I built this monster in '91-'92, my first complete instrument from scratch. It was a substantial engineering project, and I had to build a lot of special tooling and fixtures for it. It's intended to be an alternative to an upright bass, for bluegrass and folk style music. Over the years, I've refined the design in my head, and I plan to eventually develop it into a production instrument.
What's it sound like? It's a warm tone, with a moderate percussive attack curve, sort of like an acoustic guitar down in the bass range. It's not as loud as I had hoped, but I have some ideas on how to improve that. There is a pickup onboard, with a jack built into the side of the tailpiece, but it's a touchy instrument to play amplified.
I built this instrument as a 34" scale 6-string bass, tuned BEADGC, with a fretted fingerboard and roundwound strings. However, now that I've had more experience with fretless and upright basses, I suspect that the Bass Banjo would be better as a 40"-42" scale 4-string, with an upright-style fretless fingerboard and high quality flatwound strings.
I also have a lot of new ideas on how to build the structure, to make it lighter and more resonant, and less expensive to build. This one is built like a tank, and is much too heavy! I'd also make the neck removeable, similar to the Baby Bass, so it's easier to adjust and can be taken apart for shipping. The plan is to keep the features and finish simple, and aim for a selling price of about $1200.
If you might be interested in a bass like this, please let me know. If there appears to be a serious market for the Bass Banjo, I'll get back to work on it.
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Overall height: 6'5"
Diameter of rim: 25 1/2"
Depth of rim: 8"
Head: 24" Remo Ambassador Clear
Strings: 6, tuned BEADGC
Scale length: 34"
Frets: 24
Weight: 25 lbs
The Spring System:
One of the first things I found out in designing this instrument is that you can't just scale up a banjo! There's no way that a mylar head of that diameter could support the downforce of the bass strings. So, on the back side of the head is an adjustable spring system which supports a floating shoe that's directly under the bridge. The spring system is set up to counteract the download of the strings, and its characteristics are the primary resonance and damping effect on the strings. The head is sandwiched between the shoe and the bridge, and goes along for the ride, like a giant speaker cone.
Much of the development time of this instrument was in this spring system, because it's the real heart of the tone. I went through at least seven iterations of the design, including some complex combinations of maple leaf springs, brass bars, etc. What I finally ended up with was surprisingly simple. I got the best overall sound out of steel coil springs, and a very light weight alder shoe.
The brass knob on the inside adjusts the load that the springs apply to the back side of the bridge. I found that the best setup is where the spring/string balance is pushing the head slightly convex. I don't understand why, but this produces better tone and volume than when the head is dead flat.
The Rim:
The rim is built up out of layers of arc-shaped segments of maple and walnut, sort of like building a round block wall. It's beautiful looking, but it was a pain getting all the segments to fit together correctly. I don't think I'll do that again! The circumference of the rim under the head has an 1/8" wide brass ring inset into it, to give the head a really solid, precise edge. That was also a lot of work, and I'm not sure if it was worth it.
The Head:
I experimented with several different heads, and ended up using a Remo Ambassador, which is one of their heaviest gauge mylar heads. The head is cranked up very tight, much tighter than you'd typically run a bass drum. If you look close, you'll see that the brass pull rods are pulling directly on the aluminum ring of the head, with no seperate retainer ring. I'm amazed that the bond between the mylar and the ring hasn't failed! But it's been together for nine years, and has hardly lost any tension.
The Neck:
This monster is actually through-neck style construction! The center maple slab of the neck is one piece from the tip of the headstock down into the tailstock, about 5 1/2 feel long. The side laminations are walnut and maple, and carbon fiber is inset into the center slab. The neck is glued into the rim, and I built a special 6 foot long fixture that holds the entire instrument while I did the fingerboard, fretting, etc. Like the rest of this project, this construction was design overkill.
The Pickup System:
This was the toughest part of all! It should have been obvious, but mechanically coupling a 24" diaphragm to the strings creates, effectively, a giant microphone. This thing is a nightmare to amplify. Whatever method is used to pick up the sound, whether a microphone, a magnetic pickup, piezos, etc., as soon as the sound is in the air, the head picks it back up and feeds it back into the strings. I remember some experiments where it was so sensitive that, with a little amp 6' away, as soon as it was turned up enough that you could barely hear it, it was already feeding back! I tried a lot of different things, but the heart of the problem is the giant diaphragm. Sure, I could put on a magnetic pickup and totally dampen the head, but then it would lose the characteristic banjo sound.
I finally found a partial solution. I mounted a pair of Fishman disk pickups vertically on the back face of the bridge. This way, they're mostly picking up the fore-back motion of the bridge, which isn't so directly coupled to the motion of the head. This works okay for an indoor acoustic band environment, but it's still far too feedback prone to be used in a loud club. It's a tough problem, but there are some new techniques and hardware available today that might help.
I used the Bass Banjo in a studio recording with my bluegrass band in '93, and it sounded great on tape. If I remember right, the engineer used both a condenser mike and the feed from the pickup. The acoustic sound from the mike was much better, and was all we used in the final mix.
