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TIG Welding an Aluminum lap joint uphill with nothing to prop on but the hot aluminum.
Mike Zanconato shows Tips for Silver Brazing bike lugs fittings. These same tips and techniques apply for most any silver brazing task.
There is a bunch of data that states 4130 chromoly tubing .120" and under does not required a preheat or post heat treatment. But a preheat won't hurt.
There are several different techniques that work for tig welding coped tubing.
This video explores a few and shows instructive arc shots.