Retopping first Falcate braced Classical Guitar

[johnparchem]

My first first falcate classical guitar had a lower resonance than I have had in my four falcate braced classical guitars. It is a nice looking guitar with a panama rosewood back and sides. The guitar was not bad sounding but did not really have any punch on the 1st string up the neck. I also have been wanting to try a retop where the binds are kept intact.

I have not yet completed the re-top but I have got to the point where I think I can pull it off. The trick to the retop is to route new top to be the exact size to fit into the bindings of the guitar. Picture will show it best, but the basic idea is to mount the new top onto the old top spaced a bit from the top. A binding machine is then set to cut the top to the inside of the binding or binding and purfling line using the guitar itself as the index for the binding machine.

FIrst off I removed the bridge of the guitar and prepared jointed and loosely profiled the new top.





I used double stick tape to put down a bunch of spacers on the old top. I just cut up a bunch of the waste from the top.





I set the binding jig to just cut past the purflings, I used my test cut to check on the guitar. Then I routed the new top leaving the bindings alone







I cut the top out and used a bit of heat to loosen the glued down pieces.





The binding survived although I did pull a bit of the purfling when removing the top pieces for the linings I used heat to remove the rest of the purflings and will reinstall.

I made one of my classical wood ring rosettes, cheating a bit by installing premade rosette and routing out the center end grain tiles.









The new top fits right in, so I am ready to brace it using the experience I had gained on my last four classical falcate braced guitars.

[kiwigeo]

John I notice you've got the neck bolt down holes offset..is there a reason for this. One thing Ive done with these bolts is go to 4mm bolts as opposed to the 6mm bolts in Trevors books.

[Mark McLean]

Nice method John. I wish I had seen this a few weeks ago. I am re-topping a OM (my first build) on which I did a pretty poor job with the bracing. I took the top off by routing off the bindings, and then discovered that I don't have any of those type of bindings spare to replace them with. Using your method would have made life easier.

[johnparchem]

Thanks guys.

Martin, The offset hole is there because of a router slip on the neck extension when rerouting the truss rod slot. Basically I relearned there is only one direction to go on a router table.

Mark, I have done a few retops just the way you described. I saw this method posted in some other forums. I also found a tutorial on http://www.grevenguitars.com/retopping-demo.html. Greven also routes out the top on the other side of the bindings.

[Dave M]

John that is seriously clever work. I find it hard to believe that the fit was so good as to just drop in.

I have to admire your patience. I think I'd be inclined to just start again ad write the old one off!

[johnparchem]

John that is seriously clever work. I find it hard to believe that the fit was so good as to just drop in.

I have to admire your patience. I think I'd be inclined to just start again ad write the old one off!

Thanks,
It dropped in OK, as I used the same index (the rims) that I used to make the original binding channels. I was ready to pull the bindings if this did not work. The next step that I did not do was to use a router (still indexed off the rims) To cut up to the bindings or purfling on the inside.

[johnparchem]

I braced the top and have glued it back onto the body. It actually went pretty well. I bent a couple of sets of 1.7 mm steps on my hot bending and laminated them using west system epoxy. I used a a band saw and cut the two laminated braces to produce the full set of braces.





After the last messy falcate top, I lifted my go bar deck from the floor to the top of my bench. With extra light I could see well enough to cleanly apply just enough epoxy to clamp on the falcate braces.





I clamped the transverse brace in place so that I could carefully mark out the channels for the braces. With the braces marked I cut them with a razor saw in a parrot vise. I place the brace so the vise is used as the stop for the cuts. I use a chisel to punch out waste.







The brace fit well, I glued, profiled profiled it and fitted the top to the body





The top fit pretty well. I did replace the purfling because I'd pulled a chunk back earlier when I removed the top. Without glue and the top in the body I fitted the purfling. There were a few places on the top that I had to lightly sand to make room for the purfling. It did go well, so I glued on the top and the purfling.

[johnparchem]

The top went on pretty well. Probably less gaps than I have after I bind. I filled the gaps and started to french polish the top. At this point I know the guitar is going to turn out OK.





I could have used the padauk bridge I had on the the last top, but I did not take enough care when I removed it. so I laid up and profiled a walnut and a BRW bridge blank. I resawed into three veneers 4 mm, 3 mm and 2 mm thick and laminated them back together with two sheets of carbon fiber. The BRW bridge sure looked good but it was 24 grams and the walnut was 16 grams (before the final profile. I will ebonise the walnut bridge and use it. I will save the BRW bridge for a time when I have a top that is a bit too stiff.









I had a fun time with my micro-mill and did most of the final profile on the walnut bridge. I finished with a chisel.




Bridge before final sanding and ebonising.

[ernie]

Hi john what species of wood are you using for the falcate bracing on the top?? thanks ernie,

[johnparchem]

These braces are Sitka spruce, I have used Englishman as well both bent well with heat. I used German spruce but had a harder time bending the strips I had.

[ernie]

thanks john regards from KC MO ernie

[kiwigeo]

These braces are Sitka spruce, I have used Englishman as well both bent well with heat. I used German spruce but had a harder time bending the strips I had.

I'm using Sitka and Lutz...it's important that the wood is as stiff as possible. I also make my braces a bit higher than specified on Trevor's plans....9mm. I usually tap test my bracing stock and pick the stiffest pieces for falcate bracing...generally the Lutz comes out with best results. If you're having trouble bending a wood then there's always the option of going to thinner layers and using four instead of three layers on each brace.

[Dave M]

Yes I have always ended up using 4 layers for the tight bend on the secondary falcates.

[johnparchem]

It is all strung up. I ran a first plot with strings and the T(1,1) 1 = 91 Hz, 2 = 183.5 Hz and 3 = 234 Hz. With this I think I can pretty much nail 95, 180, 226 Hz as everything is in the correct direction for tuning. My plan is to enlarge the sound hole (I left it a bit small) which can move T(1,1)1 to or toward 95. Then I would tune the backs lower transverse towards to lower T(1,1),3 to 226 Hz from experience I believe that will move T(1,1)2 to very close to 180 Hz from coupling. I adjusted the matlab 4dof model I got from this site to match what I was seeing with my tap test. I modified the sound hole are to move T(1,1)1 to 95 and then lower the backs stiffness until T(1,1)3 was 226 Hz. The results were very close to my desired results. I am pretty excited as I will be able to hear a properly tuned 180 Hz classical guitar.

retop first plot with strings.JPG (65.99 KiB) Viewed 28771 times

Here is a plot with the model tuned to match my guitar

Here is the plot after modifying the sound hole area and the back stiffness as described above,

[johnparchem]

On second thought I decided to first lower the back resonance to close to the target of 226 Hz. That went well and it lowered the top's resonance to 180.1 Hz. I am waiting on changing the sound hole. Lowering the back dropped the Air resonance to 89 Hz. In between notes which is good, but almost an octave lower than the tops resonance. I fear that 195 Hz is too far away and I do not want to end up right on F# 92.5. I might add a black purfling strip to the inside of the sound hole to drop the air resonance a Hz or 2.

The guitar really sounds good especially given that I just strung it up today. I will wait a few days before I do more.

[Jim watts]

John, nice job there!
Did the trebles stay focused and clear when you drop the tops resonance to 180? I've been kind of afraid to go that low for fear of losing clarity. Probably a misconception on my part.
Also, you make reference to a Matlab script you found here. If you could point me to it that'd be swell.
Jim

[johnparchem]

Thanks Jim, At one day it is too early to tell but the trebles sound pretty good, the overall voice is slightly richer to my ear than my 190 Hz guitar. It set up quickly as I was mostly able to use the setup for the previous top. I routed the saddle slot a half mm deeper on the new bridge so I made a new saddle. This guitar has a compensated nut, so I just used the string length from the compensation to intonate the saddle.


Here is where I got the matlab model viewtopic.php?f=33&t=4636&p=57822&hilit=4dof.m#p57822

[Bruce McC]

Hi Jim

...." I might add a black purfling strip to the inside of the sound hole to drop the air resonance a Hz or 2. "

could you please explain? Are you in effect making a mini tornavoz?

[Bruce McC]

Hi John,
oops I meant John not Jim.

[johnparchem]

Hi Jim

...." I might add a black purfling strip to the inside of the sound hole to drop the air resonance a Hz or 2. "

could you please explain? Are you in effect making a mini tornavoz?

Not really a tornavoz, just a way to make the sound hole a bit smaller and look good. I do this with sound ports sometime. Once glued in place I would trim it inside and out to the thickness of the top and sound hole backer. A 1 mm strip will change the diameter of the sound hole by 2 mm which will lower the air resonance a bit may be 1 to 1.5 Hz. If I make the sound hole bigger the Air resonance will go up. I can may be sand back the sound hole diameter 4 mm or so. I think that amount will leave me with an air resonance of of 92 or so.

[Bruce McC]

Thanks for that John.

[Jim watts]

Thanks for the link John, I'll give it a whirl. I'll need to try it in Scilab.
Jim

[Jim watts]

Quick update. Jim Kirby's code runs just fine in Scilab using their Matlab to Scilab translator function.
Scilab is a free Matlab like program for those who may want to give it try.