Crazy, stupid, archtop style, 6 string bass - how to build it?

[slowlearner]

Ever since I started building a few years ago I've been egging myself onto building a big, fretless 6 string bass. I've got a headless bridge (yes you read that right), some pickups and a huge sitka spruce top. I've got some Vic ash already glued up for a back. Basically what I'm aiming to do is to build a bass version in the spirit of one of Nigel Forster's archtops...

http://www.nkforsterguitars.com/instrum ... p-guitars/



This is the rough design I have so far. The idea is to have a solid set of sides like an electric. Lots of work to do yet.



It's a LOT bigger than a P-bass when up to scale. The neck is suspended above the belly and transmits vibration to the belly via and extra bridge.



I'm not entirely sure what Nigel has done for his tops... ie, whether it's really a carved archtop or just bent. I'm not aiming for a carved top. Rather, what I'd like to do is to put a radius on the top that will act in some what like an archtop. I have no idea what sort of bracing to use... or how much radius to use. I'm aiming for an upright feel.

Love to hear people's thoughts.

[Nick]

Hi Pete, interesting project. Are you planning on suspending the pickups or mounting them to the soundboard? If you plan on cutting holes in the top and mounting them to it then that bridge pickup will bugger up the typical 'acoustics' of an archtop as it breaks the continuity of the plate as a single vibrating member ( ). But it's an all electric anyway.... it's acoustic properties probably high on your priority list, more the aesthetics. Also your bridge mounting may need to be anchored to something more substantial than just screwed to the top (I'm presuming that's what you envisaged from your sketch?). If you aren't considering using it as a true archtop you could maybe glue a solid block between the plates.
Just a couple of initial thoughts.

[slowlearner]

Sorry, should have clarified. The neck is one long beam from the headstock to the bridge (think stick). It also supports the PUs. Vibrations are transferred from that to the top. So really the top is almost completely unstressed. Does that help?

[routout]

I looked at his site perdy nice design I like I was going in that direction of a different type of build must get back to drawing a lot of up rites fascinate me also.

[jeffhigh]

IMHO that is going to be a very inefficient way of getting string vibration to the soundboard due to both the mass of the bridge and the rigidity of the neck beam. It's hard enough getting adequate volume out of an acoustic bass without those handicaps.

[slowlearner]

Umm, ok.

I know it's slightly hare-brained. Maybe I need to build a working prototype with plywood first. I'm fully aware I won't end up with an acoustic instrument that works like an upright and produces fundamentals.

[slowlearner]

Here's a Forster style archtop being played acoustically...


youtu.be/

[jeffhigh]

I think even Nigel has said when he introduced this build that it was not the loudest.
Add in all the mass of a headless bridge and the inherent problems with an guitar sized acoustic bass and you might be able to play along with a softly strummed ukulele.

[Nick]

Sorry, re-read this post I'd just spent twenty minutes writing and missed the point of what you were proposing completely so I deleted it!

Jeff's right though, with the lower string tension a bass has and a fairly chunky piece of wood that the strings are anchored to driving the top, time the string energy gets through to vibrating the top, it would have the same acoustic properties as a box of feathers I'm guessing.

[slowlearner]

The laws of physics say, unless I've got a chamber the size of an upright bass it's going to be pretty quiet anyway right?

So, What I'm proposing is not so much a fully acoustic bass, but rather, an electric bass with some acoustic qualities. Which is really what I want anyhow, coz if it's fully acoustic it'll just feedback all over the place like my upright does.

Getting back to what you've said already, using the headless bridge is just a bad idea. Ok, I can see that now. As far as the neck/beam goes, could I shave it down so it's lighter, a little more flexible and able to transfer vibration to the top. Also, re string tension, we're talking a 6 string bass here. So it's a little more tension that you're average 4 string. I'm also planning on 36" scale, which again will add some more string tension.

[jeffhigh]

What about going es335 style with a centreblock inside the body?

[slowlearner]

Kinda done that style of thing 3 or 4 times already... sure they don't 'look' like a 335 but they're chambered to all heck with a centre block.





What I'd like to do is something with a floating top and semi-decent acoustic chamber.

[slowlearner]

Right, so lets assume I'd cast the Forster style archtop idea aside...

What would you guys do?

[jeffhigh]

Personally, if I wanted an upright sound I'd probably go with some form of piezo bridge pickup rather than the magnetics(or as well as)
That way you are driving the pickup from the bridge location ala upright rather than just picking up from locations along the string.
Also consider some form of string damping to cut sustain.

[slowlearner]

It will get some piezos for sure. Good call. What about an acoustic body of some sort?

[jeffhigh]

Hard to say, depends on your priorities for feedback resistance.
I have only built one acoustic bass it was full flattop acoustic with a pin bridge, but I built this to be quite acoustically responsive especially to the fundamental and this may not suit you for a loud performance situation.
You could also do a floating bridge tailpiece design on a heavily arched soundboard

[slowlearner]

Ok, Will that involve carving a top? Or can I brace a radius into the flat top I already have for the project?

[jeffhigh]

You could do that, just have to make the appropriate decisions on neck angle etc to give you a suitable break angle over the saddle, not too much nor too little

[slowlearner]

How much radius would you use for the top in that situation?

I guess break angle would depend on string tension, right?

[Nick]

Ok, Will that involve carving a top?

Go on, give it a go!
You could carve a top but would maybe involve the purchase of more tools depending on what you currently have and the making of a carving 'cradle' to hold the top while you carve.
If you're not up for the extra expense or work, an arched flat top would be the way to go, plus I think that a flat top would give a sound more conducive to a bass sound. Just how much arching you give it I couldn't answer I'm sorry, having never built one, it will only be guess work but I could maybe give a few "educated guesses" (I hate that term but couldn't think of any other way of putting it) from observations of the various different styles of guitar I've built so far.
For bass and an uncarved top I'd actually be going with the traditional radius's around the 30 foot mark, the trouble as I see it, is that as you increase the radius you are actually stiffening the plate up. Carved tops are different because you actually thin/carve the plate out near it's edges so the whole plate is vibrating (you are creating the same structure as you see on a speaker essentially, a thin membrane fixing the more rigid cone to the frame) but a flat top, whilst it is 'looser' and free to move near it's edges where the braces are thinned down, doesn't move as one as is evident by looking at Chladni patterns. If you arch it too much the whole plate gets stronger, hence why Maccaferri tops with only ladder bracing are able to hold there shape under a great amount of string pressure (Maccaferri's traditionally have a longer scale length than 'normal') but the trade off is that you loose bottom end off the tone because you've restricted the plate's monopole (which produces the majority of the bass frequencies). So to my thinking and I know it's getting away from the archtop shape you are trying to reproduce, you'd be wanting a flat top with a larger lower bout area to maximize the monopole effect. Having said that, take a look at an 18" archtop, they have quite a big lower bout, could be a pattern to use if you want the shape to look like an archie?
Just a few thoughts to put into the grinder and add to the mix.

[slowlearner]

Wow, that's a detailed answer!

Honestly, I'd love to give carving an archtop a go...

The issue is, I've spent quite a lot of money on a bear claw spruce top for it and it's already glued up. It's just been sitting in my workshop for the last year or so. I need you use it!

I get what you're saying about too much radius. I was thinking the same pretty much. What attracted me to the Forster style archtop was the idea that I could be a very lightly braced/radiused top that would be free to resonate a lot and let the neck/beam do the work of supporting the strings.

So now we're really talking about how little pressure we could put into the top, while still having it carry some of the tension of the strings. Right?

BTW, my current design has a belly that's near on 22" inches.

[slowlearner]

Been reading the Ken Parker thread. This sort of thing could really work. Right?

http://www.martinkeithguitars.com/const ... ails.shtml#

[slowlearner]

Also found this...

https://nathanguitars.com/tag/gypsy-jazz-guitar-repair/

A Selmer style flat top Jazz guitar. Would this style of bracing be better?

[Nick]

I think you should still be looking at an X brace style, you need to get as much of that top moving as possible as you'll have lower than normal string energy to get it moving in the first place if you stick with your original intention (Ala Nigel's design) of a vibrating 'stick'. Ladder bracing doesn't utilise the whole top's monopole frequencies the best, it emphasizes a long dipole mode more.
This taken from the technical section of a Gypsy Jazz guitar fan site (gypsyguitarfans.com) -
"Ladder bracing is created when all the braces are parallel with the bridge, so if you look at the back side of the top, the braces look like steps in a ladder. There usually are not many, 3 or 4, maybe 5. They create fulcrum points where the top can easily vibrate in short, low amplitude motions that are perfect for producing high notes, which therefore get emphasis, along with high overtones from lower pitched notes. Thus ladder bracing strongly supports what is called the "long dipole", though there are more than two poles (spans) created. Highs increase the carry and make the sound more assertive. To get this, the top sacrifices the rich complex lows that a well built X-braced top can provide."
I seem to think Nigel is a member on the forum, perhaps do a search and PM him directly with questions about his observations/experiences of this design. Here you go, did the leg work for you Pete memberlist.php?mode=viewprofile&u=1196

[slowlearner]

Cheers, I've emailed back and forth with Nigel a bit. I almost visited him in the UK last year but ran out of time. I've asked him about this sort of thing in the past, but he wasn't super keen about sharing his findings. I respect that.

I'm actually leaning more towards a live top with a regular style tailpiece/bridge setup as you guys suggested. Hence the pics of selmer flat tops.