So I already built a 6-string bass...

What if, I'd do one better, a 7-string?

String spacing.

Now, this build was going to need some serious planing. First off, the string spacing is a dilemma on it's own. It can't be too narrow or ridicilously wide either. As I started thinking about it, the idea of making a hybrid string-to-string spacing at the nut came to mind. Since the thickness of the strings will be huge, from the thickest B-string to the thinnest F-string, how about making the thicker strings a bit more further apart from each other than the thinner strings. So I ended up making the spacing between the B and E string 9mm, E A and D- strings 8mm and all the rest 7mm. Seems to be working out quite ok, so far...

The Body

On this build I wanted to make something special when it came to the body of the bass. Always been a fan of Mayones basses and those very nicely shaped bodies. Made my own slightly modified shape based on the Mayones BE Series.

Center piece maple, wings alder and some nice looking elm on top of the alder. The wings are angled backwards at 5 degrees to make the whole body slightly curved for better playing comfort.

The truss rods.

Two truss rods was needed instead of one, even 6-string basses have two truss rods, so... However, the neck is obviously so stiff in itself that once I strung it up and got it in tune, the fretboard didn't get a slight curve from the pull of the strings! So I had to use the truss rods to get a slight forward bow in the neck. Thank God for those dubbel action truss rods!

Pickups.

Since a 7-string has got such a wide string spread some very wide pickups had to be used. One model that came to mind was the Bartolini G6, the same pickups models I used on the 6-string bass. The pickups are 141mm wide, which is just about right.

The specs.

Construction: 7-string (glue-on)
Scale Length: 840mm (33")
Strings: Dean Markley 022-127
Number of frets: 24
Tuners: Schaller BM Light (Gold)
Bridge: Göldo 1-string (Gold)
Frets: Sintoms 270091

Neck wood: Ash, wenge and maple (9-piece)
Body wood: Maple, alder and elm.
Fretboard wood: Merbau
Neck pickup: Bartolini G6 B
Bridge pickup: Bartolini G6 T
Preamp: Home made 3-band

Tuners

To prevent this bass from being head-heavy the obvious choice some sort of light-weight tuners. I've been using Schaller BM Lights before, although they requier some unconventional installing. However, with a few modifications the installation can be made more traditionally with some drilling and small screws.

Name of the game

Of course I had to come up with a name for this build, so I thought about the number of strings, seven, and what was the first thing that came to my mind... Seven seas.

Inlay

On this build I wanted to do a proper inlay at the 12th fret location at the fretboard. It seems to be some sort of tradition among high-end electric guitars and basses. So I got this idea of a chart compass, you know one of those compass drawings you find at old naval charts. Thought it suited nicely, since the name of the bass is "The Seven Seas".

The onboard preamp

I've been fiddling around with analog audio circuits for a while now, so naturally I hade to design and build a preamp for this bass as well. First I thought about a 4-band design, but ended up making a 3-band instead, using a specific op-amp that's specially design for audio use. The OPA2134 by Texas Instruments. Superior sound quality, very low noise (THD 0,00008%) and a true FET input stage.

However, I had some trouble getting it to work in my designed circuit, and I was scratching my head bold trying to figure out why it didn't work. Until I figured out that this specific op-amp is designed to be supplied with both positive AND negative voltage rails, not just a positive. So, how to supply this circuit with a +/- power supply from a singel 9V battery? Turned out to be fairly simple. There's a specific chip design for this, the ICL7660 (or the MAX1044. They seem to be identical) which is a switched-capacitor voltage converters that invert, double, divide, or multiply a positive input voltage. So using this chip I could create a -9V voltage rail from the +9V battery.

Problem solved. The first part of the preamp circuit itself is basically a buffer/gain stage followed by a 3-band EQ utilising a standard negative feedback loop. Of course I had to tweak the eq to suite this bass a bit better, so I made the bass control a little lower and the treble control a tad higher than usual.


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The warning LED

Another interesting circuit I found browsing all those electronics websites is a low battery warning LED circuit. This nifty little simple circuit is designed to light up a LED when the voltage of the battery drops below a certain voltage. The voltage at which the LED will light up is determined by the value of the zener diode. In this case the zener diode is a 6,2V, so the LED will light up when the battery voltage drops to approx. 6,8V.

Click on picture for a bigger version (opens in new tab)
Click on picture for a bigger version (opens in new tab)

The neck

The neck was glued together with Titebond Original using 4 different wood species. Maple in the middle, from there towards the edges, wenge, birch, wenge and ash.

And on this bass I wanted to try making a so called set-neck construction. No neck screws, just a standard routed neck pocket in the body and the neck just simply glued into that. So far it seems to work out ok, time will tell.

Color scheme

Since the name of this build is "The Seven Seas" a dark blue color is pretty much a must. No "real" tint or stain was used, only ordinary food coloring, the same as I used on "Blue Lagoon". Interesting fact is, that on maple the color ends up blue, but apply it on birch or elm it gets a greenish tint to it. Kind of a teal facet, which is nice. One of my most favourite color schemes is dark blue with gold. Black and gold is another very high contrast scheme, so the obvious choice for the color of the hardware on this build is gold.

 

No sound samples, yet. I have to learn how to tame this beasty monster before I hit that rec button.

Which may take a while...