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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.
Both Stick welding with 7018 and dual shield flux core are good processes but one is way more productive. If you are in business, that matters a lot.