Tuning my Gibson SG.

Firstly, IMO you're better off going up a gauge of string (or possibly two) if you've tuned to D - ie, if you were using 9's tuned to E, switch to 10's etc

Get a brand new set of strings and remove the old strings from your guitar and clean it. Thoroughly clean the bridge and nut, especially in the grooves.

A Gibson SG should have fairly decent tuners and shouldn't break strings, so it might be how you're stringing the guitar which is causing the problem. Take your time when putting on strings because this is behind about 90% of tuning trouble, in my experience for guitars without a locking temolo. Snip the strings about 1" past the tuning peg (don't leave the end flailing around past the machine head, you'll take your eye out and it can cause the string to slip). Generally I snip them just past the next machine head - give the E string enough lenght to reach the A machine head. Insert the snipped string and leave maybe.5cm sticking out past the machine head. Start winding the string using the machine head, don't wrap the string over, it won't sit properly and it'll twist the core, meaning you're more likely to snap the string. Twist the tuner in one hand and pull the slack string away from the tuner, back towards the bridge. When the wrapping string reaches the exposed end, guide the first wrap over the exposed end and all subsequent wraps under. Your tuner peg is more than likely shaped like )( - wrapping the first loop over and the rest of the loops under will lock the string in place, meaning it won't slip(the wrap over is naturally pushed down and the wraps under push up against the string end). No matter how long it takes, use the tuner to tighten the string, don't manually twist it over, all the whille pulling the slack away from the tuner. I can't stress it enough, don't wrap the string around the tuning post by hand, it will twist the string and make it more likely to break, always use the machine head to tighten the string all the way.

Tune the string to pitch and then pull it away from the fretboard over the 12th fret. You'll notice it goes massively out of tune. Repeat this until it stops going out of tune. Don't go nuts, just pull with reasonable force, not enough to break the string but enough to stretch it. Finally, when you're tuning to the pitch you want, always tune up to the note, never down, if you tune up past the note you want, tune way back down under it and tune up again. There is a certain amount of give in tuners which will cause the string to loosen slightly if you tune down to a note. This slack is not present when you tune up to the note.

If you change string gauges you'll need to work on the intonation of the guitar, and there was a thread about this a few days ago.

It shouldn't take more than 10-15 minutes to do this, but it's worth it. Let me know how you get on.

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