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 Post subject: double-X bracing?
PostPosted: Fri Feb 24, 2023 10:07 pm 
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Koa
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I just saw a picture posted elsewhere of the backside of a Gibson soundboard that had a pair of x-braces with the bridgeplate in the middle.

Does anyone know the reasoning behind this configuration, and if someone does, can they offer up whatever rules-of-thumb about using the configuration?

Thanks!

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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 7:23 am 
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Koa
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I presume you’re talking about something like this:

Image

I don’t have a lot of experience with double-x Gibsons but I work on ten or fifteen of these each year:

Image

These old Maton CW80s have a similar bracing design. In general, most of them that I see sound good, some sound excellent. One of the best sounding dreads through the shop last year was a CW80 which featured this bracing.

I see very few lifting bridges on these - this may partially be due to the second X, which seems to keep the bellying down to a minimum.

It’s not really apparent from the pic but the second X is much lower in height compared to the primary X. Still, many would say they are overbraced and that may be true - it’s still a factory guitar with a conservative design.

Those 70s Gibsons I believe have a reputation of not sounding that great but the second x looks pretty massive, at least in the pics I’ve seen.



These users thanked the author joshnothing for the post (total 2): Hesh (Sat Feb 25, 2023 8:06 am) • Michaeldc (Sat Feb 25, 2023 8:00 am)
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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 8:36 am 
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The Gibsons usually have a very thick plywood bridge plate and it is almost impossible to remove since it is surrounded by braces. The design is designed to avoid warranty returns.

Mario used to suggest that you use a set of pliers to reach in and break one of the lower X-braces out which would allow the top to move more freely.


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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 10:31 am 
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Brazilian Rosewood
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I've made two double X top guitars before and both of them sounded great. In fact I don't know why I have not built more but I plan to some day. The old Gibsons are way over braced though and as mentioned the bridge plate was HUGE and a tone killer.

If I had the back off like that in the pic above I would remove the huge bridge plate and make a new smaller one to fit. I would also thin the bracing particularly in the lower X.


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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 11:31 am 
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I may use double X bracing on medium sized (OM) bodied guitars
I've been using triple X bracing experimenting with larger 16"+ lower bout guitars looking for lighter, more responsive soundboard assemblies.
Top pic is in progress. Lower pic was overbraced.


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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 11:46 am 
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Brazilian Rosewood
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The concept is good, but, as often happens, Gibson messed up in implementing it, IMO. I've been using a version of double-X bracing for some time, and find it works well.

If you use any version of 'tap tone' tuning on the 'free' top you're probably trying to get as many 'clear' tones as you can. It turns out that the clarity of the tap tones depends a lot on how well balanced the stiffness is from one area of the top to another, and in different directions. I use a 'tech' version of this called 'Chladni pattern' tuning, where you can actually see the way the top moves as it vibrates. Generally speaking, the more symmetric the patterns, the better the guitar ends up sounding.

Standard X bracing is asymmetric. If you flex the lower bout of the top across the two diagonals you'll find that it's much different between them. In one direction you have to bend the tone bars, while in the other you don't; all you have to bend is the top itself. Dana Bourgeois solved that problem to some extent by only scalloping the bass side of the X-brace.

Some years ago a student and I were discussing the issue, and agreed it made little sense to try to get symmetric vibration patterns from asymmetric bracing. A while after that one of my customers asked me to make him two guitars fir duet performances that had somewhat different sounds, but would still blend. I made a 'nearly matched' pair of small Jumbos, one with standard top bracing and the other with double-X. I made them as identical as possible otherwise.

I brought them to an ASIA Symposium and had folks try them out without telling them what the experiment was. One fairly typical response was from a very good maker who carefully tuned them up, and played some fingestyle, some chord melody, and some flatpicked music on both. When I asked which one they preferred it was the double X, by a little bit. When I told them about the bracing they exckaimed:"But that doesn't work!). I just shrugged.

The overall judgement of more than sixty guitar makers was that the double-X guitar was a little bit better; the vote was 2:1. Some mentioned that the standard braced instrument sounded more 'traditional', and the double-X more 'modern'.

I think that Gibson's main problem was using a top that was too thin, and bracing that was too heavy, particularly behind the bridge, and a massive plate. This works well with 'scalloped' bracing, although I usually use 'tapered' profiles: higher at the bridge and low at the ends. I took a scalloped braced double-X top to a workshop with Dana Bourgeois a few years ago, and he said thought it worked well. I was also able to test one of his asymmetric half-scalloped tops there, and found that it does, indeed, vibrate in a much more symmetric fashion, with clearer tap tones and better formed Chladni patterns.

I'll try to get up a picture of one of my tops.



These users thanked the author Alan Carruth for the post (total 2): joshnothing (Sat Feb 25, 2023 3:59 pm) • Colin North (Sat Feb 25, 2023 12:19 pm)
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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 12:18 pm 
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Koa
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This is wonderful. Thanks, folks!

About symmetric bracing: I picked up from somewhere a comment that Larrivee put tone bars at right angles to the centerline. I got happy results using one tapered lateral tone bar on my 12-fret project that put the bridge nearly centered in the rough circle of the lower bout (dreadnought shape, OO size).

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Last edited by phavriluk on Thu Mar 23, 2023 6:23 pm, edited 1 time in total.

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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 2:39 pm 
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Brazilian Rosewood
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I have used Dana’s asymmetric scalloping on dreds as my standard pattern but not OMs. Not sure why but I thought Dana only did it on dreds.

Did he do it on smaller bodied models as well Alan?

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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 3:10 pm 
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The classic Larrivee design features a 90* x brace lap joint, with a tone bar perpendicular to center line not too far behind the bridge plate and another maybe 2/3 between x lap and tail block, very roughly. They do a few different things these days…


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 Post subject: Re: double-X bracing?
PostPosted: Sat Feb 25, 2023 4:18 pm 
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Brazilian Rosewood
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Several makers have used the Larrivee strut design, and it makes a lot of sense. I don't know if Dana uses this on anything other than Dreads.

Here's the top I worked on at Dana's workshop. I had shied away from Dreads in the past, partly because I just couldn't get the top to work right with standard bracing, but when the workshop came up I gave it a try. As you can see , it's a sort of 'slope Dread'. On the OM sizes I usually make the upper ends of the lower X extend under the main X brace to become the finger braces, and that's worked well. Dana suggested that I add in some more finger braces on this one. Other than that it's pretty much what I brought it to the workshop.

The bridge plate is skew cut persimmon; very hard, close grained, and tough. The top is doubled around the sound hole with a piece of spruce that has the grain angled about 3-5 degrees from the top center line. This follows violin practice; the long-grain stiffness of the patch is similar to that of the top, but the crossing grain helps resist splitting. The UTB, upper arms of the X, and the 'A' braces on either side run along the outer edges of the rosette, so it's all reinforced. If you look closely the 'A' braces run under the UTB at full height, and plug into an extending 'chin' of the neck block to prevent the neck shifting in when the guitar gets knocked off a table or something. Never happened to you? Just wait... It seems quite effective. All the ends where braces intersect are inlet; only the lower ends of the bottom X are not. All the lap joints have patches across the top.


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These users thanked the author Alan Carruth for the post (total 5): James Orr (Wed Mar 08, 2023 6:18 pm) • bionta (Sun Feb 26, 2023 10:27 am) • Colin North (Sun Feb 26, 2023 4:51 am) • Skarsaune (Sat Feb 25, 2023 8:38 pm) • joshnothing (Sat Feb 25, 2023 5:41 pm)
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 Post subject: Re: double-X bracing?
PostPosted: Sun Feb 26, 2023 6:15 pm 
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Koa
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I’ve been applying what I learn from Alan for over twenty years now. [GRINNING FACE WITH SMILING EYES]

I actually found double x (or even x plus v of symmetrical bracing that just fades out where they meet rather than crossing to form a second x) to have worked especially well on small bodied guitars with light bracing.

I haven’t built a small guitar in many years, but my next parlor sized guitar will probably have some form of symmetrical double x bracing.


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 Post subject: Re: double-X bracing?
PostPosted: Sun Feb 26, 2023 10:43 pm 
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phavriluk wrote:
This is wonderful. Thanks, folks!

About symmetric bracing: I picked up from somewhere a comment that Larrivee put tone bars at right angles to the centerline. I got happy results using one tapered lateral tone bar on my 12-fret project tat put the bridge nearly centered in the rough circle of the lower bout (dreadnought shape, OO size).


Mark Blanchard uses two tone bars at right angles to the center line, with the bottom one about where the lower ends of the x-braces end. He knows a few things.

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 Post subject: Re: double-X bracing?
PostPosted: Wed Mar 08, 2023 9:39 am 
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Repair folk often refer to the Gibson double-X brace as the double cross because the bridge plate design makes it extraordinarily difficult to repair or replace it since it completely fills the area within the arms of the X braces. Some of these other designs with a more traditional bridge plate are fine because you can get a tool in there to remove the bridge plate.



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 Post subject: Re: double-X bracing?
PostPosted: Tue Mar 21, 2023 11:10 pm 
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I'm still just learning but on my second build a friend donated a set of Carpathian spruce that I thinned down a little too much and hated to throw it away. I picked the brain of a more seasoned luthier and he suggested I try using lattice bracing and a spruce/rosewood bridge plate/patch combination to reinforce the top where it needed it. This was the result mid construction.

Image

The top still had a lot of flex but I was motivated to see it through though I haven't glued the bridge down yet.


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 Post subject: Re: double-X bracing?
PostPosted: Wed Mar 22, 2023 10:05 am 
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Brazilian Rosewood
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Gore's '2 degree rule' is good to keep in mind: under string load the bridge should rotate forward about 2 degrees. If it's much less than that the top is probably stiffer than it needs to be, and if it's much more, it's not stiff enough. Keep in mind that rotation is partly a function of the bridge torque; the product of tension and the string height off the top. If the bridge rotation is a little on the high side you might get it under control by lowering the saddle. This will not reduce the sound output so long as there's sufficient break angle over the saddle. Much less break angle is needed than people seem to think; 15-18 degrees should be 'enough'


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 Post subject: Re: double-X bracing?
PostPosted: Wed Mar 22, 2023 12:19 pm 
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Brazilian Rosewood
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Alan Carruth wrote:
Gore's '2 degree rule' is good to keep in mind: under string load the bridge should rotate forward about 2 degrees. If it's much less than that the top is probably stiffer than it needs to be, and if it's much more, it's not stiff enough. Keep in mind that rotation is partly a function of the bridge torque; the product of tension and the string height off the top. If the bridge rotation is a little on the high side you might get it under control by lowering the saddle. This will not reduce the sound output so long as there's sufficient break angle over the saddle. Much less break angle is needed than people seem to think; 15-18 degrees should be 'enough'


Another thing to keep in mind is the possibility for adjusting bridge rotation is to change the gauge of the strings - some guitars seem to be very sensitive to string tension changes which can be effected by changing the string gauge. If the action is already at the optimal height for the player, then changing the string gauge might be a good option.


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