7 String Guitar From a 6 String Guitar. Model Z7.2

I recently rescued a 6 string acoustic classical guitar from the fireplace because the headstock was broken off. But as the soundboard was in good condition, I thought I had to try to save something...

After this -little- success, I decided to turn this 6 string guitar (E, A, D, G, B, E) into a 7 string guitar (lowB, E, A, D , G, B, E) like a Russian guitar, just for fun and to see what it is possible to play with an extra low B string.

In fact, with this extra low B string, it become easy to play a low B, a low C and a low D , to play barre chords F, G H, and play more 7 strings chords.

Head must be modified in order to accept one more tuning peg.

Neck nut and acoustic table bridge must be modified in order to accept one more string

For the main body : a 6 string guitar to be modified (with a widest neck as possible)

For the head: an individual folk or electric guitar tuning key + Drilling press + drill bits (d= 7,5mm ,10mm, 16mm),

For the neck saddle : super glue + baking soda + little files + little handsaw

For the table bridge: hand drilling machine + caliper + a diy help drilling tool (see step 3) +drill bits d=2mm

For the tuning: a set of shims, + files + guitar string tuning rule + patience + precision

For the work : Time = about 12 hours

I repaired the broken head . I held the broken parts with clamps and glued them with epoxy glue. There are a lot of tutorials on internet showing this process, and there was no special difficulty for doing that.The result is not super esthetic (because there's sometimes more epoxy than wood in the head), but I got there, and the 6 strings guitar is playable again!

For a 7 strings guitar, it is necessary to mount an extra tuning peg.

I choosed to use a Folk guitar tuning peg. Type "guitar tuning pegs" in your favorite online store

The new tuning peg will tune the D string. This D string is just in the middle of the head. It means that the axis of the tuning peg must be slightly moved (in my case 3mm) from the axe of the neck in order that the string and the neck axis are the same.

The implantation of this tuning peg in your guitar depends on the model you have.

For the 7 string guitar, we'll keep the same gap between the lowest and the highest string we had in the 6 string guitar.

In my case, the gap between the lowest and the highest string is 42mm

For the previous 6 string guitar, the 5 intervals between the strings was 42mm/5= 8,4mm

For the future 7 string guitar, the 6 intervals between the strings will be 42mm/6=7mm

Draw the new notches on the saddle (7mm spaced in my case) 

As I decided to keep the first and the last notches , there are no new notches to do for these strings

There is no special problem to saw new notches for the 3rd, 4th and the 5th string with a little handsaw but ..

There is a problem for the 2nd and the 6th string : it is impossible to saw a new notch, because there is not enough material : the new notch we have to do is too close from the previous notches done for the 6 strings saddle.

I though to do a new saddle doing a plastic part with 7 notches according to my instructable  "https://www.instructables.com/Molding-Plastic-With-Polystyrene-Paste-Cold-Moldin/"  , but I resigned this idea because I supposed that the plastic was'nt strong enough , and that it was a lot of work to change cleanly the saddle on the guitar.

I decided to fill in the 2nd and the 5th notches of the previous 6 string saddle with backing soda and superglue. There are a lot of tutorials on this subject on internet. The result is really strong.

Protect the head and the neck with tape, put backing soda in the notch, and put superglue on it. This mix becomes hard quickly. Wait a few seconds. Do it again if the hole is note completely full. Then file and sand the saddle in order to clean it .

You can now saw and file the notches for the 2nd and the 6th string.

Important: do not saw deep notches. At this moment, they just neet to be able to guide the strings. The deepth of the notches will be tuned later.

And now: the scoop of the day: The 6-string guitar bridge belongs 6 holes, and the 7-string guitar bridge belongs 7 holes

Seriously: the position of the first and last hole of the 7-string bridge will be the same as the first and last hole of the 6-string bridge.It means that the 6 intervals betwwen the 7 strings bridge will be shorter than the 5 intervals between the 6 strings bridge

In my case, the interval between the first and the last hole is 57mm. This means that the interval between the 7 strings will be 57/6 = 9.5 mm.

The problem is:

• The holes must be perfectly spaced 9.5mm apart
• The holes must be perfectly parallel
• The holes must be perfectly aligned
• The holes must be perfectly drilled with a specific angle par according to the acoustic table.

Conclusion: a drilling tool must be made in order to drill these 7 holes perfectly .

For this drilling tool, we need

• 1x plywood sheet 200mmx 150mmx15mm,
• 1x wooden piece 200mmx 15mmx15mm
• 1x wooden piece 30mmx 15mmx15mm
• 1x threaded rod M6x100 length=250mm ( M6 means diameter =8mm, and 1 turn=1mm)
• 1x wooden button D=30mm with 10 graduations on it (1 graduation = 1/10mm)

Assemble the drilling tool as shown in the picture, and insert the tapped rod into the tapped hole (a few centimeters)

For the drilling guide we need

• 1 piece of hardwood or hard plastic 10mm x 10mm x 80mm.
• 1x drill bit D=2mm
• 3x metal rod D=2mm x40mm

On this part, draw an axis in the middle of the length, and indicate on it "top" and "bottom" "right" and "left".

On this axis, draw 7 holes along this axis (9.5mm distance in my case). precision is not mandatory. It is only to have a visual . Precision will be given by the drilling tool.

Attach the drilling tool on the drilling press, place the future drilling guide along the guide of the drilling tool and adjust the drilling tool so that the drill bit can drill in the axis of the future drilling guide .

Adjust the threaded rod in order to push the first hole of the guide under the drill bit, and drill

For the next hole, turn the threated M6 rod. In my case, 9.5mm = 9 turns + 5 graduations (because for M6 screw, 1 turn = 1mm)

And so on for the next 6 holes

We will reuse the first and the last holes of the actual bridge as a guide

Protect the acoustic table before working

Put the 2x metal rod D=2mm x40mm in the first and the last hole of the actual bridge. If not possible, file the acces of the hole, and put the other sides of the rod in the first and the last hole of the guiding tool. The tool is now in place , and the holes have the same angle than the 6 holes previous bridge

Gently drill the hole in the middle .(future D string) and put the metal rod in this hole

Drill the 4 other holes, and the bridge is now ready

Reinforcing the bridge

Another scoop: The force produced by 7 strings is stronger than the force produced by 6 strings. That's why I drilled 2 holes D=6mm through te bridge, and I screwed 2x M6 screws, and Nylstop bolts (be careful do not to drill where the wooden reinforcement under the acoustic table are!)

On my first test I used a 9-11-16-24-32-42-52 gauge metal string set.

Big mistake: the next day, I saw that the neck was starting to arch by at least 1 mm due to the tension of the strings.

This is normal: there is no metal rod to reinforce the neck, and the low B string generates enormous additional tension (about 90Kgs) . A classical guitar is not designed to withstand such tension.

So, in order to resist to the tension, I mounted 6x nylon strings for the other 6 strings (E, A, D, G, B, E ), and I kept the 52 gauge metal string for the low B string

Big mistake again: metal string does not react like nylon string at all . I'll not explain the details, but instead of being at the octave at the 12th fret, I had (by ear) 1/4 of a tone too much (almost 1/2). The result was really awfull...

So I ordered a nylon low B string in an online store, because it was not available in any store in my city (it is not a very common model) . In my case, I bought a SAVAREZ #5209R string . It's all right , like that, now...

Tuning means 2 operations: 1-The neck nut must be tuned, because as we made slight notches on this nut, and the strings are necessarily too high over the 1st fret, and 2- the bridge nut must be adjusted so that the strings are at the correct height at the 12th fret, otherwise the strings are too high , and the guitar is not playable.

The tuning process takes a long time, you have to be patient and meticulous, and do a lot of iterations. Remember: it is easy to remove material, but very difficult to reapply.

Use a set of shims for adjustment

For the adjustment of the neck nut, for each string, place the shims between the first fret and the string and carefully file the notch so that the string gently touches the top of the shim .

As a first approach, I recommend adjusting 0.8mm between the 1st fret and the bottom of the strings.

For the adjustment of the saddle of the bridge of the acoustic table, it is necessary to put a sheet of sandpaper on a table and to sand the saddle in order to have a flat bas surface and the required height

As a first approach, I recommend adjusting 3.5mm between the 12th fret and the bottom of the strings.

In order to measure the 12th fret, I recommend buying a small tool as pictured. Type "guitar string tuning rule" on your favorite online store. It is possible to find this product for worth almost nothing, and it's very practical.

After this first step, go ahead for the final tuning. Be patient. ...Take your time....

For me, for nylon strings the best tuning is

0,5mm between the top of the first fret and the bottom of the string

and

3,2mm between the top of the twelveth fret and the bottom of the string

Et voila! All you have to do is tighten the strings, tune B-E-A-D-G-B-E, and... play!

Question: How does this guitar sound?

Answer: https://youtu.be/71Ej9jZaIhs

I had a lot of fun saving this guitar from the fireplace... and to turn it in an unsual tinstrument.

As this 7 string guitar can play 4 full octaves now (instead of 3.5 octaves on a 6 string guitar), famous composers created beautiful melodies for 7 string guitars and great players performed them. Unfortunately I am neither...

I just have now ... to learn playing this instrument, and explore all the acoustic possibilities! Anyway, my biggest reward is that the instrument sounds good, and that I feel good about it.

In this instructable, the most interesting ( and the most difficult) part was to imagine and manufacture the tools that allow the machining of the 7 holes in the bridge. The rest of the process is just amateur guitar making.

I was happy to share this instructable with you. Please send me your realizations!

Greetings from France!

Bruno