Official Luthiers Forum!

Hey all,

Got a question about pyramid bridges.....

I used an Ebony model from LMI.

The top is sinking in front of the X -- following the scooped out area of the Pyramid bridge... and it has a hump that also follows the same scooped out areas of the bridge in the back... This is surprising to me because it has a hard maple bridge plate under there

I am thinking that while a pyramid bridge looks neat... it may not be the best choice to pair with a thin top... that the thin scooped out area of the bridge isn't stiff enough and allows the bridge to rotate somewhere it would have been otherwise prevented from rotating..... The top humps and valleys very nicely follows the shape of the bridge.

Here is a diagram:

I don't have an issue. I use the pyramid on most 000 and OM models . Did you scallop the braces?

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Right John, it's probably not the bridge or the plate, but the thin top and the braces.
I think maybe a cross brace behind the bridge is going to be called for.
That, or a label inside that says "light strings only".

_________________
https://www.facebook.com/profile.php?id=100008907949110

This guitar is a small body Parlor size. It was on loan to a local professional musician (Music is his day job.) He has been using it for studio recording for about a month now-- so it gets far more play than I could ever put on it. I asked for it back so I could look it over and take some measurements. It will go back on loan this week....

It has a "True" flat top -- which I think is the other part of why the bridge was easier to rotate...
and the bridge is flat to match the top. (I do try to match the bridge to the profile of the top)

The braces started out as 1/2"h x 3/8"w. They got a scooped out triangle shape, and they are slightly scalloped.

I think my takeaways from this that are going to be incorporated into my current build are:
1. Use a slight dome
2. Don't use a pyramid bridge with a thin top.

Thanks

John

How much and how fast did you reduce the height of the X-brace between the intersection and where the bridge overlaps? Seems this would be key.

_________________
Formerly known as Adaboy.......

This isn't a bridge problem. Using a bridge with a larger footprint to overcome basic problems with the soundboard is not the way to do things.

Always -- that is, ALWAYS -- have some arch on the soundboard. Whether it's a spherical or cylindrical is up to you but yes, the braces have to be arched. This is something instrument makers learned in the 18th century.

Then pay attention to the scallops and/or taper. The upper legs of the X are primarily structural and should remain full height or close to it for most of their length. Any tapering or scalloping of the lower legs should start near the lower end of the bridge plate or farther. Reducing the stiffness at the plate is an invitation to many problems.

One problem with traditional parlors is that the bridge is quite far from the middle of the X, putting added stress on the lower legs. It's very important to keep the braces between the joint and the bridge quite stiff. Your brace dimensions are more than adequate (I use 1/4" wide by 1/2" tall) but I wonder about the "scooped out triangle." It saves some mass to use a triangular cross-section but if it's too tall and skinny (or "scooped out"), it can fail in surprising ways. I ease the edges of the X into something more like a bullet cross-section rather than the triangle I use on the lower-stressed tone bars and finger braces.

I was looking through the bracing library on the Unofficial Martin site and the scalloping started above the bridge. The bridge was about in the center of the scallop so this is very interesting. Are there other factors related to this?

Uh...maybe I should have provided a link, so
http://theunofficialmartinguitarforum.y ... ml?page=11

This is from your link Hemet, I think it is most appropriate.
I still think the thinness of the top is the weakness in John's case.

_________________
https://www.facebook.com/profile.php?id=100008907949110

Don't Jim Olson and Kevin Ryan build with flat tops? How do they make it work?

I did some checking on my bridge vs some others...

My pyramid bridge is 1/16" thick at the scoop. This is from LMI.
Tutorials I have seen for making them up leave the scoop at 1/8" thick...

What thickness do you guys use in that scoop area of the pyramid bridges?

Thanks

John

With respect to the top, how thin is thin?

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

The top is 0.080" - 0.090" in the middle.... but it is soft, light Lutz... not 0.090" of super hard dense Sitka or Red. Density is right around 0.395 g/cc

The bottom 3" or so of the lower bout tapers down to 0.050"

Thanks

John

I made the top too thin on my first (only) guitar, and it's sounded wonderful for 8 months now and the bellying got so great that I made an imitation JLD Bridge doctor.


I was amazed is brought back a lot of sparkle to the top similar to when I first strung it, and I note that breedlove fit them from new I guess it lets you have a lot thinner responsive top with less worry about it bellying too badly.

_________________
Formerly JJH

I learn more from my mistakes than my successes

Yes, they do. There's no magic to it. The two best guitars I've ever played were true flat tops.

Kevin's got over 500 out there, and Jim is probably at that mark or above by a good margin, so I feel relatively confident that it can be done.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

But are they truly flat tops after being strung up?

_________________
Formerly JJH

I learn more from my mistakes than my successes

Yes, clearly a great guitar can be built with a flat top. Does anyone know the specifics of how they do it. Is the moisture in the top at "normal" humidity levels (altough luthiers have a pretty wide range for that) or does the top start out dryer?



I remember reading a thread in another forum where people were talking about this and a guy who owned an Olson or two was sure they were not built flat because he could see that they looked arched. So he called Jim and asked him what the story was and sure enough, he was told that the guitars were built flat. I seem to recall Jim saying somthing like, "I've built them both ways and I don't find any advantage to arching them."

So, I think the answer is, no, they don't stay truly flat after being strung up.

If you glue the braces in very low humidity say 30%, at 45% you can bet it will form a healthy dome by itself.

_________________
Build log

Yes, that's what I was thinking. I wonder if they do something like this.

I seem to remember one builder liking 28%RH for top bracing.

Well I didn't like to post this before, but I was lucky enough to visit Andy Manson a well know UK luthier, don't know if he's know in the states, but he was telling me his process and he thicknesses his tops and before bracing dries all the wood by shining anglepoise lamps on it all day then braces it up flat as the last job of the day, and when it re-humidifies it domes, how repeatable it is I don't know.

_________________
Formerly JJH

I learn more from my mistakes than my successes

I think I figured out what was going on... and it was related to not knowing any better...
I left about 1/32" gap between the X legs and the bridge plate. I left a larger gap on my retop Esteban. I carefully shimmed the gaps in the Esteban and the top flattened out a bit, and sound and tone improved...

Based on this, I did the same on this one 2 nights ago. It seemed to reduce the cupping and seems to spread out the belly a little better... I believe it also now has more "Tone" and sustain.

It is now back on Loan again to the local musician. I want to see how it holds up over time.

On the Flat Top....

The guy who lead our Ditson "Build Along" builds true flat tops including Flat top Dreads, J45's etc. No gyrations about heating or anything else... They are built flat on a flat workboard. He is in his 70's and said that is how he was taught to build years ago.

I am certain that you could make them dome up by doing any one of the things mentioned (Differential humidifcation, heating, drying out, whatever)... but this isn't how he builds.

His comment was that they aren't flat anymore as soon as you string them up -- they pull up.

I gotta say, there is something special about the sound of a real Flat top... It has a very warm, wonderful, full sound.

Thanks

John

That's great information. I love hearing about flat builders. I have been so prujudiced against it that it's good to force myself to think it through.



There does seem to be a "flat school" of thought. Does anyone know where this started? Did the Sloane guitar books build flat? It's all very curious.

I think I'd find it hard to give up domes. I hang guitars on the wall in the winter here in NE, stress the heck out of guitars, sometimes it's painful just to look at how stressed they are, it's just so hard to imagine that they would be fine if they were built flat.