[Bluebell]

Confused. Me too.
Here are four common system variations, that I understand.

1. A Ford 6 volt coil is actually a 4.5 volt coil that needs the in dash resistor.
2. An aftermarket 6 volt (true 6 volt) needs no resistor. (bypass the ford resistor)
3.A 12 volt coil can run on 12 volts no resister and no ballast.
4.A ballast coil for a 12 volt system is in reality a 9 volt coil that is used in conjunction with the appropriate external ballast.

The resistor in No1. is Fords way of aiding starting from cold because as the resistor heats up the voltage to the coil drops to the 4.5 volts from the 6 volts it gets when the resistor is cold.

The ballast system is another cold start aid for a key start system. When the key is in the cranking position, the ballast is bypassed, feeding the full 12 volts to the coil. when the key is released the feed to the coil is directed through the ballast resistance and so the coil is fed only 9 volts while running.

I understand that the electronic systems need a special coil?

Bubba, can you please answer these for me;
1. Are there coils with built in ballasts, or am I missing something?
2.What are the applications (when are the different ones used) for the three different primary?) resistances that you mentioned?