new ballast resistor died in 10 minutes

installed new voltage regulator and new ballast resistor at same time, engine started dying after 10 min run time, cooled it down, same thing again, put 10 yr old resistor back on and it runs fine. I think I read somewhere after this they are supposed to be done separately, something about burning in the new regulator first? any thoughts or experience with this or maybe it was just a bum resistor?