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5 string to 7 string conversion - neck strength?

Rockin Mike

Rockin Mike

May 27, 2011
2,772
424
#1
I have put some thought into converting a 5-string Ibanez Mikro GSRM25 in to a Bass VII tuned BEADGBE.

There's room on the headstock for two more guitar-sized tuners, one between the A & D, and one in the corner just on the nut side of the G. String spacing would be 11mm - same as a Bass VI, which I like. I found a manufacturer that says they can adapt one of their 7-string guitar trem bridges to work with strings up to .142 or more. This would give me a nice 30# tension low B at 28.6" scale.

My concern is tension on the neck. The stock string set .125 .105 .085 .065 .045 adds up to about 160.5 lbs of pressure (using Circle K's string unit weights as ballpark references) averaging about 32 lbs per string. 7 strings at that tension adds 64 lbs, a 40% constant increase. Whammy it up a whole step and the tension goes momentarily up to 260 lbs, a 26% increase over the constant 7-string tension and 63% over the stock 5-string tension.
5-string tension (stock) = 160.5 lbs
7-string tension = 207.3 lbs (some of the gauges are different from stock)
7-string whammied up = 260.9 lbs (whole step up, momentary)

How strong are bass necks? What's the likelihood of it breaking? What's the likelihood that the truss rod couldn't hold it straight with 7 strings?

It's a one-piece neck with truss rod. It's a cheap bass but it's Ibanez so it's not junk. Looks like the top of the neck including the headstock down to the first 2 frets is glued on with a straight scarf joint. No volute. Angled headstock.

I'm willing to risk the bass but hoping some experts can offer opinions on whether it's a high, medium or low risk.

Thank you for all advice.
 
#2
It might take some math, but you can tune the existing 5 strings up until there's 207 lbs on the neck and see what happens.

If you can dial in the relief you need and the headstock doesn't break off then I'd say go for the full BassVII treatment.
 
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#4
lz4005 said:
If you can dial in the relief you need and the headstock doesn't break off then I'd say go for the full BassVII treatment.
Click to expand...

Not faulting your methods or logic, but I read this and thought "if nothing catastrophically fails it should be fine".

Mike, would you be willing to share this manufacturer of tremolos? Maybe in PM if nothing else?

-Jake
 
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#6
207.2 lbs divided by 5 strings is 41.44 lbs per string.
I don't have the tension charts for the stock strings so I'm using the Circle K charts to approximate.
Stock string | Closest Circle K match | Note | Tension (lbs) at 28.6" scale
.125 | .124 | E1 | 41.415
.105 | .106 | G1 | 43.250
.085 | .086 | A#1 | 45.715
.065 | .065 | D#2 | 42.064
.045 | .045 | A2 | 41.490
-----
total tension: 208.934

Those numbers may look pretty accurate and impressive, but remember they are fudged because they're not Circle K strings; I don't have the Unit Weights for whatever kind of strings they are, and the closest gauge on the chart is .001 off for three of them.

What's encouraging is that an average 42 lbs of tension is not unreasonable. It's high normal. My 34" main player 5-strings sport .142 B's (42.7 lbs) and .106 E's (43.2 lbs) etc.

Unit weight (UW) is the weight in lbs of one inch of string. Different strings will vary due to different alloys being used, differences in airspace between the wraps (flatwound has less airspace than roundwound, a round core will have less airspace between wires than a hex core, and so on). The stock strings are roundwound, and they look, feel and sound like nickel steels so the UWs should be at least in the same ballpark as the Circle K chart.

Thanks to Circle K for making their chart available to make this kind of planning possible.
All my basses sport CK balanced sets because they sound and feel great.

So, armed with safety glasses, leather gloves, heavy jacket & jeans, a cup, and my trusty Snark, I approach the instrument with abusive intent...
 
#7
I think it's going to be fine. Most of the up-tunes were trivial. Even tuning the B string up a full 4th to E was no biggie as it was pretty loose to begin with.

I can see and feel a small difference in the relief, but it's in the range of fine tuning. The action would be totally playable right now with no adjustment.

I'm going to leave it tuned like this for two days just to see if the neck moves.
Tuning is normally very stable with this bass, so if it goes flat I can attribute that to neck movement.

I won't adjust the truss until after the 2 days. No point chasing a moving target.
 
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#9
I have Kahler 2415s on two of my 5-strings and they are great. I'm hoping Kahler will be able to mod one of their 7-string guitar tremolos (I have my eye on a 2217 post-style) to handle bass-sized strings at a guitar-sized spacing.

They've indicated it might be possible but we're still just discussing it. I apologize if I overspoke in a previous post.

The Kahlers have a cam, or "axle" if you will, and the bridge rotates on that. String tension is counterbalanced by springs mounted to the unit itself, so it only requires surface routing, not all the way through the body. Lots of other cool things about them including adjustable string spacing.
 
Last edited:
#10
Well it sat tuned this way all weekend:
.125 | .124 | E1 | 41.415
.105 | .106 | G1 | 43.250
.085 | .086 | A#1 | 45.715
.065 | .065 | D#2 | 42.064
.045 | .045 | A2 | 41.490
-----
total tension: 208.934

and the neck barely moved. I had tune the strings up a few cents a couple of times, but they're stable today.
Checked the relief with a capo at 1st fret and holding the string down past the last fret, it's between 1/64" and 2/64" at the 12th fret. Doesn't even need adjustment.
 
#11
Rockin Mike said:
Well it sat tuned this way all weekend:
.125 | .124 | E1 | 41.415
.105 | .106 | G1 | 43.250
.085 | .086 | A#1 | 45.715
.065 | .065 | D#2 | 42.064
.045 | .045 | A2 | 41.490
-----
total tension: 208.934

and the neck barely moved. I had tune the strings up a few cents a couple of times, but they're stable today.
Checked the relief with a capo at 1st fret and holding the string down past the last fret, it's between 1/64" and 2/64" at the 12th fret. Doesn't even need adjustment.
Click to expand...

Sounds like this project will be on the way once you hear back from Kahler. I'm interested!
 

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