y block coil

If it is a 1955 or earlier and has not been converted to 12-volts, you do not use a ballast resistor. 1956 was the first year for 12-volt electrical system in Ford's.
If you have converted to some type of electronic ignition, follow the manufacturers instructions.
 
Also look on the coil itself, esp if you aren't sure where it came from.
Some have a printed label...

"Use with external resistor"

or

"Internal Resistor"
 
Originally, all '56 and later Fords were 12-volt and required a ballast resistor. Again, though, modifications to ignition systems using aftermarket parts should alert the owner to make certain that those components are complete and compatible.
 
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