The Moss guy is wrong. The 1978-79 Midgets were the ONLY Spridgets with a ballast resistor. All others are internally-ballasted. No, it is not determined by the distributor (other than some of the electronics), it is determined by the car's wiring and solenoid. The 78-9 Midgets have a WLG wire from the solenoid to the coil which bypasses the high-resistance wire used as a ballast. This gives a hotter spark while trying to start the engine. Upon releasing the key, the ballasted wire takes over the task of powering the coil.
ALL Spridget coils are ballasted. The 3-ohm coils have the ballast IN the coil; the 1.5-ohm coils expect an external ballast resistor. There is a fancy electronic theory explaining why which boils down to the fact that physical laws governing the transformer action of the coil cause the ability of the coil to saturate the windings fast enough for high-rpm sparks goes down as the voltage goes up. Around 9 or 10 volts winds up being optimum for the output voltages and sparks per minute that we need, so the voltage to the coil is stepped down accordingly.
You need a 3-ohm coil for your 73. Pertronix does make one.
David Lieb
David Lieb