Fred Dickens back bracing pattern

Construction and repair of Classical Guitar and related instruments
mChavez
Posts: 24
Joined: Mon Jul 03, 2017 6:38 pm

Fred Dickens back bracing pattern

Post by mChavez » Wed Oct 11, 2017 11:32 pm

Hello All,

I am currently in the process of building my second guitar and, after reading about Fred Dickens' x-braced back response in the annals of GAL, I became interested in trying out this back bracing pattern.
Would anyone have any plans/photos of a Dickens-braced back? The Catgut Issue 26 (p.19) shows 2 cross braces (upper and lower bout) crossed by 3 longitudinal braces.
What brace height would be a good starting point? I understand that the upper bout cross-brace shall be stiff to reduce the upper bout vibrations? Would the central longitudinal brace replace the standard marriage strip?

Any advice is greatly appreciated!

Thank you.

Alan Carruth
Luthier
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Joined: Mon Dec 24, 2007 6:56 pm

Re: Fred Dickens back bracing pattern

Post by Alan Carruth » Thu Oct 12, 2017 8:50 pm

I used to visit with Fred when I went to my violin making sessions at Carleen Hutchins' in NJ. In spite of spending a fair amount of time talking about back bracing, I don't really remember exactly what he did. I do remember the objective, though, and worked with a number of different back bracing schemes over the years since to try to realize it.

Fred had done some computer modeling using electrical circuit analogies to get a handle on the low range function of the guitar, in the 'bass reflex' range (roughly up to about the open G string fundamental). He had noticed that guitars that he liked tended to have a monopole resonance ('main back' mode) that was not too much higher in pitch than the top monopole. When that happens you get coupling between the top and back modes that effectively increases the compliance of the box, and lowers the pitch of the 'main air' resonance, which is the other half of the 'bass reflex' action. His objective was to come up with a back bracing scheme that would allow him to 'tune' the pitch of that back mode, and also enlarge the area, which would increase the coupling strength.

You have to keep in mind here that Fred was also working with 'free' plate modes at the time (that's where I caught the bug). Since a normal Torres-braced Clasasical top will produce a very clean 'ring and a half' or 'ring+' mode there was some hope that a similar sort of thing could be done with the back. Again, at that time Carleen was exploring the 'free' plate modes of violins, looking to see if there were relationships between the top and back plates that produced 'better' instruments, so there was some thought maybe guitars would be similar.

IIRC, the first layout that Feed tried was a sort of lattice of four braces; two parallel member that ran along the length of the back, and two that went across. He was able to tune the free back to produce a well formed 'ring' mode, and that gave a strong monopole mode on the assembled guitar. The advantage of this was that you had independent control of the lengthwise and crosswise stiffness of the back, so it was relatively easy to get the correct stiffness ratio to produce the ring. What was harder, apparently, was to get the ratio right and also the overall stiffness, so that the pitch of the assembled back monopole was where he wanted it; about a semitone higher than the top monopole. That lead him into other brace layouts. I don't know if he ever settled on something as a 'standard'. The last time I saw him was, briefly, at the GAL meeting in '92, at which time he told me he'd given up on 'free' plate tuning.

In the years since I've tried out a half dozen or so different back brace schemes, many involving 'X' bracing of one sort or another. One of the tricks with X bracing is to figure out the 'correct' angle for the X for this piece of wood. The long-grain to cross grain ratio of back wood varies a lot: a quick look at my data shows values for Indian rosewood ranging from 3.5:1 to 7:1. Even with that data in hand it's hard to get it just right, and then you still need to figure out how to get the absolute stiffness to weight ratio right so that the pitch is correct.

As it turns out, it seems to be easier to get things to work well using plain old ladder bracing, but not necessarily the way it's often done on Classical guitars. Most makers seem to hold with the 'reflector back' model, which envisions the back as a rigid element that doesn't move appreciably, particularly in the low range. What Fred was after was an 'active back', and that tends to call for a lot more flexibility, particularly in the lower bout, than the usual tall and narrow braces will allow. You can shave them down, and particularly the lower one, to make the back more mobile, but that ends up with some pretty small braces that don't inspire much confidence.

What I found was that using four back braces, instead of the usual three, and making the lower two much lower and wider than usual, worked better. Thus is the back brace scheme that the Martin company has used for a long time, and, although they do now mostly make steel string guitars, the designs before the Dreadnought in the late 'thirties were meant for gut strings.

With this setup it's fairly easy to get an active monopole below the waist. Making the waist brace low helps extent that up into the upper bout some, at the expense of dropping the pitch, perhaps too low. I'll note that I had the same issue with the X-brace back patterns I tried; extending the monopole up into the upper bout to any appreciable degree required making the upper part of the back more flexible than I liked, both acoustically and structurally.

So, I'm sorry I can't tell you more about what Fred did (I miss that guy!). Maybe my own experience helps a bit, though.

Joseph
Posts: 40
Joined: Sun Aug 07, 2016 10:18 am

Re: Fred Dickens back bracing pattern

Post by Joseph » Fri Oct 13, 2017 5:44 am

Thanks Alan, I found that informatio really informative
Joseph "Soxy" Price
Professional Luthier,
Specialising in Advanced Playability and Accurate Intonation

MessyTendon
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Joined: Fri Jun 19, 2015 1:33 am

Re: Fred Dickens back bracing pattern

Post by MessyTendon » Fri Oct 13, 2017 6:34 am

Interesting Allen. I saw one of those early Martins sadly with steel strings. But it was still holding together good.

Do you know of any set's of plans for those older gut Martin guitars?

Alan Carruth
Luthier
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Joined: Mon Dec 24, 2007 6:56 pm

Re: Fred Dickens back bracing pattern

Post by Alan Carruth » Fri Oct 13, 2017 6:03 pm

I have not seen any plans specifically for gut-strung Martins, although I'm sure the information must be out there someplace. Perhaps the Martin guitar museum would have some?

I've made two Classical guitars based on Martin's 12-fret 000 model; the largest one they designed for gut strings back in 1909, iirc. About all I did was to make the top a bit thinner, and reduce the top bracing size to suit. I did, however, use tieblock bridges, rather then the pin bridge Martin would have used. The pin bridge was common in German and French guitars at the time, but these days, at least in the US, it's considered to be exclusively a 'steel string' feature. Somebody who doesn't know better might try to put steel strings on it if it has a pin bridge. We actually got an 1844 Panormo in with bridge damage when the inheritor of it tried putting on steel strings (mediums, yet!). Fortunately he didn't get them up to tension or they'd have ripped the top right off.

But I digress. Those nylon-strung 000s worked very well: I showed one to Sharon Isbin and she quite liked it. She never noticed that the lower bout was a full inch wider than a normal Classical guitar.

mChavez
Posts: 24
Joined: Mon Jul 03, 2017 6:38 pm

Re: Fred Dickens back bracing pattern

Post by mChavez » Fri Oct 13, 2017 9:46 pm

Thank you very much, Alan.
I suspected that marrying the stiffness ratio with the pitch might be a problem and going with 4 braces seems like a safer option.

The only Martin plan that I have is a Martin 1-18 (1918) that suggests using 2 lower bout braces that are 3/4" wide by 3/8" tall, while the upper bout is braced with 2 braces that are 1/4" wide by 1/2" tall. Does this sound like a reasonable starting point?

May I ask what is the logic/formula behind spacing these braces out? Just break the guitar length into quarters?

And how would you recommend to go about tuning the plate/ controlling the stiffness ratio with the two lower braces?

Alan Carruth
Luthier
Posts: 2683
Joined: Mon Dec 24, 2007 6:56 pm

Re: Fred Dickens back bracing pattern

Post by Alan Carruth » Sat Oct 14, 2017 10:30 pm

That Martin plan sounds like a good place to start. I just put them where they look right... ;)

I just use the Edisonian approach when plate tuning: try something (not too much!) and see if it moves you in the direction you want to go. If it does, do more. If it doesn't, try something else. Keep going back and trying stuff over again; sometimes a particular brace will assume more importance as you get some of the other 'dead wood' out of the way. Usually the lower braces have more effect than the upper ones, but not always.

Of course, at first you don't know what's going to work, and you'll need to get some experience to figure that out. As a general rule, it seems to me that the guitars I (and my customers) like best are the ones that have the largest number of nice looking, and active modes in the 'free' plates. You do need to figure out what stiffness and weight you need to have in the back so that the assembled modes couple properly: so that the back monopole ends up close in pitch to the top monopole, but a little higher, and not too close, which can cause a 'wolf'. Since you probably work a bit differently from the way I do, you might find that different numbers get you where you want to be.

mChavez
Posts: 24
Joined: Mon Jul 03, 2017 6:38 pm

Re: Fred Dickens back bracing pattern

Post by mChavez » Mon Oct 16, 2017 7:29 pm

Thank you very much.

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