Replace old ply back with solid wood?

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John Damon
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Replace old ply back with solid wood?

Post by John Damon »

The guitar in question is an old late 50s silvertone 620, which is a cheap copy of a j200. It has a solid spruce top, but laminate sides and back. Replacing the back would also allow me to replace the existing ladder bracing with x-bracing under the top. 2 birds with 1 stone kind of thing. The guitar doesn’t sound bad as is, and looks REALLY cool with its age and mojo. I just wonder if, sonically, I’d have a better sounding guitar if I proceeded. The sides and back appear to be mahogany/maple/mahogany.
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Alan Carruth
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Re: Replace old ply back with solid wood?

Post by Alan Carruth »

Properly done X-bracing should produce a better sound. I would not expect a large difference from replacing the back: most of the sound is in the top anyway.
John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

Thank you
John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

Now I’m wondering, would x-bracing dramatically change the sound of the guitar compared to the existing ladder bracing? If so, how would the guitars voice change?
Mark McLean
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Re: Replace old ply back with solid wood?

Post by Mark McLean »

Ladder braced guitars can sound great, with a tendency for more fundamentals and less complex overtones. A bit more of a bark. The best ladder braced guitars tend to be small bodied. It is not as strong as X-bracing, especially along the grain. To effectively brace a large soundboard like your jumbo-sized Silvertone the ladder bracing needs to be fairly heavy, which is a tone killer. Changing it to X-bracing is likely to produce better sustain and a more mids-focused sound. I agree with Al that changing to a solid wood back is not likely to make a big difference.
John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

What would constitute “properly done” x-bracing? Size, shape, pattern… good gluing technique? I figure I’ve got one shot at this, so why do it wrong? Also, how to remove the back? 🙂
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Barry Daniels
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Re: Replace old ply back with solid wood?

Post by Barry Daniels »

There is plenty of information on those topics in our library. Have you looked there?
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John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

No but I’ll go there immediately, thank you!
John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

I looked it up three different ways- and I found nothing. I see it discussed as a known entity, everyone seems to be aware of the process, but the process ITSELF is not described. I assume I’m to heat up and remove the binding, then heat up a thin paint spatula and “melt” the glue between the back and rim glue blocks? Then clean up rim to accept glue again, and re-glue? (After brace work)
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Barry Daniels
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Re: Replace old ply back with solid wood?

Post by Barry Daniels »

You may have just been searching in the current discussions and overlooked the old archives (Old MIMF Library). Here is an old tutorial on back removal.

https://www.mimf.com/old-lib/back_removal2.htm

Heat can be used but don't overdo it. You don't really melt any glue but more force the joints open with the gentle application of stress with the putty knife. I often use single edge razor blades to cut the back joint open after the binding is removed.
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Bob Gramann
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Re: Replace old ply back with solid wood?

Post by Bob Gramann »

Heat has the potential to delaminate the plywood back. If you still plan to replace it, no problem. If it comes apart, it might be hard to relaminate and reuse.
Alan Carruth
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Re: Replace old ply back with solid wood?

Post by Alan Carruth »

I'd simply rout the back out, since you're going to replace it anyway. The trick (more often used on tops) is to make the new back first, so that you can trim it exactly to the right shape before you brace it. Once you take the old back off the edge of the rim will distort and you won't have the outline any more.

Cut through the old back a little inside of the liner, making sure you go out around the end blocks. You then chip away the rest of the back material out to the binding, making sure you don't damage the binding. Clean all the plywood off the liner down to bare wood.

Presumably you trimmed the new back to the exact shape of the outside of the sides and binding. Now you trim off the edge by the thickness of the binding, so the new back will drop into place once you've cut the inlets in the liners for the back braces. Once you've gotten the new top bracing in you can glue in the back. Usually you'll need to put a line of purfling inside the binding to cover any gaps. It's a relatively easy thing then to finish the surface of the back, without having to match the old finish on the sides and binding. If the new back was properly trimmed to shape the rim will end up perpendicular all the way around.

Although I've never done this with a back, you might find things would ge even easier if you leave tha back cross braces in place, cleaning the old wood off just as you do with the liner, and just gluing on a new plate of solid wood. Torres, apparently, used to glue the back on that way, so there's ample precident.
John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

Wow, thanks for all the tips!
John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

Ok, so bracing… glue down square stock and shape afterwards? Or shape the ends first to go beneath the glue dentals, then glue down/finish shaping when glue dries? Or shape everything first, then glue in place?
Thank you in advance for any help, it is appreciated by myself and this old guitar!😃
Alan Carruth
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Re: Replace old ply back with solid wood?

Post by Alan Carruth »

Backs are usually arched a bit, so the glue surface has to be shaped to the correct curve. Often, but not always, this is something like a 15' radius. There are several ways to do this, depending on your level of experience and tooling.

The braces can be glued to the back individually, using a number of clamps, and a flexible caul on the outside; such a strip of 1/4" plexi or Masonite, about an inch or so wide. Many makers use a radius dish, which allows for gluing more than one brace at a time, using either go-bars or a vacuum. It's normal to leave the braces at full height for this, to keep them as stiff as possible, and make clamping easier.

Once they're glued on you can take the ends down to the proper height to inlet them into the liners, and shape the braces themselves.
John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

Thank you, Alan. I’m not there quite yet- I’ve removed the ladder bracing from the soundboard and cleaned it up and installed the main “X” 0f the new x bracing. They are tall, pyramid shaped, rounded tops. Had to lift the soundboard upper bout to meet the braces with the bottom of the soundboard, but it is done. The rest of the bracing (on the top) is what I’m wondering about. Here’s a picture-
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Alan Carruth
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Re: Replace old ply back with solid wood?

Post by Alan Carruth »

It wasn't clear that you were talking about the top bracing.

Did you arch the top bracing? That's often a flatter arch, about 25' radius. It can be made flat, but many of us believe that arching the braces helps reduce cracking problems down the road. Many makers make the upper transverse brace flat, which facilitates getting the neck set and bridge height right.

On my builds I start with the smaller braces on the top: the tone bars, finger braces, and 'A' braces on either side of the hole. These are glued full height, and trimmed down as needed to notch under the 'X' braces and the UTB. The ends of the 'A' braces inlet into the neck block.

A good rule of thumb is that any brace that won't be trimmed down to zero height at the end needs to be inlet under something. This generally is the case for all the brace ends between the bridge and the upper block, since bridge torque is pushing them down.

Many glues have fairly low peel resistance; once an end of a brace comes loose it's easy to work it up from there. If the brace is tall enough at the end to be stiffer in bending than the top (which would be much less than the thickness of the top at that point) downward pressure, or a knock on the top, can start the peeling.

Be sure to use some sort of patch on the open side at the crossing of the X braces. The X crossing is also under a down load from the bridge torque, and this tends to spread the lap joint open. The notch is a big stress riser, and it's not uncommon for the brace to start to split there. Tying the open end of the joint together can prevent this. I like to use a short (1" or so) piece of spruce from the top for this: the actual tension load on the patch is not large, and it doesn't need to be thick, but you need to have some glue surface. Flatten the top of the braces out if they've been rounded off to glue the wood patch down. Martin relied for years (and maybe still does) on a cloth patch, and these can work well. I believe the original ones were linen circles, such as were used in muzzle loading guns; a heavy cotton will do. Soak the patch in glue and work it on over the junction, making sure it conforms well out to the edges, and use paper towel to take up the excess glue. I've used this on guitars, such as Taylors, where the braces have been rounded over before they were glued to the top , producing gaps on both sides of the joint and leaving no flat spot to glue a wood patch.
John Damon
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Re: Replace old ply back with solid wood?

Post by John Damon »

Nice, thank you!
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Barry Daniels
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Re: Replace old ply back with solid wood?

Post by Barry Daniels »

I would replace the kerfing at the end of the braces to reinforce that area to prevent the delamination Alan described. A triangular piece of wood on the upper ends of the x-brace goes a long way in adding strength.
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Barry Daniels
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Re: Replace old ply back with solid wood?

Post by Barry Daniels »

For the bracing on the top you should add the normal braces of an X-brace pattern, including the upper transverse brace (UTB) and a couple of braces on either side of the sound hole.

A couple of finger braces from the lower legs of the X-braces to the edge of the guitar would also be appropriate. And I would also add one tone bar in the lower bout.
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