Back contours for an A5

[Bob Schmidt]

I am currently working on an A5. As per past suggestions I bought Adrians F5 plans and tha GAL snakehead plans. I am targeting the back thicknesses from the F5, but the contours are pear shaped and forward of center compared to the A model. Should I modify them to shift them back towards the center, or even go with the contours from the A model.

[Rob Roy]

Someone more knowledgeable will likely chime in. From what I've read, the back graduation is left a little thicker in the area approaching the neck block. This is both a structural and tonal consideration. I did read that one builder found that leaving a little extra beef there contributed to the mid-range bark that the old Gibsons were known for.

I would leave it on the heavy side and string it up in the white and see how you like the bass response, especially if you're aiming for a strong bluegrass sound. You can always take off a little more around the recurve area if it's too tight and not as resonant as you like. …but you can't put it back.

[HoGo]

The traditional Gibson arch was always high towards neck block and didn't strictly follow the outside shape. This implied the thickness above neck block and sudden dip in inside carved area where the block ends. The recurve disappears towards the neck block on the outside and plates are thicker there as well. I suppose they used similar starting patterns for tops and backs on oval holed instruments (thick under fingerboard and no separate fingerboard support piece) and they didn't evolve too far away from that form even when the elevated boards and backs without the "ridge" down center seam appeared in 20's. You can carve recurve all around the back (or top) and graduate to "speaker" cone but that will shift sound away from traditional. You may like it or not...