Either the mosfets used in the amp are 13n10's - or more likely IRF-520N's; but RM Italy calls them 'RM-3'. But they are the same thing as the other mentioned mosfets and will work the same way. In either case, they are about a buck apiece, or just slightly more. These are not high end RF devices. In fact, these mosfets weren't even originally designed for RF use, they were designed for switching power supplies. But some goobers discovered that they do work as RF devices @ 27mhz.
Disconnect the coax and the DC power. Jumper 12 Vdc to the coil of K1 and read the resistance across the contacts, When energized the contacts should read nearly ZERO Ohms,
Hey Tallman; isn't the freq range unusable for the IRF530? I don't recall. The spec sheets use a 1mhz test and doesn't show max freq/rf; so I don't recall if these devices will even work @27mhz. We know the IRF520 will.
You have to examine all of the parameters like gate capacitance, and rise time will be the most important. The way IR use to number their parts was like the 520 would handle 50 or so volts Drain to Source. then the IRF530 would work with 100 VDC. It was more for the voltage with stand rating. Those changes usually meant differences in response times and input capacitance. Compare the rise times of the two parts. Different Manufacturers will have different specifications for the same part number. IRF will be different from SGS, or Vishay and so on. And sometimes they will vary from batch to batch from the same manufacturer. Also they say some of these readings are typical.
