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Hello, dear readers. Today we are going to explore a bit of amateur lutherie. We will take an old, battered and broken flat-top guitar and rebuild it, creating a beautiful and playable resonator guitar.
Before and after
This Guild Model F-40 was given to me by my father around 1965 or so. I truly loved that guitar. It had beautiful tone and a feather light touch that was a joy to play. But in 1984, I made a terrible mistake. I moved from Oklahoma in a driving snowstorm to Arizona, arriving in 80 degree heat with no humidity, and I had forgotton to detune the poor guitar before the move. I opened the case to find the sad mess in the before image. The weather change combined with the string tension had broken the bridge and torn it right out of the top. The celluloid pick guard had come off and rolled up into a ball, much of the binding had come loose, and there were severe cracks in both the top and the back. This guitar was ruined. But I kept it all these years because I loved it and always believed I would someday find a way to salvage it. That day finally arrived!
A closer look at the damage. Cracks in the top plate run everywhere. There is no fixing this short of completely retopping it, which is an expensive job that would require a professional luthier.
But there is a solution!
We can convert this guitar to a resonator. By doing so, we can cut out the damaged top area, and since a resonator derives its tone from the cone system and not so much from the tonewoods of the body, we can repair the remaining cracks with internal patches. Best of all, this is a job that an amateur can tackle with reasonable expectation of success.
There are two basic types of resonator, the biscuit cone and the spider cone. For an amateur project, I strongly recommend using the biscuit cone design. The sound well is much less complicated and the final setup is less fiddly. You can find all the needed parts online. I bought mine from sellers on Amazon and eBay. You will need to get all the parts before you start cutting because the cuts must precisely fit the parts.
Here we see some of the parts and the pencil line I drew around the cone for the sound well hole:
The first step (and one of the most important) is to determine the original scale length of your guitar. The scale length is the distance between the nut (the piece the strings come over as they leave the tuning pegs) and the bridge. Notice that the bridge is NOT mounted straight across the guitar body. It will be tilted so that the high string end is closer to the neck than the low string end. That tilt matters, so try your best to get an accurate picture of it. (If your guitar has an adjustable bridge, that won't be possible and you will have to determine the final tilt by experiment). Measure the scale length from the center of the nut and bridge as shown below:
Also measure and mark the exact center of the bridge, which should also be the centerline of the fretboard. The point where these two lines cross is the point your cone must be centered on. Be careful here. If you miss this, you will end up with a guitar that, if it's playable at all, won't sound good. My cone came with the biscuit detached. There is a screw hole in the center of the cone for securing the biscuit. I placed a steel rod in a hole through the center point in the top, slid the cone over it through the screw hole, and thus was able to pencil around the edge of the cone to get it centered.
Notice that the original sound hole is probably bisected by the line around your cone. That's a problem, but we can solve it. The purpose of the wooden 'filigree' piece in the after photo above is to cover the unwanted edge of that sound hole.
Now you will need to cut out the top along your line. Very important: The hole must be no larger than the cone! We want the cone to just fit through the hole with no clearance to spare, because the mounting holes for the top plate are so near this edge that the screws will tear out if you haven't left enough material.
Here we see the hole thing!
I used a Dremel Moto-Tool with a small titanium cutter to cut the hole. There were braces glued under the top, and those caused me considerable difficulty by making the bit jump. Be extremely careful here. I deliberately cut inside the line so that I could finish the dimension by sanding the edges out to meet the line. My problem with that sound hole is about as bad as it gets - most of it remains to be filled in. Save the piece you sawed out, you'll need some of that for filling the hole.
Now sand out the edges of the original sound hole so that you have a nice gluing surface and have eliminated any thinning they did (usually the edges will have been thinned into the hole). Then put a piece of cardboard under it and trace the hole onto it. Cut that out and use it as a template to cut and sand a precise fit from the waste top piece as shown below. (If you look closely, you can see I botched this step a bit. The inner edge isn't a perfect match for the curvature of the main hole. That caused me some grief later, so try to avoid making the same mistake). Do not glue this patch in yet, just set it aside for now. You need the extra hole open to insert the sound well later.
Now comes the hard part. You must make a sound well that holds the cone, and the depth of that well must be exactly right. The good news is that it doesn't much matter what you make it out of. A good, tight grained plywood is fine. I used a piece of 3/4" birch countertop plywood purchased from Home Depot as the main body. I hate to admit it, but I forgot to take pictures of the sound well itself, so my description will have to do.
First you must determine the exact depth required. To do that, set your cone with biscuit attached on a flat surface. Build up a stack of any material handy, putting the cover plate on top (careful, don't damage the delicate cone) until you find the height that puts the top edge of the biscuit JUST below the edge of its hole in the cover plate. You're looking for maybe 1/16" of clearance, no more, but enough that it doesn't touch the cover plate. The stack is shown below. Do not forget to take the thickness of the guitar top into account! In the photo, that's the purpose of the two pieces of thin wood under the cover plate.
Now use the cone to draw a line to cut out the center of the plywood piece and whatever other pieces you needed to shim the height. Then draw another line about 3/4" outside that to be the outer dimension. Cut these out carefully so that you end up with a ring, the inside diameter of which your cone will just fit through. The outer diameter is not critical and can be left rough, but the inner one needs to be sanded as smooth as you can get it. If this assembly is more than one piece of wood, glue the pieces together with carpenter's wood glue. You might want to paint or stain the inner diameter with either black or dark brown at this point so that no whiteness will show through the cover plate holes later.
Now you must make the shelf the cone sits on. For this, I highly recommend getting some tonewood. Curly maple is an excellent choice here. This piece should be about 1/8" thick because it needs to hold the string tension yet be a bit flexible to the vibrations. Again draw a line around your cone, but then draw a line about 3/8" inside that line, and cut it out. Then center that on your ring and cut the outer dimension to fit. This is then glued to the bottom side of the ring so that the 'shelf' projects into the sound well. Test it after the glue dries to make sure your cone sits on the shelf with no excess clearance, but not binding anywhere either. The cone is not fastened in any way - it must float on the shelf or the sound will be severely degraded. String tension alone holds it in place. Don't put any paint or other finish on the shelf, it needs to be bare wood.
Now test fit the sound well in the guitar. It should be possible to get it in by using the original sound hole, but if it won't go, you can saw it in half and then glue it in as two pieces. It's likely that you will run into trouble with those top braces again. I used a sanding drum on my Dremel to sand the braces down where I needed clearance. Also cut and fit the two sound holes at this time. Their placement is largely a matter of taste. Try for a fit that the covers will snap into, otherwise you'll have to glue them. And check what is at the bottom end of the guitar. With any luck, there will be a solid block of wood in there. If there isn't, you'll have to make one and glue it in. This is what holds the tailpiece, and it needs to be mechanically strong. Temporarily fit the tailpiece and drill a hole for its screw. (Don't use the wood screw they supply with it. Get a drywall screw that has metal wings instead. That is stronger and can be tightened later if it ever works loose).
When all the fitting is done, glue the sound well onto the guitar top. Also glue some bits of scrap from the 'shelf' under the edges of the original sound hole to provide support for its patch, and glue the patch into that hole. Now you can sand the thing and decide how you want to finish it. I discovered that the original top was too badly damaged to finish in wood tone, so I opted for black lacquer instead. This is another traditional resonator color scheme and will allow the use of wood filler to fix any deep dings.
Below we see the guitar with the sound well installed, the black finish done, and the sound hole covers installed. The tailpiece is loosely fitted at this time. If you want an electric pickup, now is also the time to install a jack for it. (I recommend a piezoelectric pickup of the type that glues to an interior surface). I have also rebound the neck using ABS binding material. If you need to do binding, buy about three times as much as it should take. You will ruin a lot of it getting it right. Fortunately that material is cheap! In the photo, the cone has been set in the hole to check once again for proper fit. Note the tilt of the bridge. Take the cone back out and vacuum out all the sawdust you have put inside the guitar.
Now it's time to address that old sound hole. What you do is largely up to you, but you'll obviously want to cover up the patch somehow. I decided to use a piece of 1/8" West African Padauk wood to make a decorative plate. Seen below is that piece being made. Again, cardboard is your friend. Make a template by tracing around everything, cut that out, and trace it onto your patch piece. Once I had a proper fit, I then sanded the piece down and added ABS binding to the edges. I made the edges grade down thinner where they meet the sound holes and cover plate. I finished this piece in tung oil (I recommend Formby's Tung Oil Finish, available at any hardware store), and glued it in place. Tung oil is also an excellent finish for the fretboard, and is what I used. You apply this, let it dry, rub it down with 4/0 steel wool, and repeat that until you're happy with the result. This takes a lot of time, don't rush it. The resulting satiny finish is well worth the effort.
Below, we see a view of the refinished and rebound neck. The nut has been removed because we need a taller one for a resonator. A new nut is carved from a bone blank, available from the same online sellers that have the other parts. (Tip: This material gives off a terrible stench when cut or ground. Do this outdoors, or you will surely regret it). Use small triangular files to make grooves for the strings, and test their depth with actual strings until you have an even fit. You don't want one string sticking up here as that will make it impossible to play. Glue the new nut in place with cyanoacrylic glue. The headpiece has also been refinished, being careful not to ruin the Guild inlay. The specks of white sawdust seen are from a last-minute refitting of the ABS binding.
You also have to cut string grooves in the bridge. This is another fiddly operation, so take your time with it. Notice that the bridge is slightly beveled, higher on one side than the other. The high side goes toward the neck. Put the cone in the hole, set the bridge tilt angle, and put the first and sixth strings on the guitar, tightening them just enough to sit firmly on the bridge. Adjust their positions so that the strings lie just inside the fretboard edges at the body end, and mark the positions with a fine pencil. Let the strings off and carefully file grooves there. Now using something like an engineer's ruler, find equally spaced positions for the other four strings and file those grooves too. Like the nut, these grooves will have to be deeper for the heavy strings so that the strings are level across the top.
Time to put it all together!
First thing to do is fit the cover plate and make screw holes for it. With the cone in place, put the cover plate over it and adjust it very carefully until the biscuit is centered, not touching, and the hole pattern on the plate is straight. Very carefully mark through the screw holes using a sharp tool. remove the cover plate and use a 1/32" drill to make holes. Clean up the sawdust and put the cover plate back. Secure it with 1/2" #4 round-head sheet metal screws (less likely to split out than wood screws). Tighten the tailpiece screw if you haven't already. Now string it and presto, you have a resonator! Well.... almost. Try tuning the first and sixth strings to the same note a couple of octaves apart. Now use a bar across them up at the body end of the neck. Are they still the same note? Probably not. That's the tilt of that bridge! If the high string is flat relative to the low string, you need to turn the cone a bit farther counterclockwise. If it's sharp, then the opposite. Take your time, this is worthwhile. A guitar that sounds sour at various positions is not fun to play.
And here it is:
I am now the proud owner of a custom built resonator guitar, and more than that, I have a showpiece of my very own craftsmanship. Which is considerably better than my playing, but that's another story.
If this article inspires you to take on a similar project, good luck! You CAN do it.
NOTES AND TIPS
1. Finishes: On a standard stringed acoustic instrument, the finish is part of the tone. You cannot just slap an enamel or varnish on it and expect good sound, because the finish will muffle the tonewood. Fortunately this is not true of a resonator. For the parts that are to be wood-grain finished, try just lightly sanding the original finish with 600-grit wet-or-dry paper and spraying it with a good clear urethane. The results will amaze you, and the finish is quite hard and scratch resistant. Test this first on the remains of that piece you cut out. Caution: Spray urethane in very light coats and let it dry between coats. If you get it too thick anywhere, it will crinkle up and make a mess.
However, NEVER put urethane, varnish, or any other such material on the fretboard. It will rub off under the strings leaving nasty looking divots. Only use an oil finish here. Tung oil is lovely and easy to wotk with, but you can use other drying oils if you prefer.
2. The biscuit cone has a well under the biscuit, with the screw hole in the center. If you just try to screw the biscuit on there, the well is likely to collapse! I made a spacer from a piece of 1/2" wood dowel. Get it exactly the right length and drill a hole through it for the screw. Bamboo would work well here, but I used an oak dowel. Put a thin film of contact cement on each end before assembly so it won't rattle if the screw ever works loose. I also like to use just a bit of the same cement around the edge of the biscuit where it meets the cone, but don't use very much. Remember anything that adds weight or stiffness to the cone will muffle the sound.
3. Strings depend on your playing style. If you have used a mid-height nut and intend to play finger style with a bottle slide, you can use ordinary round-wound strings. But if you have used a full height nut and plan to play lap-style with a heavy slide bar, you need flat-wound strings. The round-wounds will 'talk' as you move the slide. Strings can be either ones made for a resonator, or ones made for a steel guitar. Important: Since your guitar is probably round-necked, it doesn't have tremendous structural integrity. A high nut will stress the neck and could eventually cause it to warp. For that reason, I recommend using light or extra light strings. They require less tension to tune and will be easier on the neck.
4. If you need to fix body cracks as I did, get some 1/16" thick curly maple. If the crack is flat, you can just glue and clamp a strip of the maple on the inside to fix it. If the crack has curled up, you'll need to steam it and clamp it flat between wood pieces before you can glue it. If you have no way to steam it, you can try using cloth dampened with hot water, repeatedly applying these until the wood is wetted enough to flatten under pressure. It's more work, but it will probably do the job. Be sure to keep it clamped until it's completely dry, or it's apt to curl even more. Give it a couple of days to be sure. If you have to sand the exterior of a fixed crack so far that the wood under the finish is exposed, you'll find a small can of wood stain very handy. Get some that roughly matches the finish color, When painted over with the urethane this will look fine.