Using bass strings on guitar

Would it be safe to use a 0.065 bass string as a B string(tuned to Bb) on a 7 string guitar with a scale length of 26.5?

-
It would be useful to know how the rest of the guitar is strung - you need a matched tension over all strings to be kind to the neck. Also it'll give us an opportunity to match to the existing strings. I know that is not your question, but there are ramifications. – Tim Jun 29 '13 at 8:36
@tim the guitar is strung with 10-46 tuned to standard half step down – user10444 Jun 29 '13 at 10:59

You can look up string tensions in the D'Addario String Tension Supplement, however your scale length is not covered in their tables.

They do, however, provide a general formula

T (Tension) = (UW x (2 x L x F)2) / 386.4

.. where UW is the unit weight in lb/in, L is the scale length in inches, and F is the frequency of the note in Hz. Presumably if they used metric measures, there wouldn't need to be a fudge constant.

B♭ is 58.27 Hz.

The unit weight of their 0.065 string is 0.0007956

So the tension of your proposed string is:

0.0007956 * (2 * 26.5 * 58.27)2) / 385.5 = 19.68lbs

That's comfortably in the range you'd expect of an electric guitar string, so should work fine.

To work out the ideal string to go with your other strings, work out the tension for your other strings, and use the formula in the opposite direction to find the unit weight that gives you B♭ at that tension:

UW (unit weight) = (T x 386.4) / (2 x L x F)2

-
Amazing answer thank you very much for all the clarity! – user10444 Jun 28 '13 at 18:48
I'd wonder if the bass string would even fit in the bridge of a guitar.... – Dave Jacoby Jun 28 '13 at 18:49
@VarLogRant It actually fit, my guitar has Floyd Rose bridge and the tuning peg is weirdly large. I didn't wind it though after reading slim's answer. – user10444 Jun 29 '13 at 11:02
The low Bb is 58.25 Hz not 116.5 Hz – David Axtell Moore II Jun 30 '13 at 7:21
Ooooops! Fixed. Don't tune any of your strings an octave too high! – slim Jun 30 '13 at 8:07

The low Bb (the 7th string I'm assuming you'll tune lower than your open Eb right? The Open A is 110Hz); so… Bb is .5 X 116.5Hz (58.25Hz actually, recheck the chart on the D'Addario string site).

This changes the math of course… 0.0007956 * (2 * 26.5 * 58.25)2) / 386.4 = 19.62lbs… It's just about right, if you want to go totally "Gonzo" tune it to an Ab (1 finger barre power chords in the two low strings, bwhahaha!!! fast), seriously… you can also do low I-V bossa/samba bass lines with a single finger if you tune your 7th down a 5th instead of a 4th.

Ab would be 0.0007956 * (2 * 26.5 * 51.9)2) / 386.4 = 15.57lbs

As a comment on detuned 7-string playing… you might want to consider a little heavier on the bottom end than a standard set of "10's", sometimes the 6 & 7 strings can get a little "floppy" and if you press too hard (especially with jumbo frets), you can get intonation problems (sometimes things just get a little too loose, but the 26.5" scale should help, just mentioning it so you're aware of it). I used to use a set of "11's" with a plain third, and it was over a decade ago, I've forgotten what I used for a low A string (55Hz, I didn't normally detune).

-
I usually downtune a half step down, I find the set of 10's nice. By downtuned playing you mean tuning lower than that? – user10444 Jul 2 '13 at 10:07

Given that your bottom E is .046", a .060" or .062" will do the job. A .065" will maybe give a little more grunt to your sound, but would feel a little tight. That string rarely gets bent, so it doesn't need to be too loose. Any thicker than .065" won't do the neck any favours in the long term.You mention 'bass string'. An ordinary guitar string will be better, as a string for a bass guitar (if that's what you mean) is way too long. I'm not sure if the make-up of a .065" guitar string is any different from a .065" bass guitar string - probably not. Just shorter.When you have a successful set-up, please let us know: theory is all very well - practice is better.

-

Leaving aside the fact that anytime someone asks "[is] it safe," I start thinking about escaped Nazi officers posing as dentists :-), I would recommend as a general rule against using any string that you're going to tune to a higher open pitch than its design pitch. After adjusting for the open-string length, of course (a bass E-string would have the same applied tension if used as an A-string on a 3/4 length neck, roughly speaking). If you do try to tune a string high, it'll have a rather short lifetime. Anecdotally, the lifetime of a cello C-string that I mistakenly mounted as a G-string had a life of less than 30 seconds :-( .

-
With bass guitar strings, I'd worry more about the lifetime of the instrument's neck, than the lifetime of the string. – slim Jun 28 '13 at 16:19
@slim so you're saying guitar necks are wimpier than cello necks, or guitar strings are tougher than cello strings? just joking :-) – Carl Witthoft Jun 28 '13 at 16:21
I'm assuming electric guitar strings, which work at much higher tensions than nylon cello strings. – slim Jun 29 '13 at 6:27
@slim who uses nylon? All cello strings I know are wound metal. there may be a few gut string specialists out there, but not in measurable numbers. – Carl Witthoft Jun 29 '13 at 21:26
@Carl Witthoft, that's just the outer winding, the inside is gut or nylon (preferably gut, of course); I always used gut, it's been 20 years though... but here's some "Pirastro Gold's" for sale bing.com/shopping/… Great strings, I highly recommend them, I always found their tone much richer than nylon (quite a bit more expensive of course). But yes... electric steel core strings (electric guitar/bass/steel string acoustic) are a lot more tension than violin/viola/cello/double bass or... classical guitar – David Axtell Moore II Jun 30 '13 at 7:31