A lil searchin found this:
https://vk2qr.wordpress.com/2014/11/25/mx-p50m-hf-amplifier-review/ "I have taken the time to check which connections are made to the FT-817 and they are as follows. The Black or in some cases Blue control wire from the amplifier is connected to ground or GND (-VE) on the radio and the Red control wire from the amplifier is connected to TX GND from the radio. So to operate the amplifier in transmit mode it is a simple matter of shorting the two control wires from the amplifier together when transmitting and open circuit them when receiving."
Is this the amp you have.....?
rgg