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The best tip that I got for making good, clean plumbing solder joints is to not use the flux that Home Depot sells. Go to a plumbing supply house and use what the pros use. I use Everflux.
Saw on AvE's YouTube channel where he made an emergency voltage divider with a pencil. Pulled 5 volts out to charge a cell phone. It was one of those "why didn't I think of that?" moments.