#1] High voltage inside. Make sure you know how to drain this voltage before touching anything.
#2] Get a Cobra schematic so that you're attaching wires to the right spots.
This is a general discription:
#3] The OT's side with 3 wires attaches to the high voltage supply leg (center wire), and the other 2 attach to the complementary output tube plates' load resistors. These must be oriented correctly.
#4] Of the wires on the other side, one will be earth ground and the others probably go to an impedence switch, which feeds the speaker(jack). Again, they must be oriented correctly. Contact Mercury for the proper pinouts.
#5] Be careful !!
This is a typical connection: http://www.schematicheaven.com/ampegamps/v4poweramp.pdf