It's probably the relief you're seeing. It keeps the strings from buzzing against the neck. If it's too extreme let a tech look at it.

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Life...... It's sexually transmitted and always fatal

If its got a truss rod, adjust it.

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1984 Squier Contemporary Stratocaster
1986 Peavey Envoy 110
1967 Kingston Acoustic

The first thing you need to do is put some strings back on it. It'll probably fix itself.

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Tune Low, Play Hard & Floor it!

Why take the old strings off if you didn't have any new ones to put back on?

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Dennis in CR
Rockin' since 1963

+1 I misunderstood your post. I thought you restrung it and it had a slight bow.As Voodoo said, It will probably fix itself when you get some tension back on it.

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Life...... It's sexually transmitted and always fatal

When you put a new set on make sure that you use the lightest gauge available,which should be .009 - .042.The problem with the relief should remedy itself,neck adjustment is something that a beginner should be very very careful with as turning the trussrod too tight can cause damage that would be very costly to fix.It's best to buy a couple of books on guitar self-maintenance or look up some of the instructional sites on the web.

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'65 Strat,65 Mustang,65 Jaguar,4 more Strats,3 vintage Vox guitars,5 Vox amps,'69 Bassman with a '68 2-15 Bassman cab,36 guitars total-15asst'd amps total,2 vintage '60s Hammond organs & a myriad of effects-with a few rare vintage ones.

On an acoustic, you need to know if the neck is the problem or the bridge saddle is the problem. Sometimes people start cranking the truss rod when all you need is a saddle adjustment. I don't mess with my Martin. I'm less than an hour away from the factory. I let the guys who built it adjust it. I wanted the action a little lower and they did it for me while I waited. Very cool people there. Screwing the pooch on an acoustic can be an expensive mistake.

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Life...... It's sexually transmitted and always fatal

You're going to make it worse...

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Uneven pressure on the neck could possibly put a twist in it. You would have been better off leaving it be than putting on two strings.
Twelve string guitars are designed for the tension of twelve strings. Just go out and get a set, put them on, tune them to pitch and you'll probably be fine. Wht complicate the situation?
We're only trying to help you.

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Life...... It's sexually transmitted and always fatal

By only having two strings AND tuning them very high, you're going to twist the neck and absolutely lay waste to it.

_________________
Favorite bands:

Melvins
The Jesus Lizard
Cows
Big Business
R***man
Minutemen
Flight
Minor Threat
Big Black
Shellac
Karp
Scratch Acid
Wipers
Pixies

You need all 12 strings in either a set of 9's or 10's. This is what we were warning you about in your other thread about strings for a 12 string. You might have killed the neck leaving the strings off of it for a few days. Acoustic guitars are a totally different beast. 12 strings even more so. First thing Monday morning take it to a luither not a tech. You might need a neck reset after all of this.

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Your pain is the breaking of the shell that encloses your understanding. It is the bitter potion by which the physician within you heals your sick self. Therefore, trust the physician and drink his remedy in silence and tranquility.-Khalil Gibran

One should never do what you did. Change out strings one at a time.

Taking all six or tweleve strings off at once will not hurt the guitar. Taking them all off and leaving them off will do the harm. The only time you shouldn't take off all of them at once is if you have a non attached bridge like a mandolin or an archtop or a floyd rose system.

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Your pain is the breaking of the shell that encloses your understanding. It is the bitter potion by which the physician within you heals your sick self. Therefore, trust the physician and drink his remedy in silence and tranquility.-Khalil Gibran