Introduction: Acoustic Guitar to Electric Bass Guitar Conversion

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About: Hi, I have a hobby to build some DIY Audio stuff, simple gadgets, etc. Hope you will find my instructables useful! :)

I've got my first classic guitar as a present on my 15th birthday. As years passed by, I've had some low-budget electric guitars and a semi-acoustic one. But I have never used to buy myself a bass. So a couple of weeks ago I decided to convert my old classic guitar to semi-electric bass guitar.

The Idea:

The main idea is to convert classic cheap 6-string guitar to the 4-string semi-electric bass guitar. By the semi-electric, the meaning is additional electronic circuit, to make converted bass be used as an electric one. As physics can tell us, these two different types of guitars, and they operate in much different ways. As we can see in a real life, bass guitar's fretboard much longer than classic's and fret separating wires (That makes them look like rectangular fixed-place separate blocks) are wider, thus areas of bass frets are bigger than classic's. There are a lot of inspiring articles and YouTube videos that describe acoustic-to-bass guitar conversion. This Instructable provides a simple guide, how to convert classic\acoustic guitar to the semi-electric bass by the following algorithm: (Very good guitar terminology description can be found here).

  1. Re-arranging Fretboard and neck adjustment: Creating compliance to the bass guitar physics, thus new converted guitar will sound as better as possible.
  2. Rebuilding guitar's head in order to comply 4-string bass basis: Unused tuning pecks removal, expansion of remaining pecks' hole diameters.
  3. External bridge adjustment: Instead of making increase in neck length, I preferred to adjust distance between the bridge and nut - make it longer. This step is crucial, because of its physical properties - conducted experiments on guitar conversions show us, that just substituting acoustic (EADGBE Types) strings to bass strings (EADG Types), doesn't make new converted instrument sound good at all - this "crappy" sound comes as result of insufficient strings length and their adaptation according to the guitar neck shape.
  4. Drilling and Grinding: In order to attach additional parts to the guitar, there is need to plan all the variations on the guitar shape - Holes for electronic circuits, expanding holes for four bass strings and cutting resonator enclosure for pickups enclosure
  5. Electronics Assembly: Designing electronic circuit, testing and assembling on the brand-new bass guitar.
  6. Final Assembly: Assembling all the guitar parts altogether.
  7. Final Testing: Testing converted guitar operation.

So, let's proceed to conversion!

Step 1: Parts and Instruments

Mechanical Parts:

  1. 1 x Old Classic Guitar - Most important part in the project
  2. 1 x 5x5cm Rectangular\Multi-angular Metal Shape - Stainless steel is preferred
  3. 1 x Acoustic\Electric Bass Strings Set
  4. 1 x Wooden Rectangular form - Used in string support attachment
  5. 1 x Plastic Thick-sided Rectangular Film - Attached to the rectangular prism

Electronic Parts(Optional):

  1. 2 x 500KOhm potentiometer
  2. 1 x 1/4" female mono audio jack
  3. 1 x Dual bass pickup (Or two single ones that can be connected in series)
  4. 1 x 10nF ceramic capacitor
  5. Two Mettalic rectangular slices (LEGO style)
  6. 6 x In-drilling screws
  7. 2 x Small washers
  8. 2 x Small nuts
  9. 1 x 1/4" Jack washer and nut set

Instruments:

  1. Hand sized grinding file
  2. Thin grinding file
  3. Electric screwdriver
  4. Drilling bits
  5. Small sized hammer
  6. Bass tuner device or smartphone app
  7. Ruler
  8. Cutter
  9. Multimeter

Step 2: Guitar Preparing

Part One: The Bridge

First thing we need to do, is to remove all the strings. Seems obvious, but I've started whole process with the strings still attached to the guitar, what does not bring any comfort to this. Now it is a time for measurements. There is need to determine center of the bottom of guitar's body, where the strings holder will be attached. Speaking of which, before proceeding to attachment itself, we have to prepare the bridge by following these steps:

  • Measure the width of the neck
  • Determine the maximum pitch between the strings, so there are equal distances between them
  • Drill 2-3 holes at the vertical axis of the guitar bottom on the bridge and guitar.
  • Drill 4 holes on the top of the bridge, according to the measurements that were taken before.

Attach the bridge to the guitar, make sure it looks symmetrical to the sides. Now, let's proceed to the next part.

Part Two: The String Holder

In order to ensure that space between strings and the truss rod is sufficient, we have to place the string holder. As it can be seen from the picture, I've made a wooden one with cut-out slot with attached rectangular piece of plastic to it. The width of the plastic determines exact distance between strings and the truss rod, so make sure that its width doesn't exceed the desired one. String holder merely glued to the guitar, since there is a constant stress applied by the strings.

Part Three: Removing Tuning Pegs

This part is quite optional, but we are interested in building a bass guitar, so two of pegs that are centered should be removed from the head.

Well, that was easy. Let's proceed to the hardest part which defines the whole acoustics...

Step 3: Frets Placement Method

First of all, remove all the fret wires from the fretboard, so the guitar will be complete fretless. As you can see, there are some of unwanted slides near the areas where frets were placed. Remove them by sharpening file, make sure you that align whole fretboard well, it has to be plain and smooth.

Prepare at least 11 new frets (that were pulled out or the new ones) in order to make possible full-octave per each playable string. Make sure that length of each fret we are going to add, matches exact slot on the fretboard, because if it is shorter than desired fretboard location, 1st and 4th strings just wouldn't work well.

Here comes the tricky part: We can obey the physics and calculate all the distances between frets mathematically according to the guitar mechanical parameters, but it will take loads of our precious time. But there is an much easier way to measure distances on all the frets, using just single string and a guitar tuner:

  • Prepare a permanent well visible colored marker, G-string (which will be the highest pitch on the guitar) and a guitar tuner. If you don't have one, you can use any smartphone app. I strongly recommend Guitar Tuna, and Soundcorset both of them are convenient to use and their precision is very accurate.
  • Attach G-string to its position, tune it to the G2 (Approximately 99Hz).
  • The most important step: Pick a small ruler, place it perpendicular to the neck, apply pressure on it and start retrieving sound from the string. As soon as you achieve the next tone (For example G is initial tone, and you achieved G#) stop here and mark current location of your ruler.
  • Repeat the previous step several times, until there are a total of 12 regions of the separate frets - i.e full octave playing ability for each string.

Remove the wooden fill on all the marked areas at least 3/4 of the new frets to be placed' size.After you succeed, align the neck again to remove all the unwanted wood slides. Insert all the new frets in the new slots while each fret has to be in appropriate length according to the slot. I recommend to place fretboard marks on positions 3,5,7,9 and 12 in this step, since bass' fretboard width differs from the classic one, it will be much easier to get used to play bass with these marks.

Step 4: Electronics and Schematics

This step is optional and can be skipped. But I was enthusiastic about making a bass semi-electric.

Bass guitar electronic circuit that was implemented is very simple: It contains single tone control filter, two attached pickups, volume control and a 1/4" female output plug for the PL cable. Pickups have to be attached together with a slight shift to the sides with a very strong glue, to capture the oscillations of all the four strings on the bass. In this design, I've used a double pickup with only two wires - ground and signal. Solder the filter and volume control circuit according to the schematics, make sure that wires that you use are long enough to be placed inside the body of the bass. solder output 1/4" plug to the circuit' volume control output. Make sure that electronic circuit is properly grounded -> all the potentiometers body should be connected to the ground wire.

After all the electronics, drill all the needed holes on the front body of guitar - Volume control, tone control and output plug allocation. Attach metallic holders to the pickups so it will be attached on a fixed position on the sound hole. If there is a humming noise when guitar is plugged, there is need to ground the strings by grounding the bridge.

Step 5: Final Testing

After all the hard work is done it is a time for testing, by playing all the notes on all four strings. With tuner's help, there is a possibility to determine problematic regions and fix them if any of the notes shifts up or down by the pitch. If all the frets are placed at their true positions, we've done a great job, and our acoustic bass is ready to be played!

In order to test electronics, just plug you brand new bass to the soundcard/bass amplifier and check it out, with tone\volume adjustments. If there are issues with humming noise (We hear 50Hz on the line), recover the circuit and make sure it is grounded properly.

Hope, you'll find this Instructable useful,

Thanks for reading!