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I've been putting this off, but it's time. I need to figure out how I'm going to brace a really different sort of instrument. Behold...



This, my friends, is a bass guitar. Or it will be. I hope. Bear with me here. I realize what a phenomenal waste of wood this is, but several years ago, before I really knew better, I bought a big piece of 12/4 African Mahogany to build a big hollow body guitar. I goofed it up really bad, and a couple of months ago I took a look at it, and decided with some work it could be saved. It's hollowed out, and will use a soundboard with an acoustic bridge. Weird enough? Just wait.

It will also have six strings, tuned EADGBE, one octave below a standard guitar. (Any Fender VI fans out there?) It will blend a guitar humbucker in the neck position with an under saddle piezo transducer.

Now I realize how unlikely it is that anyone has actually built an instrument like this before, but it's all theory. I have a WRC top, and my goal is to produce something that closely approximates the sound of an acoustic bass, maintaining realistic expectations. But I'm a little concerned about the strength of the soundboard all that string tension. On the other hand, I'm concerned that the back and "sides" will be too thick as to allow the soundboard to vibrate correctly. Then again, piezos tend to be pretty forgiving, in my experience.

I think I'd like to do something like fan bracing, but I realize x-bracing would give me more strength. Or I could just glue a couple of sticks down the middle like on Taylor's T5.

What would you do? Also, on an instrument like this, do you even think it would matter much?

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David Morris

You might want to do some calculations as to what the string tension on a 6 string bass might be to aid you in your bracing scheme. If it were me and since it's going to be mostly electric, I would error on the heavy side of things. This was one calculator I found that could be potentially used for a bass. I dig your design!
http://wahiduddin.net/calc/calc_guitar_string.htm

The bracing will matter a lot. For a big acoustic tone I would use x bracing. For a feedback resistant stage axe, fun to practice with no amp, I would use two bars.
I hope this next idea doesn't sound too wierd. It worked for me on an archtop guitar that I wanted to sound a lot like a 335 but more acoustic.
I made the back easy to remove and put a wooden brace from the heel to the endblock to take the string tension. I could then string up the guitar with the back off, play it through an amp, or hold the back on to hear the acoustic sound. I could glue on more braces and carve the braces and hear the changes immediately. Its not like I could copy any other guitar. My method is better than guessing. It doesn't look like you'll be reaching through that soundhole to carve a brace.
If i ever do another one I'm going to attatch the back with a couple dozen tiny brass screws. Obviously the lining is going to be a little unusual.
Good luck! Do you know how to tune the soundhole? Bigger soundhole= higher pitch of the air resonance. If you blow over the soundhole like it was a big pop bottle you can usually figure out about what note the box is tuned to. If you cover the soundhole with a magazine and thump the bridge you can hear about where the wood is tuned. The relationship of those two notes is the biggest factor in giving a guitar its personality.

Let me say up front that you can't take my advice as anything near expert.

I do these things by intuition, and I often ask others for advice, as you have. This is an archtop...yes? If I were building it, I would close the angle of the x-brace so that the space between the brace tails would be narrower towards the neck and tail. Wider towards either side/waist area. I'd try to land the upper ends in the upper bouts, and the lowers ends farther down in the lower bouts. Then I'd think about cross struts boxing off the pickup rout area. I'd maybe think about a similar strut in the lower bout area, depending on the thickness of my top, the graduations, etc.

Now....if this is a flat top, just forget I even responded, because I know next to nothing about flat top instruments.

Either way, I have probably stirred the pot here, and I'm sure some genuinely knowledgable people will chime in and then you and I will both learn a lot.

Good luck with it. Looks to me like it ought to be a killer axe if you can pull it all together, and I'll be you can!

Patrick

Actually, and I should have mentioned this, I'm planning for a flat top, meaning flat - no radius. I don't have a radius dish right now, and for this, I'm not sure how much value a radius would add. I'm open to suggestions, or course. I could certainly make radius dish, but a lot of successful builders use flat tops.

I think I agree with Mitch that I ought to err on the side of caution and use heavy braces. Even my pure acoustic guitars are amplified 99% of the time. This is intended for playing through a big effects unit that can handle a number of instruments, and has a good EQ (that I use a lot). As always, it will be a balancing act. I love the idea of shaving the braces after stringing it up. It won't work on this one, but it's a great idea.

Philosofriend, I've had an idea I've wanted to try for a while, and I think this instrument is nutty enough to be a good guinea pig. The single hole on the soundboard won't be ideal for the air exchange I'm gonna need, and rather than get into a bunch of theoretical calculus or something, I think I'm going to turn the access panel on the back into a sort of sliding window. Sort of like an adjustable sound port. I realize it won't behave in the same way a typical soundhole would, but at worst, it will be a weird idiosyncrasy. I have to put an access panel on the back anyway. It might as well be useful.

Good advice, guys. Keep it coming if anybody else has thoughts. And yes, this is likely to come to fruition. The body and neck are almost done. I've held off on the soundboard.

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David Morris

I don't know if this helps.. but I'm building an electric flat top at the moment. And I put a solid strip down the centre. Here's pictures...

Neil - I've been waiting for some updates on that. It's a beautiful design, and I'm looking forward to seeing it realized. Are you planning on adding any acoustic pickups?

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David Morris

That will really be something!?!?
IMO: A full length X like on big archtops. And depending on top thickness, probably a cross piece where the neck/FB meet the top.

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You just pick up a chord, go twang, and you're got music. -Sid Vicious

It will certainly be something. I'm not sure what, but something. I think murrmac is onto something with putting a second crossbrace South of the bridge plate. You guys have convinced me. I'll do the x-bracing. Thanks!

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David Morris

I think murrmac's got it.

If the soundhole were in the normal location, I'd do a thin top with standard X layout with 3 fingers and 3 tone bars, and large bridge plate. But with the soundhole there, finger braces on the other side would only unbalance it worse, so I think the top just needs to be left thick, in which case the fingers and tone bars are all unnecessary. And then it's already heavy, so the behind-bridge brace is probably better than the large bridge plate.

I like the upper transverse brace being pretty far down. Isolates the pickup from the vibrating area, and makes the vibrating area approximately round, with good centering of the bridge.

I'd taper the X legs some, and scallop to nothing at the ends. Maybe 1 inch scallop at the upper legs (leave a little height at the linings if you worry about them coming unglued), and lower scallops starting at half way between the bridge brace and the perimeter, reaching zero height at maybe 1/2-3/4" from the linings. After you get the box assembled, thin the soundboard perimeter all the way around quite a bit to free up the monopole some more.

And I'd do it in western red cedar. It's lower density than spruce, so you can get more top stiffness for less weight.

But that's all just instinct... I've never done anything like it before. I am planning an ABG with the 3 finger/tone bar and normal soundhole layout, however

Thanks for weighing in, Dennis. You make some good points.

Ok, since you guys did so well on that one, let's move on to the next theoretical challenge. That little hole in the soundboard isn't going to be enough, imho, and as I mentioned earlier, I'd like to try putting an adjustable sound port on the back. It will work like a sliding window that can be opened a little or a lot. That way I can tune it to where I like it, or shut it off entirely if I need to mitigate some feedback. My big question is where to put the sound port. I've got some ideas.



Position A represents what would be most convenient. I think I want to locate all the controls in the upper bout, but it's a negotiable point. If I use position A, it can double as a convenient access point, and I'm going to need some way in there. Positions B and C I have no logical support for other than they intuitively seem like they might be good spots. All we're looking for here is airflow, but I'm concerned position A is too far away from the action. Anyone have an idea?

I'm also gonna have to decide on some manner of gluing that bridge down, so I guess that's a consideration too.

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David Morris

This instrument would be a close cousin to a Gibson style guitar called a blues hawk......take a look.

You might want to do a little research on this instrument.... ( it wasn't a bass )

kevin

I always reminded me more of one of the Gibson Byrdlands with the florentine cutaways, like this one:



But I see what you mean. The shape isn't really all that original. I just tweaked some common design elements. Aesthetically, at least, I seem to be a traditionalist.

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David Morris

thats a very harsh/amaturish sunburst transition on that byrdland there! ...is that how they all were?

Isn't that what they call the darkburst? I'm no expert of historic Gibsons. I will say it's not uncommon to see a finish like that.

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David Morris