assortment of archtop doubts

[Fernando Esteves]

Hello guys, I've some doubts regards archtops

1) I've an Adirondack soundboard blank for archtop, but it has a dark stain in the middle and would be ugly to do a 17" archtop in natural/burst...
That leads to 2 options: I learn how to make the stain have the same color as the other parts (which I'm not very confident is possible) and make a smaller archtop, like the L30.
If anyone has a guess on the color, let me know (I attached a picture of the wood, the other parts oxidized and the difference is not very big now, but when you sand it, it is very different)

2) L30 archtop size: the height of the archtop center compared to the edge has the same size as a full archtop or is it proportionally smaller? (I've attached a image explaining the doubt)

3) The Gibson L30 and Epiphone Olympic have about the same width, but the L30 is flatback and the Olympic archtop. What should I expect in difference on the tone with each option?

4) I'm planning to do a small hole beneath where the bridge will be located, with a reinforcement on the bottom of the top, to pass the cord of the piezo pickup on LR Baggs Anthem pickup, do you think it would be a problem, even I doing the reinforcement? (I really think the archtop pickups with the cord over the top to be ugly as hell).

5) Is there anything as ideal position of the bridge on an archtop? The bigger body I'm trying to design by myself and not sure if there is a optimal position from both ends.

6) Thickness: on Benedetto book, he says about max 3" thick, but the Olympic is about 3.5" and the L30 about 3.75", because they are smaller, I think. Would be a problem to do a whole 4.5" or 4.75" like flattops?

I'm sure will appear new doubts, sorry about that! =D

[Alan Carruth]

1) Paint it black.
2) The height of the arch should scale to the thickness of the top, not the height of the arch. A smaller span be be a little thinner and retain the needed stiffness for a given Young's modulus along the grain (E-long), but if you go with a 'standard' thickness the arch height should be the same. Cross grain stiffness doesn't seem to matter as much in the long term, as cold creep reduces it's contribution, at least in flat tops. On archtops you have to pay a lot of attention to thew arch contours, particularly in the recurve, to keep from developing long term sinking.
"3) The Gibson L30 and Epiphone Olympic have about the same width, but the L30 is flatback and the Olympic archtop. What should I expect in difference on the tone with each option?"
I thought we were talking about the top: are you introducing a new variable here?
4) A properly reinforced hole should be OK.
5) The bridge goes on the highest point of the arch, and, of course, the proper distance from the 12th fret. Don't worry too much about the upper bout.
6) Body is the sum of the rib height and the top and back arches, pretty much. It makes a difference in the timbre, of course, but it's not something that's amenable to simple classification. You have to think about the relationship between top area and depth, for one thing.

[Fernando Esteves]

Thanks for the answer Alan!
I plan to use 009 or 010 strings, so the top might be thinner than an usual archtop

Yes, the back is a totally new route, I've many many doubts!
I'm planning to go with Mahogany or South American Cedar, do you have experience in the difference from an arch back to a flat back?

I've read the Benedetto book some time ago, but just bought the dvds, gonna give it a look before start putting the ideas at work

[Alan Carruth]

I used mahogany for the B&S of my first archtop, and later for the first successful nylon string archtop. It works fine.

I've had a hand in two flat top/arch back guitars over the years. The sound is somewhere between a flat top and an archtop, but I'm not prepared to get more specific than that. Both were a long time ago, and I don't have the sort of data I'd like, for one thing. In any event, the back has a lot less influence over the sound than the top.

One advantage to using an arched back and top is that you can 'tune' them together. The modes are pretty much the same, and matching the pitches of the 'free' plate ring modes before assembly works well. Flat backs are a lot less predictable.

"I plan to use 009 or 010 strings, so the top might be thinner than an usual archtop"

You settle on the thickness based on the down bearing load of the bridge: basically the tension and the break angle. Bridge height per se is not a factor, since you could get a tall bridge by using a lot of overstand on the neck. You don't need a lot of break angle: iirc Benedetto shows something like a six degree break angle, and with a notched saddle you don't need much more than that.

On my arch Classicals I've used a 3mm top thickness, so the arch height measured from the bench (the lower surface of the edge)to the top of the arch at the bridge location was 15mm, which is what they use on fiddles. I made the mistake on my first few arch Classicals of making a thin top and a high arch, and got really odd modes. The 'ring' modes in particular had a habit of splitting into two modes that were not closed rings. Then I read a paper by John Schelling on 'The Violin as a Circuit' in an electrical engineering journal where he talked about scaling the arch to the thickness. I chose 3mm somewhat arbitrarily as being thick enough (I hoped) to support the load with the low density spruce I wanted to use, and the arch height came from that.

One of the tricks there is that I use 'curtate cycloid' cross arch profiles, with the lowest point of the arch right over the inside edge of the liners. Scraping the edges down from there, so that it ends up rising out to the liner, invites having the top sink over time, particularly if the recurve is made too wide. I've seen some nice archtops with really tall bridges due to top sinking.

If you have not tried it yet, the Android app 'Luthier Lab' will plot out contour maps for arched plates based on the cycloid archings, given the lengthwise curve and outline. I've been plotting mine the old fashioned way; with holes punched in disks, but the app is probably quicker.

[Fernando Esteves]

Nice to know about the app, will take a look!
Thanks