FRETS.COM Thumbnail Article
Please click on the small images to see the large views.

From guitar to octave mandolin
8-String Conversion
© Frank Ford, 12/3/99; Photos by FF, 12/99

Every so often, a really fun custom job comes along. This one got my attention because it is a modification of a new modestly priced instrument for different functionality. Who knows, it might just point up the possibility of new limited edition Martin instrument.

The object is to create an octave mandolin with a much larger body than the usual Celtic style instrument. The low "G" is not far from the low "E" in standard guitar tuning, so it stands to reason that an auditorium size guitar body would sound good as an octave mandolin. For this conversion, I was not asked to make the neck narrower to accommodate four courses of strings because the player has large hands and finds himself cramped on the fingerboard of the more standard Celtic "bouzouki" or octave mandolin.

In this article, I've omitted some detail or procedures that I've covered repeatedly in other articles on FRETS.COM. Likewise, I've expanded on some features of the job that I haven't covered yet elsewhere.

The "subject" is a new 1999 Martin 000-15. It's all mahogany - with solid top and back. It sounds very good as a fingerstyle six string guitar. I hope it's as successful as an octave mandolin!

This is Martin's newest style bridge, with the bridge pins aligned at equal distances from the saddle. I like this design because it keeps a consistent "break angle" over the saddle.

A regular six string peghead. I'll keep the upper and lower pairs in the same place. There's room between them to add my extra tuners.

First, though, I'll have to plug the second and fifth tuner holes. I looked around the shop for a piece of mahogany to match the guitar neck. Here. I'm using a plug cutter to produce a couple of cylinders to fill the tuner holes.

Because the glue line and plug will show behind the peghead, I'm mixing some pigment with regular epoxy to match the neck stain as closely as possible. That's my "color sorting block" in the background.

Simply pressing the plugs in place, allowing the epoxy fill any gaps around them. They're a loose fit, because the holes are 10 mm. and my plugs are 3/8" in diameter. Close enough, though, for a cosmetic fill.

Holding my violin knife flat against the finish, I can slice off the excess without scratching the finish, which, by the way, is a catalyzed "cross linked" polymer, and MUCH thinner than Martin's nitrocellulose lacquer finish. I have to be super careful not to scar it, because touchup is near impossible.

The face of the peghead deserves a good treatment. This model came with no pehgead veneer. With this thin finish, it's a snap to sand it with a block to get a good prepared surface for my veneer.

Speaking of veneer, I happen to have a nice scrap of very thin Brazilian rosewood veneer, just right for this peghead. To make trimming and finishing easier, I want to use the thinnest veneer I can find.

I need to clamp it flat, so I use a piece of plywood and a whole fleet of Quick Grip clamps. I like these clamps because I'm using hide glue, which has a good quick "grab" so it won't slide around. Hide glue needs to be clamped quickly, so these clamps are ideal for the job.

While the peghead is under clamps, I can turn my attention to the bridge. I'm heating the bridge with my L.M.I. electric heat blanket.

Once the glue joint is heated, the bridge separates from the top easily.

Notice the original gluing area. The finish extends under the bridge a bit more than 1/8" all around.

I'll scrape the finish right up to the very edge of where the bridge goes, so I'll have the best gluing surface possible.

I'm changing the location of all the holes, so I'll overlay the bridge plate with thin maple to avoid any structural failure that might occur where a new bridge pin hole intersects an old one. I'll clamp the overlay with hide glue, of course.

I'm filling the holes with hardwood plugs. With the overlay underneath, these plugs can't loosen even if my new bridge pin hole intersects an old one.

Time to make the new bridge. I have an assortment of bridge blanks made up in advance. About once a year I machine up a whole bunch of these blanks, which are simply rectangular, with Martin's consistent one-inch radius cuts at the ends.

I use my half pencil to mark the outline of the old bridge on my blank. I cut it out, splitting the line, and sanding the bridge right down to match the old one. I chose a Brazilian rosewood blank that was a reasonable match for the Indian rosewood fingerboard, and my pehgead veneer.

I can rough out the profile with my 1" x 42" belt sander.

There's nothing like the flexible portion of the belt above the platen for rounding over the back edge.

See? It looks fairly even, considering I work freehand.

Here's the roughed-out bridge.

Now, for bit of block sanding to correct the shape, and final sanding with 220 grit.

I have two of these Baldor buffers. On one of them, I have an 8-inch buff for black stuff. I use it for polishing banjo parts, using metal buffing compounds. Here, I'm using a bit of black plastic buffing compound to buff the rosewood bridge. I don't want to use my white finish buffing wheel, because it tends to pack white compound into the pores of bare wood.

A quick buffing immediately reveals the smallest flaw in m final sanding job. Then, I can touch up my sanding, buff again, and I'm done. The buffing wheel also does a nice job of waxing the bridge since wax is the binder in the buffing compound.

I'm using my Japanese backsaw to slice off the tops of the bridge pin hole plugs. By pressing down firmly, I can make the blade bend to conform to the top of the guitar in the bridge area without touching the finished part of the top.

My sharp 3/4" chisel is just the tool to "snick" off that last little shaving to make the tops of the pins perfectly level with the guitar top.

To keep the bridge from sliding as I clamp it down, I use several layers of masking tape. The easy way to prepare the tape is to slice about four or five layers deep with my big knife.

Then I can peel off a thick layer with nice clean edges.

Holding the bridge exactly where I want it, I place pieces of tape to form little fences so the bridge won't slide.

Hot hide glue is my favorite for bridge gluing, and I use it whenever I can.

Letting the glue gel after clamping, I can "roll" off most of it, and clean up the rest with warm water.

OK, back to the pehgead. Scissors help me trim the end grain quickly.

I use a "paring" cut whenever it's handy.

And, by watching the grain direction, I can slice of most of the excess all around with my knife. After trimming the veneer as close as possible with my knife, I'll us a sanding block to bevel it a little, trim it flush, and (if I'm careful or lucky or both) not scratch the edges of the peghead finish.

To trim the nut area just right, I simply hold the original nut in place, and scribe deeply along the back edge.

By repeating this cut, I sever the end of the veneer without a chip.

A final sanding (220) grit, and I'm ready for finish. I'm brushing on some dark walnut paste wood filler.

Wiping across the grain helps keep the filler in the pores. The filler takes a day to dry.

So, it's back to the bridge again, this time to lay out the new bridge pin spacing. I plotted the saddle location as a reference first.

Drilling the holes for the bridge pins is a perfectly reasonable job to do by hand. After all, if I'm a bit out of alignment, I can make it up as I ream the pin holes to fit the tapered bridge pins. After this operation, I took the tape off, calculated the saddle location and routed the slot for the saddle.

With a bit of shellac to seal the surface, I laid out the new tuner spacing on the peghead. Measuring the spacing from the original upper and lower holes, I discovered that they were 3.25" on center. A quick bit of math, and I have my 1.08" spacing between each of the new holes.

The peghead is looking pretty good now, with the appropriate logo decal in place, and a few wipes of orange shellac to deepen the color a bit. Next, I'll spray a few coats of lacquer to finish the surface and embed the decal. The idea is to finish just the surface of the peghead and avoid confronting the original finish on the edges.

All done! After a few coats of clear lacquer, leveling, and a final coat of low gloss lacquer, the pehgead looks just about right. I've made the new nut and installed the gears. From the front, you'd never know this wasn't an original eight stringer.

If you look closely, you can see the little plugs where I filled the second and fifth string tuner holes. They're really not that noticeable, what with eight tuners, though.

The bridge works just fine, with eight strings spaced in four courses. This octave mandolin is strung with unision strings in all positions. With light strings, the overall tension is within reasonable limits, too.

Here's the full view. All in all, it sounds good, with a much stronger bass response. and a bit more subdued treble than the usual octave mandolin. This "0008-15" has good balance through its entire range.

Back to Index Page