What he said...
It does look like 1/2" copper and it does look soldered...
This sounds like it's a temporary thing, so another approach is to affix a 1/2" threaded fitting on the end of pipe...and buy a piece or pieces to convert that to a hose fitting...and when done, just screw on another 3/8" valve like you have to it. Use a few wraps of teflon tape.
As desperate as this sounds, in comedy, 'likes' and 'views,' 'friends,' and 'retweets' really help. If I've ever helped you, please help me by friending, liking, viewing, and retweeting. It makes others think I have a following...it'll take you a minute or less. If you could, I'd really appreciate it!
A recent set--NSFW audio: https://youtu.be/_dcxcZfD6dg
Mangofy your E46!