Wood Body, Biscuit Bridge, Resonator Construction

[franklin]

Hello Michael,

I am intent on building a wood body, biscuit bridge resonator. I am very familiar with acoustic guitar construction, however, as you well know, many of those principles do not apply to the resonator. I've searched high and low but there doesn't seem to be much information on the subject out there. I own several metal body Nationals and understand the methods used in attaching the neck, I'm wondering if the same is used in a modern wood instrument. I plan on making my own ply for the top, back, and sides, but could use some direction in the proper thickness for each, as well as a radius generally used on the backs, assuming the tops are flat. I have the Gourmet Guitar series DVD featuring Mike Lewis with Michael Messer I'm proud to say, I have Bob Brozman's book on nationals, and I have scoured the web, but hard information continues to elude me. Any help or direction you would be willing to offer would be greatly appreciated.

Thanks so much,

Franklin

[LouisianaGrey]

You're right, there's very little information out there about building wooden biscuit bridge instruments. You can make them like the metal bodied Nationals but personally I can't see the point of attaching the neck stick to the neck when it's a wooden body unless, like National, you have your machinery and jigs all set up to do it that way already. Because I like to have the back free to vibrate I use a stick attached to the neck and tail blocks to support the soundwell and I glue the neck on with a standard dovetail.


Ideally the tops on mine are a bit under 6mm, just enough to be covered by the edge binding. Back and sides are what you would normally expect for a regular acoustic guitar, although I tend to leave the backs a bit thicker (say 4 mm) and brace a bit more heavily too.

You can radius the back or not as you see fit, I've done them both ways but nowadays I tend to go for a 20 foot radius. Don't forget the neck angle is going to be about 4 degrees rather than the one and a half that you would use with a regular acoustic.

[franklin]

Interesting....I assume your tops are a ply, how about the backs, I take it you don't subscribe to the theory that the body should be stiff like a speaker cabinet, while the cone does the work. Is that an added layer on the top around the well. Thank you.

[LouisianaGrey]

I'm always tweaking things to see if I can make them any better so they all vary slightly. That particular one had a top made from mahogany about 5mm thick with extra reinforcement (6mm ply) in the soundwell area and under the fretboard. The ply is glued to the top and the well is glued to the ply rather than the ply surrounding it. Back and sides of that one are solid mahogany - I usually use solid backs and sides. I believe the back and side wood colours the tone, not as much as on a regular acoustic but it still has an effect.

I'm not sure the speaker cabinet is a particularly good analogy for how these things work - maybe OK as a generalisation, but I think there are differences too. If it was that good an analogy then you could make the guitars sound better by using speaker cabinet tricks like putting foam, fibreglass or steel wool inside to damp the overtones. I don't think so, do you?

I certainly believe the top should be very stiff and the soundwell should be firmly attached to the top and adequately supported so that the top does not move. However I have found that the guitars sound better if the backs are free to vibrate a bit.

[tark]

Hi Franklin,

Pete's right (see - I'm doing it to you now ) the speaker cabinet analogy seems to have been spread by guitar mag. journalists who sadly don't know a great deal about how guitars are built and how they work. I may even have used it in print a few times myself before I knew better.

It's only accurate to a point - the cone or cones are a bit like loudspeaker cones and the body of the guitar does act as a Helmholtz resonator, like a bass reflex cabinet. However loudspeaker cabinet designers go to great lengths to try and ensure that loudspeaker cabinets are as heavy and rigid as possible. Then they'll stuff the insides with damping material to cut down on internal reflections and resonances. Most modern loudspeakers are very inefficient. With a resonator guitar you want to make it as acoustically efficient as possible without losing too much sustain (i.e. as loud as possible).

It would be totally impractical to build a guitar body out of one inch MDF. Guitars, even wooden resonator guitars are built of lighter material and body resonance and coloration are an essential part of the sound. They give the guitar character and warmth. Unfortunately getting all the constructional details just right to produce an outstanding guitar is very tricky and luthieres like Pete spend a long time learning how to do this through a process of continual experiment .

Resonator guitar bodies are often made of slightly thicker woods than a conventional acoustic and plywood is often used, but the body definitely does not behave strictly like the ideal loudspeaker cabinet.

[Frank Cash]

Sorry, didn't really mean to get off task when I mentioned the speaker cabinet thing, only meant I have noticed that there seems to be two camps, those that opt for a stiffer body usually using ply construction, like National Resophonics, or Fine Resophonics, and those that do not, using lighter construction and a more resonant body. I'm sure both are completely valid, just different, producing different results in the way they sound. Thanks, and keep em coming..

[tark]

Frank,

You are right about the two camps and it's not limited to resonator guitars. Greg Smallman, the famous Australian classical guitar builder talks about making the back and sides of his guitars as rigid as possible and getting the top to do all the work (he builds the back and sides out of plywood or rather laminates that he lays up himself). Many of the composite guitars, most notably Ovation bowl backs, have very rigid backs.

At the time the Dopyeras started making wooden guitars the US economy was in a bad way so they were trying to make cheap guitars, which may be part of the reason for using plywood. I think many of the makers building resonators in all solid woods today are doing it for either cosmetic reasons or commercial reasons (they can charge big bucks for a solid koa top) rather than necessarily to get the best tone. Certainly the woods and thicknesses used in a resonator body aren't perhaps as critical as they are for a conventional acousitc guitar although they still have an influence on the sound.
The flexibility of the back of the body in particular seems to have a big influence on the low end.

[Franklin Cash]

Thank you, I thought I wanted to use laminated tops and backs, that I would lay up, for strength and stability, not economy, and I thought I had heard Michael Messer remark that he felt a laminated body was appropriate to mimic the sound of a National, or Fine Resophonic, which may or may not be a pleasing sound to a particular set of ears. I suppose I truly enjoy the sound of Mr. Messer's machine and was aspiring to copy it, understanding completely of coarse the role his talent plays in the sound that is produced by anything he might pick up and play. That being said I am very grateful for any and all information I can coax out out of you fellas. Thanks again, franklin

[davey]

I have Gibson 'Hound Dog' here which is a really good guitar, it has a deep Bass, sweet middle, and sparkly highs.

It appears to be solid timber of around 5mm as the grain on the unbound edges correspond to the grain on the top. The are at least 3 sound posts inside like in a violin but no neck stick. The posts are around the edges of the soundwell, which is open i.e. not enclosed. The guitar is also very resonant, covering the f holes deadens the resonance to a loud but dull sound.

The neck is a lovely piece of figured Maple with a perfect action, the sides appear to be Maple Ply and the top and bottom - who knows ? Might be Birch, with a satin mahogany finish. It's as good as any vintage one I've ever had and frankly much easier to play. If you came across one have a good look at it, it works incredibly well.

[LouisianaGrey]

The "Hound Dog" is, of course, a spider bridge instrument, which is why it has an open well and doesn't have a neck stick.

[davey]

Right you are, I'm off topic as usual.

As a matter of interest, I've always had the impression that spider bridge resonators are less temperamental than biscuit bridge types. I've had a dozen or so of each, mainly 1930's ones.

Ever time I've left a national unplayed for a month or two it seems to buzz when I
start playing it again. The Dobros seem not to mind so much.

Cheers

[franklin]

Thanks Davey, you bring up an interesting point, traditionally spider bridge instruments or Dobros, as the Hound Dog is referred to, were built with a sound well extending from front to back. The Hound Dog has an open construction and the cone is supported by four posts extending to the back. It does use three ply laminates for strength but according to you and reviews I have read It resonates nicely. I suppose its construction is not so different from a National biscuit bridge using a stick to reinforce the top with supports under the stick extending to the back. Thanks again.

[tark]

franklin said:

Thanks Davey, you bring up an interesting point, traditionally spider bridge instruments or Dobros, as the Hound Dog is referred to, were built with a sound well extending from front to back. The Hound Dog has an open construction and the cone is supported by four posts extending to the back. It does use three ply laminates for strength but according to you and reviews I have read It resonates nicely. I suppose its construction is not so different from a National biscuit bridge using a stick to reinforce the top with supports under the stick extending to the back. Thanks again.

As I understand it the traditional Dobro (meaning a single cone spider bridge guitar and not to be confused with the single cone biscuit bridge guitars that carry the Dobro brand) has a sound well formed from a cylinder of plywood that runs from the top to the back of the guitar. This cylinder is pierced by a number of large holes to allow air moved by the underside of the cone to circulate through the body. The neck does have a neck pole, but this stops short and is connected to the upper side of the sound well.

You'd expect this form of construction to introduce its own resonances due to the perforated sound well and to have a stiff back due to the sound well joining front and back. It should sustain well, not have a particularly pronounced bass and perhaps have a slightly nasal dry sound.

The more recent design with just support posts rather than a full depth sound well won't have the resonances imparted by the sound well and won't have that well stiffening the back. This construction should sound less nasal and have more bass and volume, but might have slightly less sustain.

On a tangent to the above - It's interesting that George Beauchamp and the Dopyeras should have disagreed so much over how to produce a cheaper guitar than the tricone design. You get the feeling that Beauchamp (the businessman)was going for something really cheap and simple with the single cone biscuit design and didn't care so much if it was a great sounding guitar, while the Dopyeras (the inventors) were focusing much more on performance with the large cone, spider bridge design, even if that meant their design wasn't that cheap and simple to produce. Funny that they both turned out to be right.

[LouisianaGrey]

"The neck does have a neck pole, but this stops short and is connected to the upper side of the sound well."

On all the ones I've seen with a traditional soundwell the short neck stick is screwed to a wooden bracket on the back of the guitar. There used to be a picture on the Gibson website of the innards of a Jerry Douglas model that showed it clearly but now they've dropped it they don't have the picture any more. This one is the best I've got - although this doesn't have a traditional soundwell the principle is the same, and in fact an old dobro mandolin I worked on used the same method.


Modern soundpost-style builders use various methods of attaching the neck, some with a stick, some without.

I use the soundpost method, as do most recent custom builders, and I can't say I've ever found any lack of sustain. It does give a different tone, though, and a customer of mine who wanted a more "traditional" sound retrofitted a full soundwell to his guitar with good results. Soundwell dobros generally sound more Mike Auldridge than Jerry Douglas though, as always, there are exceptions.

[tark]

Hi Pete,

Thanks for the corrections and clarifications on the Dobro spider construction. Screwing the post to a back bracket makes more sense. I'll admit I had only ever seen simple line drawings of the insides.

For some reason the Dobro community seems to be slightly more interested in modifying the construction and tweaking the sound than the rest of resonator land.