That amp uses either RF Sense keying (COR)
OR remote keying through the RC jack on the
back of the amp.
http://www.ko4bb.com/Manuals/08)_Misc_Ham_Equipment/Mirage/Mirage_A-1015-G/A-1015_Amplifier_Early_Model.pdfIf you absolutely must key the amp through it's phono
jack, use the ACC connector on the back of the 703.
You will need to setup a small relay to key an amp,
as shown in the 703's Manual on Page 16