Alright, for those interested in the thread; I've found my answer. Old, linear style power supplies ran at 60Hz, but also imposed a high frequency, high-voltage, low current wave on the same wire. This is the "high-frequency" circuit. It kept the argon gap ionized, which gives it low resistance. Once that's happens, the source puts a low voltage, high current, low frequency current across the gap, which heats the metals.
In a typical welder, you might see the HF circuit running 1Mhz, 5000 volts, 15ma. The "skin effect" that occurs at these higher frequencies makes this high voltage fairly non-dangerous to humans. Once the arc is established, welding current can flow through it. Typical welding current would be more like 15vac, 60Hz, 110 Amps.
Inverter style machine typically run the AC output at more like 100Hz.
