If a coil is labeled "12 Volts" or "12 Volts NOT for use with Ballast Resistor" or "12 Volts NO Ballast Required" etc on a 12 volt tractor YOU DO NOTTTTTTTTTT USE A BALLAST OR THE SPARK WOULD BE VERY WEAK....
If a coil is labeled "6 Volts" or "12 Volts for use with Ballast Resistor" or "12 Volts requires Ballast" etc on a 12 volt tractor YOU NEED AN EXTERNAL SERIES VOLTAGE DROPPING (12 to 6) BALLAST or else the coil overheats and the points burn rapidly.
Om many 12 volt tractors a 6 volt coil plus an external ballast is used so during cranking the ballast can be bypassed (like an extra I terminal on starter solenoid etc to feed coil direct bypassing ballast while starter is engaged) which helps cold weather starting.
If your ballast was by passed someone likely put on a 12 volt coil (instead of correct 6) and did away with the by pass circuit. It will still work okay as is with poorer starting spark in cold weather as if wired correctly with 6 volt coil and solenoid by pass
Contrary to what many lay persons believe, you most likely will NOT find any discrete stand alone "resistor" hidden away inside the coil can. A 12 volt coil has around 2.5 to under 4 ohms primary resistance while a 6 volt coil has more like 1.25 to 2 ohms
Myself and Bob M and Janicholson have explained this hundreds of times on here do a search for more info if needed
That help?? Any more questions?? See URL link below for more