I would go back to square one - what is the actual problem?
I noticed there was a problem when I realized I was avoiding the 12-string because it didn't sound good anymore. For some reason, I didn't try to analyze why. I recently took it out, tuned it up, and played it, paying attention to what I was hearing. When I'm fretting chords it sounds out of tune. So I checked it with the tuner. Open strings are in tune, but when I fret a string it goes sharp, even on the first fret.
To measure the action: fret the low E string at the first fret and then measure the distance between the top of the 12th fret and the bottom of the low E string - repeat for the high E string
Low E = 4mm
High E = 3mm
How do you play? If it is a 12 string I am guessing you will be strumming - in which case you will actually get a good sound (and more volume) with a higher action so do you really need it much lower?
You guessed it--I strum it. I had already tightened the truss rod some, and it does sound somewhat better now. When I check with the tuner, I can tell each string still goes out of tune when I'm fretting it. The lower strings tend to get progressively sharper the higher I fret. The high strings stay about the same amount of sharp throughout.
There is no problem at all having a completely straight neck or even a reverse bow
You will just need a way to measure the effect of adjusting (tightening) the truss rod. This may help lower the action in the middle of the neck but may lead to rattles and buzzes at the headstock end of the neck - where you are most likely to playing a 12 string I would have thought??
Well, I now have my measurements of the E strings, so I can compare after I tighten the truss rod, if that's what I should do.
I would like to see a photo of your bridge saddle (the bit of white plastic type material your strings rest on at the bridge)
Attached
I would be very surprised if you can't get enough of an improvement from adjusting the truss rod and saddle height. I can guide you through doing both of these to see how it goes but I do need to understand "the problem" and the current height - as a really low slinky action on a 12 string may not actually be desirable depending on your playing style, and I am guessing you will not be playing many speed lead runs at the 12th fret on a 12 string acoustic!?!?!?
Does it even matter what the action is above the 5th fret on a 12 string?
Perhaps you would be better off looking at filing / replacing the nut to reduce the string height at this end of the neck to make the action in this region a bit lower?
I would still look at the neck first, then the saddle, then the nut
I've attached the photo, but it sounds like I should probably tighten the truss rod some more as a next step? Is it ever okay to tighten the truss rod without loosening the strings? Not that I could do that--the end of the truss rod is inside the body.
Thank you, thank you....
PS: Sorry, no pic after all. I get this message every time I try to post with the attachment:
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