Replace old ply back with solid wood?

[John Damon]

Thank you, I’m on it

[John Damon]

Finished. Glued the original ply back yesterday. Bunch of mistakes were made. Top is way too stiff. I can’t even flex it by pushing down on it. I used a 1/16” piece of Pauduk for the bridge plate. There’s a string peg hole that goes through a main brace. Braces are too thick and tall, too bulky. There’s no bass at all. When I strung it up and hit the low e string I knew something was wrong. I’m going to attempt to use a thumb plane through the sound hole to remove mass from the bracing. I feel awful, it’s a really cool old guitar…

[John Damon]

Man, it sounds like those horrible acoustic telecasters from the 80s… like the body is only 2” thick

[John Damon]

Anybody have any suggestions? I’m open to anything that will help this old beauty!

[Barry Daniels]

It is over-braced. Thinning the braces through the sound hole is not easy. Could you pull the back to work on the bracing? We can give you some brace dimensions to shoot for if you can figure out the best method to access them.

[Barry Daniels]

What wood did you use for bracing?

[John Damon]

I used quarter sawn spruce. I could pull the back again and put on the solid one I was going to use. Ok, so it’s over braced, I wasn’t sure if it was that or the Pauduk bridge plate.

[Barry Daniels]

How big was the bridge plate? The 1/16" thickness is actually on the thin side.

[John Damon]

The bridge plate is about 2 1/2”x7”. And it’s in pieces, as the main x braces run through the area. Can’t really see much in the picture because of my excellent gluing technique

[Peter Wilcox]

I think that besides the x brace being too thick and probably too high, the center of the x is placed too far toward the tail, toward the most resonant part of the top, and the vertical angle of the x needs to be widened, again to avoid the most resonant area.

[John Damon]

Should I pull all the bracing and start over?

[Barry Daniels]

I think you would have better results if you used smaller and properly positioned bracing. Having a string pin hole go through one of the X-braces is also a near fatal flaw. If you take it a bit slower and take our suggestions, I think it should turn out better.

[Alan Carruth]

Peter Wilcox wrote:
"...the center of the x is placed too far toward the tail, toward the most resonant part of the top..."

There are a number of different ways to profile the braces, and they produce different timbres. One and of the continuum is 'tapered' bracing, which is tallest around the bridge where the stress is highest, and tapers out toward the edges. When properly done the top can perhaps be a little lighter in weight overall, and the sound will tend to be more treble balanced with more sustain and headroom. The other end of the scale is 'scalloped' bracing, which emphasizes bass and 'punch' and the expense of treble. Because scalloped bracing tends to be lowest where the stress is high it calls for a bit of judgement in removing material to avoid making the top too week over the long term. Often this weakness doesn't show up until months, or even years, later. The majority of production guitars use 'straight' bracing, which is only tapered down at the very end to plug into the liners. This is easy to do, structurally safe, and can produce a good sounding guitar is it's not over heavy.

Trevor Gore has proposed a rule of thumb for knowing when the top stiffness is right: the '2-degree rule'. String tension causes the bridge to rock forward, and he has observed that good instruments tend to rock about 2 degrees. If it's much less than that, the top/bracing is too stiff and can be taken down some; much more than 2 degrees means it's too weak and need beefing up. It ought to be possible to use some sort of stiffness measurement based on static deflection under a load to gauge this during construction. Dave Hurd, in his 'Left Brain Lutherie' goes into the methods for doing this, although I don't remember that he set out upper and lower limits. Many builders do this by 'feel', but until you've made a few you won't know what it's supposed to feel like...

[Peter Wilcox]

Should I pull all the bracing and start over?

If nobody else will answer this, I'll try. Yes, I would take it off and redo it more properly. It might be difficult to get the old braces off, unless you used hot hide glue. If not HHG, careful use of a chisel or small plane will be time consuming but will work. I, being lazy, would use a small trim router held up by wood strips along side the braces (to avoid damaging the top) to get most of it off, then finish with a chisel or scraper. You'll probably need to take off the bridge plate too.

Other folks may have better ideas.

[John Damon]

SINCE I’m this deep into it, maybe I should replace the plywood sides with solid maple sides I have. Any thoughts on this? I’ll be looking for advice on size/shape of bracing for the top soon

[Barry Daniels]

I wouldn't recommend that unless they are seriously compromised. Solid wood sides aren't likely to make the guitar sound any better than the old plywood ones. Just get it back together and then build a guitar from scratch.

[Alan Carruth]

Shades of the 'Vin Fizz Special'. That was the first airplane that made it across the country in less than a month: 32 crashes in 29 days, iirc. When it landed in California the pilot had a broken arm, and the only two pieces of the original left were the gas cap and one wing strut. You've got to draw the line someplace, and replacing sides would be the place here.

[Matthew Lau]

Just chiming in to say this is a fascinating thread.

When I was first taught to build a guitar, my teacher told me to get a broken old guitar and replace the bracing.
I'd x braced it...and he told me to scrape it all off, and ladder brace it. It was an Airliner/Sears & Roebuck guitar.
The final guitar ended up having a strong fundamental, and working really well for blues.

John, I'd recommend finishing this up....and then maybe starting with a Stewmac kit.

[John Damon]

Here’s how it’s going so far. This is a picture of the bracing that I had put in and some more so you can see how big they are. The side laminate has delaminated a bit in an area. I have to glue that back together before I do anything else I think.

[Barry Daniels]

I see a lot of issues here. Where to start? The X-brace is poorly located. The bridge pin holes should all fall between the braces meaning the X-brace intersection needs to be a lot closer to the sound hole. The bridge plate should only be located in the lower quadrant of the X-braces. All bracing in the lower bout of the guitar should be much lighter. The ends of the braces should taper down to nothing instead of hitting the guitar sides at full height. The brace between the two lower legs of the X-brace should not touch both braces.

I would shave down all the braces and start over. I think you can get a lot of tonal improvements if you follow our suggestions.