OM string tension

[arbarnhart]

I am curious about what range of string tension is considered "normal" (if there is such a thing) for the OM. If you don't know yours, if you post the scale and the guages of strings (or the brand/type of strings and I will look it up) and I will calculate it. The first instrument I built (my little domra) I have strung/tuned loose, about 12 lbs per string using standard mando strings tuned an octave lower on a 22" scale. Amazing little "bending blues" box; you can push a string quite a ways. My regular mando has tensions in the 17-20 lb range. I was looking at some octave mando strings online and ran their gauges and was coming up with tensions in the mid 20s. Seems a bit much.

[Chris Baird]

Around 200 pounds total.

[Dolamon]

For a "normal" eight string with a 537mm scale (21.25inches - pin bridge type) I try to keep it a bit under the 200 pounds. The norm I try for is in the 185 to 190 pounds with relatively level tension across all four courses. This instrument thrives on Elixer: .052, .035, .022w and .014 plain. The surprise for me was the E course ... I worked my way up to this from .012 through .013 over a period of months.

It seems getting a relatively level playing tension across all four courses is the art. If you can get the tension within 3 or four pounds - you'll end up with a balanced sound and feel. But - every instrument is a bit different and the high tension may be destructive on some lighter made models ...

Re the string set above ... I use sets with the G all the way down to .044 for longer scale instruments.

[arbarnhart]

Wow - that's more than I expected but means the numbers I ran are dead on. I am now planning to go shorter, about 18" scale, and use mandola strings tuned to OM at around 125# total to see how the instrument and I handle it. If I go to mandola tuning, the strings would be at 200 on about a 17" scale. Hmmm... What to do...

So what's the downside of low tension?

[Dolamon]

The downside of low string tension ... small volume output and lots of unwanted buzzes. The tension required on virtually any string instrument is pretty much a given factor. To me the 18 or 19 inch scales tuned in C are virtually unplayable due to the longer stretches in the 'fiddle' keys. It's odd to me - the same CGDA tuning on a tenor banjo works with low tension strings but this is on a single course, drum top. Every time I've tried a banjo set on a double course instrument, nothing but heartache and buzzes.

The same scale length tuned to GDAE with light weight strings is (to me) a recipe for disappointment. It seems you really need a lot of tension to move the top around.

[Chris Baird]

The best method for figuring out what strings to use is to start off with something you suspect to be low tension. Then bring the tuning up a half step at a time until the "tubby" loose string tone goes away. Of course you want to make sure you don't go too far. Just listen for the right tone quality and also pay attention to the way the strings feel. Once the ideal tension is achieved you can then ascertain the total tension from whatever tuning you've come up to. Now that you have the ideal total tension the task it to put together a set of strings to give that tension in a normal tuning.

[arbarnhart]

Using the calculator at greenmanhumming (http://www.greenmanhumming.com/html/StringCalc/Multistringcalc.html#the%20applet), I find that my regular mando's total tension is 151 lbs total:

nt Hz # gauge # tension
scale 14.0inch (355.59999999999997mm)
G 196.0 0.034 18.62887 lb
D 294.0 0.024 20.458688 lb
A 440.0 0.014 17.063604 lb
E 659.0 0.01 19.554802 lb
totaltension = 75.7103732 lb

If I go plug in OM tuning and use the mandola strings I picked up and go to 19" scale, here is what I get:

nt Hz # gauge # tension
scale 19.0inch (482.59999999999997mm)
G 98.0 0.049 17.019512 lb
D 147.0 0.034 18.342272 lb
A 220.0 0.024 20.546872 lb
E 330.0 0.014 17.6368 lb
totaltension = 73.55647900000001 lb

One thing to note is that G is really .048, but the calculator has no data for a .048 BW string. If I change it to .047, it drops the tension from 17 to 15.675.

To get to 200 lbs on this set of strings would require a tad over 22".

A side note or two. There is some relevance to my regular mando's numbers. I copied the shape and dimensions of the top to make the OM top, but I made the sides taller (sound box is a lttle deeper). This will be only the third stringed instrument I have built and the first with 2 courses. I am more what you might consider a folk instrument maker as opposed to classical luthier at this point. The neck is stable (been dry for years) rock maple with piece of 1/8" x 1/2" bar steel epoxied into a saw kerf in lieu of a truss rod under a walnut finger board. I am quite confident that it is as strong as a commercially made model, but I cannot adjust the neck (should not have to; it should stay dead flat well beyond the numbers we are throwing around). I harbor no illusions about this instrument being much more than another learning experience though I do expect it to be quite playable as a "knock around" instrument.

I want the lower range of an OM and a wider fret board, but I would like fret 6 within 6 inches of the nut (which means a scale of 20.5" or less).

Here are some other interesting numbers. I was looking at a cheap 18.5" scale "OM" (they called it that) and sent the vendor a query about strings for OM tuning and expressing my opinion that it was really a big mandola. Their reply: It can also be tuned to octave tuning,...we recommend .013,.017, .026 and .036 gauge strings for octave tuning, thanks!

So I ran those numbers:

nt Hz # gauge # tension
scale 18.5inch (469.9mm)
G 98.0 0.036 8.664078 lb
D 147.0 0.026 10.14116 lb
A 220.0 0.017 10.978908000000002 lb
E 330.0 0.013 14.418084 lb
totaltension = 44.197820799999995 lb

Apparently they employ some highly trained people #http://www.mandolincafe.net/iB_html/non-cgi/emoticons/tounge.gif