Mac, by looking at those examples they appear to have a lot of heat put into them so I'll give my thoughts on welding small diameter tubing.
My experience is very, very similar to Graham's. I have no experience with a "micro" torch and have done ALL of my TIG welding with my #20 series torches over the years. I did a lot of chassis work on sand drag quads and sand rails where I was using everything from .5" diameter/.049" wall 4130 chromoly to .875" diameter/.065" wall 4130 tubing for the ATV chassis and upwards of 1" diameter/.065" wall to 1.5" diameter/.083" wall 4130 chromoly tubing for the sandrails. The 20-series worked great in all situations. A few years ago I replaced my standard 20 series with a CK 20 series FlexLoc style torch (230-amp version) and absolutely love this torch and cable setup.
I also recommend that you try using smaller diameter TIG rod. The old standard of using similar thickness TIG rod as the parent material kind of goes by the wayside when you get to the thin stuff. I found out the hard way and was burning holes in the tubing or getting excessive burn back at the edges when I was first starting out building chassis using thin wall chromoly. I began wondering if I was having to put so much heat into the parent material to get the rod to melt was the issue so I went to smaller diameter welding rod and BINGO, that was the answer. I found that it was easier to push more smaller rod into the molten puddle than it was to try and just dip a "little" bit of the larger diameter rod into the puddle.
For pretty much everything on these chassis I was using .045" tig rod. For the 4130 chromoly I was using ER80S-D2. When I would get down to the .049" wall thickness 1/2" diameter tubing I would use .035" rod and when I got up to the 1.5" I would sometimes jump up to the .062" diameter TIG rod. Depending on the joint and whether I was using .083" or .095" wall thickness would usually depend on whether I would jump up to 1/16" rod or stick with the .045" rod.
Also, try cleaning your joints with either a red Scotchbrite pad or emery cloth and then a final wipe with acetone of the joint as well as your TIG rod. Believe it or not, this final wipe does help and is quick and easy so really doesn't affect proficiency for what you gain in overall weld quality.
I use a gas lens on everything and bounce back and forth between either 3/32" tungsten and 1/16" tungsten depending on the wall thickness and amperage using at the time. The majority of the time my TIG torch has a 3/32" tungsten and #8 cup but there are times I will step down to 1/16" tungsten and maybe a #5 cup but always a gas lens. When I need more stickout and/or welding stainless I have a couple of the Furick #10 and #12 cups that I purchased to try a few years ago and I really like them for gas coverage, especially on stainless steel or in those situations where I need more stickout then normal. I like to keep my stickout no more than half the diameter of the cup but that can be fudged a bit on occasion and by situation. For tungsten I switched over to using 2% Lanthanated several years ago and now I use it 100% of the time.
**On edit, one more thought came to mind.
When setting up joints, I use a method called "blast tacking". This is where I turn up my amperage almost another 50% more than I would use for welding a particular joint. Once the components are placed in position I hit a few areas with max pedal and the arc jumps the gap and "blasts" a high energy burst to the metal. This will create a very small tack with no filler. I prefer to use this method when I can so when I start my welding sequence I can weld directly over the tack without having a big bump where the tack weld deposit was made when using filler rod.
Also, where you place the tacks is important to help the flow of weld around the joint. I like to look at 3 to 4 tacks placed around the sequence of where I will be welding that won't intersect a start or stop point. I have found that if you have a tack holding a joint together and then you light up on that tack with your arc to start the first weld in the sequence, as soon as the tack goes molten it can allow the joint to move and a gap open up. I will start other than on a tack.
Same thing when plate welding. I may put a tack on the end of a joint where it is easy at first but then I will add another tack inboard of the end about a quarter of an inch or so. This way when I light up on the end and that outermost tack goes molten the joint stays where you put it. At the other end of the joint sometimes I will add a bit of a larger tack before beginning the welding sequence. This way as I progress towards the end with the weld and the heat is trying to run off the end of the part, that larger tack will act as a dam or sink to prevent the end of the part from melting back.
Hope that all makes sense, it was hard to get my fingers to relay what I was trying to say.
