ICOM provide two RCA/Phono sockets for an external amplifier’s PTT and ALC connections but NOTE the PTT connection shorts to earth which is NOT what all amplifiers want. My RM Italy HLA300 wants two connections connecting together, not earthing. Some rigs, like my Kenwood have a remote socket that provides this capability but the ICOM does not, hence you need to have an external relay trigger by being earthed and its isolated contacts then turn on the amplifier. I have used a reed relay for this job as the current involved is low for its contacts and the current draw from the relay coil is minimal. As the reed relay is also very small it simply sits in the cord between the ICOM and the amplifier. The best way to power this relay and trigger it is to wire it to the 13 pin accessory socket on the ICOM between the “SEND” connection and the 13.8v supply pin as shown by COM in their larger manual (from the CD). The signal diode across the relay is to protect the rig from any counter-electromotive force from the relay coil.
The ALC feedback from the linear can also be wired from the ACC socket if needed.
ICOM include a 13 pin plug with leads already wired from each pin, if you are not happy soldering multiple contacts into a tight 13 pin DIN plug, you can simply extend the required wires and cover the joins with heat-shrink wrapping. The colour coding of each wire to which pin it is connected is given in the IC-7300 manual.