Cowman, some time back I answered your question in GREAT DETAIL so I have copied and pasted it below SINCE ITS SUCH A GREAT QUESTION.
Youre correct, a coil labeled "12 volts use with external resistor" assuming the "resistor" drops 12 volts input down to 6 for the coil, I LIKE YOU CONSIDER A 6 VOLT COIL. I bet if you used it a 6 volt tracotr with no ballast of course, it would operate fine and not overheat.
Heres a copy of my previous discussion:
First of all, 6 volt and 12 volt coils ARE NOTTTTTTTTTTTTTTT THE SAME what the parts dude told you ("all coils are the same") IS WRONG AS RAIN. A coil designed and intended for 12 volt NOMINAL use, one that will still produce a spark when battery voltage is reduced or when its raised due to charging, has "around" 3 ohms (range say 2.25 to under 4 ohms) of primary resistance while a 6 volt NOMINAL coil has about half that, "around" 1.5 ohms. THESE VALUES VARY SOMEWHAT BUT THIS GIVES YOU THE IDEA.
NOTE I’m talking typical old points and condenser ignition tractor coils here NOT after market or high energy coil or coils for elec. ignitions.
One of the main design limitation parameters in old tractor Kettering points and coil ignition systems is limiting the current the points must swtich to the 4 amp and less range, otherwise the points burn up too fast. THEREFORE: In a 6 volt system, the 6 volt coils low voltage primary winding resistance is around 1.5 ohms, meaning the current (the points must switch) is 6/1.5 = 4 amps In a 12 volt unballasted system, the coils low voltage primary winding resistance is more like 3 ohms, meaning the current (the points must switch) is 12/3 = again 4 amps. Now, some 12 volt tractors still used a 6 volt (1.5 ohms) coil BUTTTTTTTTTT they added a series 1.5 ohms EXTERNAL VOLTAGE DROPPING (12 to 6) RESISTOR which dropped 6 volts leaving 6 on the coil as designed for, so the points still only switch 4 amps and alls well. NOTE: Coils labeled "12 volts" or "12 volts NOT for use with ballast resistor" are 12 volt nominal coils. HOWEVER a coil labeled "12 volts requires ballast resistor" or a coil labeled "6 volts" ARE 6 VOLT COILS and require the ballast on a 12 volt tractor but not if only a 6 volt tractor. OLD WIVES TALES: Some mistakenly refer to some 12 volt coils as being "internally ballasted" HOWEVER if you disect that coil, on most, you WILL NOT FIND A DISCRETE STAND ALONE RESISTOR HIDDEN AWAY SOMEWHERE INSIDE THE CAN. The primary winding resistance of around 3 ohms is achieved due to more windings/turns or higher resistance wire NOT A RESISTIR HIDDEN INSIDE THERE. If a coil overheats it’s because its drawing more current then it was designed for and cant effectivelty dissipate the heat.
Nuff said, hope this helps John T retired Electrical Engineer
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