Can I tune my guitar 3 whole steps down?
It's a big downtune and I've no idea why anybody would want to do so.
Your strings are going to flap against the neck like crazy.
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You can do whatever you want. But you might need to change strings.
The math is complicated, so I'll skip the full explanation. Basically, the pitch of a string depends on its length, tension, and the weight of the string itself.
Let's say you're using .046 for your low E, and you've got 16 pounds of tension on the string when it's in tune. Dropping it 3 whole steps takes you down to Bb... and it takes the tension down to 8 pounds. It's going to be pretty floppy. To keep the original tension, you'd have to increase the mass of the string by about 41% - which means using a string in the range of 64-65 gauge. You'll have to make similar changes with all your other strings.
My advice: if you really want to play in that range, get a baritone guitar. That's about halfway between a guitar and a bass in scale length - and you'd only need to downtune it by a half step, which won't require changing gauges.
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I also have a regular 6 string Guild D55......I use light Phosphor Bronze EXP's on it & have no problem with tunings like:
Drop & Double Drop D, Open D, DADGAD, DADDAD, Open G.....it works well.
When I was playing Hedges type tunes I was playing mediums & find little difference except playability is better with the light....tone is a miniscule amount better with the mediums, but the playability trade-off is worth it for me...I have crappy hands.
You will be able to if you put thick enough strings on
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