I use purple tungsten's for AC welding at work and at home, I've had good results with the 3/32 and 1/8 inch from Midwest tungsten co.

Probably what you need to do is leave the regular insulator on and one of the gas lens insulators will piggy back to it, I have a 26 torch on my dynasty set up that way

I'm not sure sure on an htp, but you may want to turn your frequency to 120 and your balance to about 60. You don't look to have very much cathotic etching going on, and it looks like your cups dragging in your weld

I would cut a slot in it with a thin 3" cut off wheel , then bevel the edge lightly wire brush it good, preheat it to drive off moisture light up on it when both sides puddle stab filler right at the base of the puddle an move out putting the rod right in the little hole in the base of the bead...

Sometimes I've found it to be moisture on aluminium

Today I made an aluminium man way ladder

Just saying hello