I got my set of light gauge strings today (had been using medium) for my SG-12. I started with the 7 string, lowered the bridge screw 2 turns after removing the string, then put the new one on. Everything was going well until tuning... Using the 6th fret to compare against the 8 string (I'm in mirrored 4ths) and when I get it tuned up to B the string unwinds/stretches and drops back down to Ab. I try tuning it up again, and it happens again...

Have I b0rked the string and need a replacement?

If so, how do I keep this from happening the next time?

It kind of sounds like maybe you didn't have enough of the string through the tuner. I had issues with my first string change on my ten string so I can only share from that experience.

When I change a string set I follow this procedure every time and it always works:

Beginning with string one, I unwind and replace strings one at a time as I move across the fretboard.

As I replace strings, I feed it through the tuner until I have a few inches of slack at the bridge. I then put a slight kink in the string at the tuner and begin winding with one hand while holding the other end taught until the ball reaches the bridge. The kink helps the string grab on while winding and locks the string in place. I usually wind the string both above and below that hole in the tuner; this also helps the string stay put by laying the string over itself. I usually wind up with a 2:1 ratio, for every two full turns I raise it above the hole for one turn, etc.

Once you have enough tension on the string to hold it in place, find somewhere near the middle of the fretboard to gently pull upward to stretch the string. Don't over do it, just a little pull. It is said to help the strings settle and keep in tune until they where in fully.

Tune the string NOW. As long as you have enough string at the tuner, your slippage won't occur. The bad news is that you will most like need a replacement for that one, unless you haven't cut it yet. You should be able to repeat this procedure until you are finished. I bungled my first string change; it sucks, but I learned not to over think it. There are a few things I would suggest to keep in mind as you go forward.

You will most likely never have to change any setup components to change strings, such as bridge screws, flaps, etc.

The string tension is WAY less with these instruments and if you have a background with guitar or bass, do not have the expectation that you will have that kind of tension, you will break strings. (don't ask me how I know).

Good luck, I hope this helps!
Steve

Yeah, you may be right. I followed the example of the previous 7 that was on there which had no full windings around the tuner, and I guess you can't do that with the light gauge!