Christopher "Double-Bass Guitar", Archtop Acoustic/Electric

[D-Smith]

A few weeks ago I started working on an Archtop Acoustic/Electric Hybrid Bass. I think (hope) I just finished the most important part of the design, the top plate. Here's a picture of where it stands as of now, and a quick and dirty recording, then I'll explain how I got to this point. The recording was made with the top on the test rig which was not designed to be played, so I only had a few notes to chose from on the A and D string!

 

[bholder]

Great design. Nice sound. Neck still looks a bit chunky for my tastes. (just kidding)

 

[D-Smith]

The body outline is an enlarged version of the electric bass I build. Top will be Adirondack Red Spruce, back and sides most likely Mahogany, and neck will be hard maple. The whole body was enlarged based on the lower bout going from 14" across to 16". Still a 34" scale neck.

The main design goals were:
- emulate the sound of a double bass
- be close in size to a standard electric bass guitar
- amplify well
- be loud enough unplugged to sound good practicing.

No attempt to make this loud enough to play acoustically with other instruments, but solo practice should be pleasant sounding and easily loud enough.

Other design goals:
- to use standard extra long-scale electric bass strings. I did a build thread on a large archtop bass I built and finding strings for that was next to impossible and I did not want to make that mistake again.
- be aesthetically pleasing
- somewhat original
- light weight
- not too difficult to build.

Like a double bass, this version is fully acoustic, meaning the top alone will have to withstand all the downward pressure of the strings. In order to determine the arching of the top, how thick the top needs to be, neck angle, soundhole size and placement, and bracing, I needed a way to quickly test different tops without building a complete instrument for each one, so I came up with this test rig.

Tops drop into the rig and rests on the lip. Ten turns of the tuning pegs to loosen the strings and the top comes out so I can make a change to it or try a different top.

Here's a picture of the cnc carving a top. This is the first rough pass. This was the first top I tried. It is a bevel top. There is no arch in the center area, it's flat. I had a feeling this might not work, or would have to be heavily braced in order not to collapse under the string pressure, but I carved it anyways.

Here it is finished:

Unfortunately, I was right. It sounded pretty good, but too much deflection. Over time the top would have sagged more and more and eventually caved in.

Since I had already carved the top I used it to experiment with different bracing patterns and sound holes. Most interesting finding was X-bracing killed the sound, while parallel bracing did not. X-bracing did support the top better, less sag, but soundwise was bad. Parallel bracing sounded good but still had too much sag. Not what I expected, but was actually the best thing I could have hoped for. With parallel bracing I can integrate the bracing right into the carving of the top. In other words, top and braces carved out of one pieced of wood instead of carving the top then gluing in braces. That saves a good bit of time. That would not work with X-braces because the angle the braces run across the soundboard would make the grain too weak.

The next couple of pics are the latest top and hopefully the final design. The center section was arched to resist sagging. I was able to leave the appearance of and outside bevel, which IMO looks nice. The tailpiece will be like those on violin family instrument, held by a cord/gut/wire that goes around the endpin. I inset a piece of ebony that the cord will go over so the cord does not damage the spruce top. I'm sure there's a name for it, but I don't know what it is so I'm calling it a "riser".

Hard to show the arched center area, but the top has basically no measurable sagging. you can see the carved ebony riser and please notice the high tech coat hangers I've employed as a temporary tailpiece!

 

[emjazz]

Love what you do...excited to see this!

 

[Geoff St. Germaine]

Interesting. I have a fair bit of experience building fully carved archtop guitars, so my comments are based around that.

Your observations about X vs parallel bracing are interesting. On guitars I've used both and generally carve the tops differently for the two. An x-braced top would normally be slightly thicker around the bridge than a parallel braced one as the narrow spacing of the parallel bracing across the entire top tends to stiffen the area around the bridge quite a bit more and make it more resistant to sag. It's curious that you saw the opposite effect, but this would certainly depend on the dimensions and profile of your braces. There's also the fact that the area around the bridge is not really carved and is more similar to a flat top instrument, which requires substantially more bracing in order to create a top with similar deflection.

The "riser" you're referring to is normally called a saddle.

Cool project! I've toyed with building a large 17 or 18" archtop bass, but I've never been able to arrive at anything that made me think I could get enough out of it volume wise to justify building such a large bodied instrument. I'm more of the belief that if I'm going to wind up amplifying it anyway that I could do the tone shaping to arrive at a similar tone in a more compact package in other ways. The premise of your prototype here is actually more the way I have been thinking though in a smaller format. Effectively a conventional solid body instrument with something of a suspended carved top.

 

[D-Smith]

Your observations about X vs parallel bracing are interesting. On guitars I've used both and generally carve the tops differently for the two. An x-braced top would normally be slightly thicker around the bridge than a parallel braced one as the narrow spacing of the parallel bracing across the entire top tends to stiffen the area around the bridge quite a bit more and make it more resistant to sag. It's curious that you saw the opposite effect, but this would certainly depend on the dimensions and profile of your braces. There's also the fact that the area around the bridge is not really carved and is more similar to a flat top instrument, which requires substantially more bracing in order to create a top with similar deflection.

The "riser" you're referring to is normally called a saddle.

Cool project! I've toyed with building a large 17 or 18" archtop bass, but I've never been able to arrive at anything that made me think I could get enough out of it volume wise to justify building such a large bodied instrument. I'm more of the belief that if I'm going to wind up amplifying it anyway that I could do the tone shaping to arrive at a similar tone in a more compact package in other ways. The premise of your prototype here is actually more the way I have been thinking though in a smaller format. Effectively a conventional solid body instrument with something of a suspended carved top.

Thanks for the observations and comments. The first two tops I carved were bevel tops. Arched around the edges but flat in the center. I was pretty sure it was not going to work, but gave it a shot. I'm not sure anything I did with those two tops can be generalized to archtops. The next tops, including the one on the test rig in the picture, have the same arched area around the edge as the bevel tops, but the center that was flat is now arched across the instrument.

I built a 20" archtop bass and your conclusions are exactly the same that I came to. It sound good and has more real low end bass and is louder than any acoustic bass guitar I've run across, but it still isn't loud enough to justify what I would have to charge someone to build one.

 

[D-Smith]

Decided this was the final design for the top and carved two more yesterday. Plan is to build two basses. One fretless for sure, the other might have frets. I'm going to put a few frets in the test rig today and see how that sounds.

Currently the test rig has LaBella Deep Talking Bass Flats, 1954 version, .52"-.110". The first set of strings I put on there were some old dead D'Addario round wounds. They sounded pretty good, but the heavy LaBella's really drive the top a bit harder, have a thicker sound, and sound more like an double bass. I have some black tape wounds I'm going to give a try. Should be interesting to hear since a lot of people put those on to try to get a more "upright bass sound", but I'm not sure how beneficial that will be on this instrument. So far this top is giving me a good upright sound without losing all the high mids and highs that the tape wounds seem to lose in their goal to sound like an upright.

This is the inside being carved with the integrated parallel bracing.

 

[Dadagoboi]

Really love how you're thinking and developing this bass!

 

[D-Smith]

I'm not sure if this picture helps show how the arching on this instrument works. The goal was to have an arched top that would support the pressure of the strings, but at the same time not lose the "bevel top" look, where there is a distinct line that defines the bevel from the center. This is the CAD drawing I drew to carve the top. At the widest point in the lower bout there is a 3/16" arch in just the center section (the solid purple area).

 

[Jisch]

Great design, thanks for sharing the pics, very cool.

 

[Matt Liebenau]

Cool build. It's interesting that the braces are integrated into the top carving. Never have to worry about regluing a loose brace, I guess.

 

[D-Smith]

I finished the most "technically" challenging part of the build a couple days ago, cutting the heel of the neck, and mortises for the neck/body joint. The neck is set at a 3 degree angle from the body and the angle is cut into the heel. Long story short, cutting an angled joint on the cnc required a lot of thought. More than my brain felt like doing at times! Getting all the angles right and figuring out how to cut them and stay accurate between two jigs was tough. Here's a picture of the jig I came up with and the joint cut on the first neck. The neck is on the jig fingerboard side down.

Here are a couple before/after going on the jig pics of the necks.

I was able to use the neck jig from the last build to do all of the other work. All I needed to do was update a few programs for the new neck. The neck pocket in the body will have mortises also and maple inserts to increase neck/body joint strength. I've got the headstocks done and the necks carved. I'll post pics of that soon. Starting the bodies on Monday. I've got the blanks glued up and ready to carve.

 

[T_Bone_TL]

please notice the high tech coat hangers I've employed as a temporary tailpiece!

Exactly what the low end Lidl has from the factory, as far as I can tell. Well, OK, they use coat hanger wire to attach the tailpiece, rather than as the tailpiece.

 

[D-Smith]

After cutting the heel on the modern cnc we go right back to late 1800s with a good old Stanley #8. Laying out the headstock angle is a critical step and I think the best tool to get the headstock flat, at the right angle and in the exact location you want, with the lowest chance of a major screw-up is a hand plane. Plus, in this case there is no machine time setup, no jigs, etc., so it's probably just as fast as any machine method, but with more accurate results.

I took the pictures after I had finished, so please pretend the ears are not glue on in the first couple of pictures.

First, use a square and a knife to lay out the end of the fingerboard, or fingerboard plus nut depending on how you design the neck:

Next I use a template to mark the profile on the side of the neck and cut out the rough profile on a band saw.

Use a pencil to darken the line you made with the knife:

Now rough plane the headstock flat. Keep an eye on the profile line on the side of the headstock you drew earlier. Make sure you plane even with that line. As you are planing, you will see a line form where the plane is starting to cut the headstock. We'll call that the "plane line". If you take a regular lead pencil, sharpen it so the lead point is very long, than lay it almost sideways and rub the edge of the lead along the plane line, it makes it easy to see. The plane line will get closer and closer to the knife line as you take wood off.

*

*In accordance with the Truth in Advertising regulations Christopher Bass is required to disclose that although this picture is an accurate representation of an actual neck having it's headstock planed, it is a staged picture.

Keep an eye on both lines and make sure that the distance between the plane line and the knife line is equal. This will ensure you are making the headstock square to the neck. Like the picture above, if the distance between the two lines becomes more on one side than the other, make a few extra passes on the larger side until it is even again. Same with the profile line on the side of the neck. If you notice you are getting closer to the line near one end of the headstock, take a few extra passes near the other end. Use a ruler or straightedge to make sure the headstock is flat from front to back, side to side and corner to corner.

At this point don't plane right up to the knife line, leave about 1/32"-1/16" of distance between the two lines

Glue on the headstock ears.

First plane the ears flat with the center, then plane the whole thing right up to line, again keeping an eye on the side profile, an even distance between the plane line and knife line, and flatness of the headstock.

That's about it. Next up is making and inlaying the heastock plate.

 

[D-Smith]

Worked on the headstock plates and inlay Friday. One nice thing about this build is other than the neck angle jig from a post or two back, and a very simple jig I need to make to hold the back/body while carving, I'm using all the jigs from previous builds. I posted the first four pics, plus a bunch more, in the last thread I did.

This is the jig for making the headstock plate and routing the inlay pocket in it. This is the vacuum table I have attached to the back half of my cnc table. It's got a 5 hp industrial vacuum pump hooked up to it. The vacuum goes through channels under the top of the vacuum table and comes up through the small holes on the table. You plug up any holes that don't fall under the jig you are using with the red plugs, then gasket off the area under the jig.

This is the jig for the headstocks. First you cut the headstock plate from whatever wood you are using. Thickness the wood, in this case, 3/32". Then cut the fingerboard end straight. For these basses that meant cutting a straight line at an 8 degree angle so when it is glued to the headstock, which has an 8 degree angle, the front edge will be perpendicular to the neck shaft.

Put the plate on the jig with the nut end butted up to the edge of the pocket and turn the vacuum on. The first operation, on the right side of the jig, cuts out the truss pocket hole, and lays out the holes for the tuning machines. It also cuts two larger holes outside the headstock area to position it accurately on the left side of the jig.

After the first operation is done the plate is moved to the left side and the pocket for the inlay is cut.

These are the plates I made for the three basses I'm building now with the mother of pearl inlay completed, ready to go on the headstock.

 

[BassHappy]

I love this place!

Smithcreek, your work is so inspiring. There are so many different ways of doing things, and I must say - the spock-like scientist in me loves the perfection of the CNC. So clean and so perfect - and it fascinates me how you use it. At the same time, someone else here might be performing tasks with a simple hand planer or a collection of jigs and rigs they fabricated themselves - and I am also amazed at that.

Nice work, I am glued to this thread, soaking it all in and thirsting for more!

Did I mention I love this place?

 

[D-Smith]

... the spock-like scientist in me loves the perfection of the CNC. So clean and so perfect - and it fascinates me how you use it. At the same time, someone else here might be performing tasks with a simple hand planer or a collection of jigs and rigs they fabricated themselves - and I am also amazed at that.

Thanks man! Like I said, in many cases hand tools are more accurate than even the cnc. Forcing yourself to learn how to do most things by hand is a good habit to get in. Once you've learned to do it well by hand you can make the best decisions about what will benefit from adding a machine. This is a mandolin I built before I had a cnc. Other than band saw, drills, dremel, maybe a couple other things, it was done by hand.

 

[BassHappy]

Thanks man! Like I said, in many cases hand tools are more accurate than even the cnc. Forcing yourself to learn how to do most things by hand is a good habit to get in. Once you've learned to do it well by hand you can make the best decisions about what will benefit from adding a machine. This is a mandolin I built before I had a cnc. Other than band saw, drills, dremel, maybe a couple other things, it was done by hand.

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Hey Smith

Just gorgeous!

Part of the fascination is with seeing how many ways there are to attack the same tasks or challenges. You are exhibit A - it doesn't really matter which path you take up the mountain, as long as you eventually make it to the top!

 

[D-Smith]

You are exhibit A - it doesn't really matter which path you take up the mountain, as long as you eventually make it to the top!

I know you're not implying it, but to be clear, I'm not trying to glorify hand tools. In fact as I'm typing, the cnc is radiusing a fingerboard for me and I'm glad I don't have to sharpen my plane blade numerous times to do it by hand, but adding to your analogy, "it's good to have alternative paths, even if they take longer, in case your main path is blocked." Make yourself learn some basic hand tool skills and you will save time and avoid frustration in the long run.

 

[D-Smith]

To glue the headstock plate on I start by putting the tip of a knife in the line I cut in the last step, then slide a square right up to the knife and clamp it with a c-clamp.

Slide the headstock plate up to the square and line up the centerline of the neck and headstock plate. I hold a top and bottom caul in place with a couple spring clamps. The top caul has two holes that fall outside the headstock outline to use as a guide for drilling holes about 1/2" deep, through the plate into the headstock.

Take the cauls off, apply glue to headstock and reposition the plate and tap the drill bits into the holes. The less glue you use here the better. Too much will make the plate want to slide around, and even though you have it aligned with the square and drill bits the clamps can easily exert enough sideways pressure to move it a hair. A little wax on the drill bits before gluing and they will come out pretty easy after the glue has dried.

Take the square off once you've tightened the clamp and clean up any glue squeeze-out on the neck surface.

I forgot to take pictures, but you just put the top and bottom cauls back on, hold them in place with spring clamps. I use about 7 c-clamps to clamp it all in place. You don't need a lot of pressure on the clamps, "sparkplug tight", and make sure to apply it slowly and evenly to each clamp, going around five or six times, just a little tighter each time. This should give the glue a chance to grab and keep the plate from shifting.
Done, and ready for a rough cut on the bandsaw.