How many turns did you have on your secondary side, and what size wire was it? The less turns, the lower the voltage, but the higher the current you will get out of the winding. Assuming that the transformer is 100% efficient, it isn't but assuming this will make it easy, then the VA (volts*amps) going into the primary side will equal the VA available on the secondary side.
When welding, you don't so much care about the voltage as you do the current. It's the current that will do the welding, and the current will also determine how fast you can weld. The faster you can weld, the less heat build up you will get. The videos I've seen only use a few turns at most on the secondary side and those turns are done with some really BIG wire.
Don