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  #1  
Old 06-17-2017, 04:01 PM
gregom gregom is offline
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Default 12 string archtop

I have only built one arch top guitar (6 string)and am interested in building a 12 string acoustic. I think I want parallel braces . The one I have currently finished has dimensions (17in) and x bracing out of the Bedenetto book. I have limited knowledge other than the one I have completed. This one does not have a bright bass tone. Other than that I am happy with the way it turned out. I want a nice bright tone.
My question is what, if any, thickness are changed with the top & back and bracing for a 12 string arch top.

Is there any dimensional information on 12 sting arch tops out there ?
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Old 06-18-2017, 06:59 AM
MC5C MC5C is offline
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If I was to build a 12 string archtop, the first thing I would consider is of course string tension. With a flat-top guitar, string tension (simplistically) tries to rip the bridge off the guitar, and at the very least tries to flop it over, creating a dip in front of the bridge and a rise behind the bridge, all of which can be braced for. It also tries to move the neck block and the top of the guitar in the upper bout. On an archtop, the string tension is terminated at the tail block and tries to squash the body in on itself, and it creates a significant downward force on the top.

I would address the string to tailblock loading issue by installing a longitudinal brace (I have used a 1/2" maple dowel) directly between the top of the neck block and the top of the tail block. It floats under the top, doesn't touch, and will take up a substantial portion of the string tension that normally gets taken up by the top alone.

The other issue is the downward force of the strings acting through the bridge on the top. This can vary widely, but with "normal" archtop geometry and 6 strings is usually between 35 lbs and 55 lbs. So double that for a 12 string and you would see 70 to 110 lbs of downward force on the top. I don't know about you, but for me that is a lot of force. I would do two things to mitigate this. The biggest thing is to reduce the break angle of the strings over the bridge. You do this in two ways, first by reducing the neck angle and lowering the fretboard closer to the top, thus allowing a lower bridge height, and second by raising the pivot point of the tail-piece as it is fixed to the tailblock. A normal break angle over the bridge is around 12 - 14 degrees, if you reduce the break angle to around 7 - 8 degrees you reduce the downforce significantly (I think linearly, so 14 degrees to 7 degrees would be half the down force). Others have done some work on this, and 7 degrees is about as low as you can go and keep the strings firmly on the bridge when playing. I've done some work on this, and found that reducing downforce from 55 to 35 lbs significantly opened up and improved the responsiveness of the top. The downforce is like a pre-loaded spring - if you crank the spring down, everything gets all stiff and bound up, and can't react as well to the tiny forces from the string vibration.

So I have written a tome. To summarize, brace the body separately from the top to accomodate the string tension load, and reduce the downforce on the top with string break angle over the bridge. Bracing of the top can be anything, and I would think about putting the soundhole in the upper bout rather than F-holes.
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1935 Dobro model 25 resonator
1943 Paramount (made by Kay) mandolin
1946 Epiphone Zephyr electric archtop
1957 Hofner Senator archtop
1962 Gibson Melody Maker electric
1963 National Dynamic lap steel
1996 Landola jumbo
1998 Godin Artisan TC electric
2003 Epiphone SG electric
2010 GoldTone PBR-CA resonator
2015 Evans electric archtop
2016 Evans archtop

Last edited by MC5C; 06-18-2017 at 07:05 AM.
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  #3  
Old 07-18-2017, 06:45 PM
Backcountry Backcountry is offline
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Default Here's one

https://reverb.com/item/4768175-hofn...1960s-sunburst
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