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I decided to add an X-Brace cap on my first guitar as I totally forgot to install one when building it in the first place. I was really surprised to find that the guitar's sound has improved, have you ever encountered such a small change in this particular location or any other for that matter to make such a noticeable difference. I don't think it is my imagination as my wife said "wow" as well. Any thoughts? I didn't weigh the piece but it could not have been more than a few grams.

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Gwaltney Guitars

Without the cap the top stiffness is radically asymmetric: 'out of balance' in a sense. I'm not at all surprised.

I've never made a guitar without one so I can't be of much help, but I would imagine it does stiffen the top up a bit.

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I didn't realize they were so common. What type of material did you use?

[QUOTE=Alan Carruth] Without the cap the top stiffness is radically asymmetric: 'out of balance' in a sense. I'm not at all surprised.[/QUOTE]

Alan,

As the X brace cross sits in the centre of the top, why would a cap absence make the top stiffness "radically asymmetric"?

Would this mean the top would have a "different" out of balance sound depending on which way you notched the X braces to cross them?

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

I don't know what an X brace cap is, Greg. Do you have any pics?

I put a dab of epoxy in the joint but I don't think that is quite the same thing.

Larry

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It's the little rienforcement that goes over the joint of the xbraces. You still want that joint as TIGHT as possible, and I always put some HHG in the joint before I put them together, but a cap goes over the top. Some people use wood. Some cloth (like martin, Gibson ect), but something to reinforce that joint.

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John Mayes
http://www.mayesluthier.com

[QUOTE=Dave White] As the X brace cross sits in the centre of the top, why would a cap absence make the top stiffness "radically asymmetric"?[/QUOTE]

I had no understanding of this, either, until I read about it here. Unless there's a cap over the gap created when the X is lap-jointed, the brace with the cutout facing up behaves as if it's only as tall as the wood left over after making the cutout, making for a very short, weak section of brace right in the middle of the top. The cap restores the integrity of the full-height brace. I guess it's analogous to drilling a hole through the side of a brace--it retains the strength of its height, even though you've removed some mass.   The other half of the X, meanwhile, retains its full strength because the top itself acts as the "cap," leaving a joint of asymmetrical strength. Close, Alan, or am I just thumb-twiddling?

Greg we are trying to keep these "secrets of Luthiery" secret. What are you doing blabbing it to the world?

Ha, I just bought my son an Ibanez Artwood series for Christmas, even though I'm building him a GA Venetian Rosewood and Cedar. Guess what? It has this xbrace cap glued on.

I promptly voided the warranty with my Dremel, and hotrodded the parabolic braces into scalloped. Made this guitar sound great, now that really makes a difference.

   Somogyi advocates caping the X brace. I just put a little Madagascar cap that came from a fingerboard cut-off on my first. Looks pretty cool, if I do say so myself...

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I'd like to be able to prove, just for once, that money wouldn't make me happy...

I just used a piece of spruce like the brace itself, seemed appropriate with like expansion/contraction properties. The guitar is louder, livlier, and I believe it is more balanced....and the bass is deeper/sweeter as well. I didn't notice much difference in the treble, if anything the treble may have lost a TINY bit, but it still sounds great and overall the guitar is much better. It just sparkles, amazing really.

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Gwaltney Guitars

O.K., so now I have to ask, are you serious about the scalloped bracing vs. the parabolic. I built this one with parabolic bracing, but I do wonder sometimes what scalloped braces would do.

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Gwaltney Guitars

Consult the venerable old Pre-War Martins, the Dreds. One of the coolest guitars I ever strummed was a Santa Cruz DPW drednaught pre-war bracing, scalloped of course. One incredible instrument. I use it on all of mine, straight off the HD28 pattern that came with a Martin Kit.

Intersting, Santa Cruz makes many guitars with parabolic bracing too. Wrong thread, I know.

[QUOTE=GregG] I just used a piece of spruce like the brace itself, seemed appropriate with like expansion/contraction properties. The guitar is louder, livlier, and I believe it is more balanced....and the bass is deeper/sweeter as well. I didn't notice much difference in the treble, if anything the treble may have lost a TINY bit, but it still sounds great and overall the guitar is much better. It just sparkles, amazing really.[/QUOTE]

Greg, sometimes when I do a simple project like you are describing I take it for granted that most people know what I'm talking about and in this case that is probably a pretty good assumption. But being very new to guitar building I have never heard of a brace as you are describing and wondered if you wouldn't mind describing what using a piece of spruce might look like in a little more detail.

I have been able to find a cloth example but haven't seen a wood one used before. Did you cut a small piece of spruce and glue it on top of one of the braces? How long? How thin/thick? Did you have a flat area to glue to or did you get it on there as close as you could? My braces come to a small peak so very little to glue to.

Appreciate the help

Larry

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Here's a pic of my x-brace cap.

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I'd like to be able to prove, just for once, that money wouldn't make me happy...

Very cool

Thanks

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Hey Larry, I tried to do like Somogyi says. Glue in the braces square, then you add the cap, then shape...

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I'd like to be able to prove, just for once, that money wouldn't make me happy...

Yeah I get it thanks. I guess a guy could shape braces first leaving a flat spot for a future cap but I really like the way the cap blends in. Nice.

Larry

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larry;
Alain's cap in the picture is (to the best of my knowledge) atypical. Normally it is not an X but a single piece glued across the "open" section of the cross. (across the cut lines). Some folks use a disc of wood which ties the entire X together.
Once I figger out how to post pics, my feeble attempts at description will improve dramatically.

The light is going on!

Larry

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Thank You and Best To All

I have a vague memory that Alan ever mentioned that this little piece of wood can change the top frequency to some extent but I'm not sure if it's lower or higher. Would you mind recapping that again please, Al? Thank you.

Yeah,

I used a single strip of spruce approx. 1" long and 3/16" wide x 1/8" tall. A little piece I'm telling you.

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Gwaltney Guitars

Great to hear from you Bruce!

[QUOTE=ParamesB] I have a vague memory that Alan ever mentioned that this little piece of wood can change the top frequency to some extent but I'm not sure if it's lower or higher. Would you mind recapping that again please, Al? Thank you. [/QUOTE]

I'm not Al, but I'm pretty sure when he chimes in he'll tell you about mode shape changes as a minimum. If I remember, without a good solid cap, mode shapes tend to be quite assymetrical, not resembling "well behaved" mode shapes well.

If I were to venture a guess beyond that, I might speculate that the wooden cap may increase lower order resonances in frequency slightly. Maybe. But, more importantly, the cap would allow the top to act more as a cohesive unit with mode shapes reflecting that. For higher order resonances, I expect it would depend on where geometrically within the mode pattern the cap falls. For a particular mode it could have no effect, or a substantial effect, and probably harder to sort out, as most higher order effects are.Pete Licis38727.4882175926

Thanks Pete.

Yes, the brace with the cuttout facing 'down', into the body, will be much less stiff at that point than the other one, because the top bridges the gap that faces 'up'. You might not notice much difference in stiffness in the completed guitar, but on the 'free' plate the change can be dramatic. As was pointed out, it's mostly the shapes of the 'free' plate resonant modes that are effected, with the lower order ones becoming much more symmetric with the cap on.

It can be argued that the 'free' plate modes will all be changed when the top is glued on, and that therefore they are not important. My experinece is that they are, and that, in particular, the shapes of those modes can tell you a lot about how well the guitar will work. 'Closed' and symmetric modes seem to make the best guitars.

Whether or not you agree about the sound, there is one thing that little cap does for sure: it helps keep the brace from splitting. That downward facing opening is a beaut of a stress riser, and it's right where there is a a lot of downward force on the top due to the bridge torque. I've lost track of the number of cheap guitars I've fixed with split braces at that point. Even the little cloth patch helps, but there's nothing like a wood one for peace of mind.