Did you buy the thing new? It will give you the input current. Most spot welders draw around 25 amps @ 220 volts. The cord is probably very short and that is how they get by with light cord. at 50 feet, you will have to quadruple the wire carrying capacity.
8 gauge is the very minimum you can use at 50 feet. The extension cord will probably cost more than you paid for the welder. I use 4 gauge on my 50 foot cords. They cost $15 a foot.
