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Another peculiar problem I encountered with this 12-string was that the string heights need to vary from string to string both at the bridge and at the nut. At the bridge you want all strings to have the same approximate height so they can be strummed equally. In order to do this you need to sink the bigger strings – the E, A, D and G – farther into the bridge so that the tops of the strings are uniform with the tops of the matching octave strings. At the bridge, the big strings are sunk down into the saddle more than their octave mates, but at the nut the opposite is true – the smaller strings need to be sunk down to the approximate bottom of the bigger strings next to them so they hit the first fret at the same time when pushed down. That means 24 height adjustments, each taking six to twelve filing and testing excursions.
For cutting the nut, I alternated between the side of my fine jeweler’s file and a broken piece of a jeweler’s saw with masking tape for a handle. Be sure you point the slope of your string trough down towards the end of the headstock, so your point of contact is highest at the front, or fingerboard side of the nut. Without this slope, your strings will sound thuddy. It’s the same for the bridge; be sure to slope the bridge back toward the pin holes, so the string passes your compensation at the highest peak and goes down from there on both sides of the peak. This will insure that you have a lively sound to your strings, not mud.
We’re almost done, but two cosmetic concerns need to be addressed: the three holes in the butt of the guitar from the tailpiece screws and the indentation on the top from the tailpiece.
For the tailpiece indentation I used an old trick for wood swelling my dad taught me years ago. Place a large drop of water in a wood dent, making sure that the surface is level. Also be sure to have your close-up glasses on and a steady hand. Heat up a soldering iron and steady the iron at the top of the water drop you placed in the indented trough. The water will sizzle, but continue holding the iron steady – don’t lower it into the drop or you can burn the top. It will stop sizzling when the swelling is completed. Remove the iron and wipe the surface with a paper towel. Repeat this procedure across the length of the indentation. If there is still an indent, get some Sherwin-Williams nitrocellulose lacquer and drop-fill the trough, waiting a day or two between applications.
When your level has risen above the top of the surrounding surface, stop and wait a week before sanding it down, as it can continue to shrink. Mask off the top next to your drop-fill and wet sand with 320 grit paper. Slowly sand and wipe the fill using a light bulb reflection to inspect your progress after every three or four sanding swipes. When you are about there, go to 400 paper and do the same thing, then 600, 1000, 1500 and finally 2000 grit paper before removing your masking tape.
The final visual detail is filling the screw holes. Mahogany doesn’t come in toothpick sizes since it tends to splinter and fall apart in small pieces, so a tedious plan B is required. First cut some small slivers of mahogany into tiny squares and glue them to the cut-off end of a toothpick with Super Glue [Photo 16]. Once it has set up, the entire mahogany plug is soaked in Super Glue to form a bonding agent in the wood and keep it from splintering when shaped in the next step – have some Super Glue remover handy for this part.
Once the mahogany plugs are dry, a Dremel tool with a small, circular, fine-grained surface wheel was used. I placed the Dremel in its router holder and clamped it to a workbench, then used a rheostat to slow down the speed of the Dremel, slowly spinning the toothpicks and grinding them in either circular or oval profiles while comparing them to the holes needing to be filled [inset photo]. Leave the plugs slightly too large and gently tap them into place – I used the chuck side of a drill bit to drive the plugs in with a small hammer. Be sure to send the plug in far enough so that it’s flush with the original wood under the finish, and try to have the grain in the plug going in the same direction as the original wood [Photo 10].
Once the plug is installed, turn the guitar on end, add a drop of Super Glue into the hole and let it set for a few hours. I like to tape the guitar to the table with masking tape and put a piece of carpet under the headstock on the floor. Use aniline dye mixed with some lacquer thinner to match the color of the finish. If you have trouble finding aniline dye powder, try International Luthier Supply in Tulsa, Oklahoma. Try to achieve your color in the lower layers, then when dry, drop-fill the clear over the top. Again, get the little bump above the surface and wait a week to complete your sanding and polishing.
A New Sound
Although I had added enough extra bracing and a bridge plate to do what I considered an adequate job, I was still determined to stabilize the bridge, à la the Bridge Doctor. If you think about anything long enough before you start, a solution will usually present itself. It eventually occurred to me that I already had two bridge nuts ready, willing and able to accept some matching all-threads from top or bottom. All-thread is just that, a 20” threaded rod with no head, generally cut to the desired length. A plan was then hatched to make my own version of the Bridge Doctor. I returned to the hardware store and procured some nuts, lock-nuts and washers for my allthread, and two nylon bushings or washers to inlay inside my spruce span. I also picked up two square 6/32” nuts for inlaying into my spruce span so I could adjust my allthread inlaid dowel.
It took care of my structural problems and bridge pull, and for that I give it high marks. As for the tone of the guitar, the sustain is markedly improved and the midrange is enhanced and louder. But as I suspected, the top flutter is inhibited to some extent, so bass response is slightly diminished – perhaps just 10-12 percent, but enough to be audible.
I should note that after a month of string tension, my epoxied bridge rose up. I slowly removed the bridge with heat again and used a lightbulb at close proximity to soften the epoxy, which I then scraped off and sanded. I re-sanded my top and re-glued the bridge using Franklin Titebond. I clamped the bridge in three places, continuously wiping off the weeping glue with a damp paper towel. I am happy to report that the bridge now holds firm and sounds even better.
This guitar is certainly happier now that it can be used for performing again. When compared side-byside to a new Martin D-28 12-string, the old Gibson is louder, has better bottom- end and more richness – something that left me rather surprised. The sheer age of this instrument must make up for the traditional shortcomings of a mahogany guitar – perhaps because these ribboned Brazillian mahogany bodies started with 20-year-old wood, they may have some extraordinary tonal mojo.
Would I recommend performing this conversion on any mid-sixties Gibson B-45 12? It was way too much work but ultimately the guitar was worth it. This guitar – previously banished to a closet because of its disappointing sound – now sings.
|John Southern is a professional photographer, musician and luthier from Tulsa, Oklahoma. You can reach John at firstname.lastname@example.org.|