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.
Upcoming comedy shows: March 3 at Broadway Comedy Club, NYC, 8 p.m. March 5, Tierney's Tavern, Montclair, NJ, 7:30 p.m. See Facebook for other shows and PM me if you can make one...love to see you there!
Mangofy your E46!