There was an old Lincoln Tombstone AC welder. Wanted to upgrade by adding a DC rectifier to the system and then wanted to be able to easily switch between DCEN and DCEP (DC Negative/DC Positive). And that would easily allow me to switch back to direct AC if needed. Old and new technologies are together again. And I didn't have to buy a new DC arc welder to burn out the 7018. Yes I know they have the 7018AC. But they're just not like DC.