Several fellows have asked about using the FTdx3000 with the Elecraft PKA500. I think you will find K2WU's observations helpful. These are also available on the Elecraft Reflector.
I use the 15 pin Linear jack on the back of the FTdx3000 to accomplish a few things. First of all, I use Yaesu pin 2 (TX GND) as the key line for the KPA500, rather than the Elecraft's separate phono cable. Secondly, I used Yaesu pins 4, 5, 6 and 7 to supply the band data to the amplifier rather than rely totally on the KPA500's auto-sense of the frequency. And, of course, pin 3 is the common ground for all of this. At least on the FTdx3000, ignore the KPA500's instructions about Yaesu pin 8 (TX INH)--just just leave it alone.
Also, I use Yaesu pin 11 (TX REQ) and ground out to a separate push-button switch. When pressed, this forces the FTdx3000 into a CW carrier (regardless of current mode), and I've set the menu #178 to let it output only 10 watts.
So in use, I put the KPA500 to operate, press my push-button switch which puts 10 watts out to the KPA500, which then generates about 200 watts into my tuner, which I quickly adjust if necessary. Let go of my push-button switch, and I'm good to go with about 25-30 watts preset on the FTdx3000 power.Jim, K2WU
