When I had my old transformer machine (BOC ADR 300) for Aluminium welding I found that at lower amperages (less than 120ish amps) I needed to keep the tungsten closer to the work than I was comfortable with to keep a stable arc; welding really thin stuff (16swg) the arc needed to be shorter still if it wasn't to 'dance' to either side of the joint when tacking. As I don't have by any means a rock steady hand, I always kept a row of ready sharpened Tungstens on the bench for the inevitable dip when doing this.