Step 11: Dimmer Control and Welding Thin Wall Tubing
That gave me very fine control over power. Marc Lander and I did some very nice welds as seen here. After a few we got good enough to do the same welds with 3/32" 6013 rods and no dimmer and not burn holes.
More tricks - I used my left hand to feed a piece of mig welding wire into the weld to add more metal in and soak up the heat. Here's Marc doing that. Any wire is fine for this, coathangers are traditional for muffler work. Sand off the paint first if you don't like fumes.
Stopping to eat lunch helped a lot also. Your welds won't be good when you're shaky and tired.
I got my variac for free, don't buy one for this, they cost as much as a welder.
A solid-state dimmer that's rated for inductive loads does the same thing and costs a lot less.
If you're feeling particularly fancy, you can add in your own scr-based switching circuitry to vary the power, like this guy did.