@aaronpk Not that I'm an expert at soldering... but I believe you have to 'tin' your wire first - before you solder it to the thing you intend to connect it to. "Tinning is a process of using a soldering iron to melt solder around a stranded electrical wire."

Yeah in any case that 12 gauge is way too thick to fit into the barrel jack so I need a smaller wire

LOL... to be fair, I haven't soldered anything since I was 16 years old.... but I guess it's a bit like riding a bicycle?!