i am thinking of getting a 7 string and tuning it down 1/2 a step (to Bb). i very much doubt i will go any lower than Ab, so what scale length would be the best to do this?

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27,5" would be cool, but it will be a little too long for your personal taste....

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my real question is - is 25.5" long enough for Ab?

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Yes, you'll just need thicker strings than you would for standard tuning.

There are plenty of longer scale 7 strings around. Jeff Loomis' Schecter for a start.

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Yeah I just use 66 gauge strings on my 25.5" seven tuned to G. So it's fine.

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my SG is 24.75 with 70 guage strings and sounds great in Bb so you'll be fine

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The big question would be.. do you want an extra string, or tune lower?

If you just want to tune lower, you might as well try a baritone.
An extra string isn't always the most comfortable neck to play on.

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i got a yamaha with a 24.75 scale length and i love it. altho i dont use low tunings

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try out some ibanez 7 strings, the wizard necks and wizard II necks are amazing on 7 strings. I have a s320 with a x2n7 and paf7 and ill never give the axe up. I play in A but i go down to G with boomer tnts and a 60 on top and it feels so nice.