should say, were) of your RF amp?

It is "present tense" - there plenty of them sail around the World.

Marine MF/HF Radio - JSS800 from JRC. Power output 250Wt Result of two such Amplifiers combined. Total # of such transistors -22 (2 in driving pre-amp)

Mayb they can source them for you. You can look at the specifications and see if one of the others would be a suitable replacement.

Aren'y you saying it's present tense for other folk's radios, but past tense for yours? At least until you fix it?

BTW, watch out for NEC's GaAs FETs, which had the same part numbers, but were microwave transistors in dramatically-different cases.

I am fixing them - "present continuous tense" - and want to find alternative source of these transistors. :-)

Besides the complementary pinouts, the thing that's unusual about these RF MOSFETs is their high voltage rating. Most of the RF power MOSFETs I use and come across these days have much lower voltage ratings. They have quite a bit of capacitance and so they must be run at low impedances with transformers or tuned networks to match thir low Z up to the 50-ohm world. As a result they don't need to work at high voltages. This is true even for moderately high-power RF transistors. So my question is, what's different in these transmitters that they need such high-voltage power MOSFETs?