This is from a discussion in the Virtual Harp Circle group concerning what to do if you don't have the needed information to know what replacement string to put on a harp:
 
Harp players worry about keeping with the original string chart for their harp. But in my case, I'm continually messing with those charts, trying this and that, to dial in the sound and feel of my harps. This is an indication that the harp is neither as precise nor as fragile as some would have you believe.
 
If you're in doubt about what to put on, and you have a mess-o-strings from which to choose, take a squint at the broken string (or its neighbors if it's missing) and pick something close. Put it on and pull it up to pitch. If it's too stiff and a bit dull, then go one notch lighter (smaller diameter). If it's too jangly or slack, then go a notch thicker. (Tone, however, is not as reliable a determnanat ina new string as feel. It takes a few days for a string to conform the the points its bending over. Until it does, it'll sound dull compared to its long-strung neighbors.)
 
To allay one's fears about doing this; that is, to see if it's significant or not, let's mess around with one string as an example. (Important note: We're only talking about monofilaments here, not wound strings.)
 
Let's pick a string in the middle of the harp. The E above middle C on my SweetHarp29 is 21.4" long. In nylon that's 45% of tensile strength and it's normally got a .045" string on it. And the whole harp has 1102 pounds pressure on it. Okay so far?
 
If I put a .040" nylon on it, the string goes from being at 31.8 pounds tension to 25.1, a decrease of 6.7 pounds. Remember - it's still at 45% of its tensile strength.
 
If I put a .050" nylon string on, it goes up to 39.2 pounds, an increase of 7.4 pounds, and is still at 45% tensile strength.
 
But what does that mean, adding or subtracting 7 pounds tension? You need to look at the whole harp. The whole harp normally has 1102 pounds tension on it. When I put the .050" string on, it now has 1110 (rounding up). The difference is well less than 1% of the total tension. If your harp is built so close to the edge that a change of tension of under 1% is a problem, I would immediately put the harp down and quickly back away from it.
 
What if you go to carbon? Well, the main difference is that it's now at 70% of tensile strength. And since it's normal to go one size down for carbon from nylon, instead of a .045" I put on a .039". The tension goes from 31.8 to 39.5 pounds. Essentially the same as going from the .045" to .050" nylon (except it's nearer the breaking point).
 
Have you ever had to tune a string up half a step? Maybe you don't have levers, or you want to record without lever tone? On our E string, if I tune it up to F (just an example), I go to 50% tensile strength (from 45%) and up to 35.7 pounds, an increase of 4 pounds–or less than 1/2 of 1% overall tension. So that's not a problem either.
 
Remember, I'm talking about a string here or there. I emphatically do not recommend increasing either the pitch or diameters over the whole harp! And in changing diameters, I am only suggesting going up or down one size. In fact, this is how we fine-tune the sound and feel of our harps. The changes we're talking about are normally quite subtle. Do I go up a diameter at the E or D? Do I switch to Carbon on the C or B? These little changes are often enough to smooth out a transition from one material to another; or to give a "light" instead of a "medium" feel; or to fix a string that didn't have the same resonance as its neighbors.
 
We harpers, in general, are way too cautious about messing with our strings. And frankly, the abjuration from some harp makers or others to only use this or that brand, etc., or not to change diameters is a bit like the Wizard of Oz behind his curtain. It's time harpers got empowered concerning their strings. Time for Toto to pull aside the curtain!
 
If nothing else, remember this: 1. changing the diameter of the same material does NOT change how close it is to its breaking point; 2. Increasing or decreasing the diameter one size on one string of the same material is an insignificant change in tension to the entire harp.

(I wrote this as part of a discussion on the Virtual Harp Circle on Yahoo)

It might be good to clear up the way we talk about strings. To my way of thinking, "light" or "stiff" stringing is the result of two very different factors, and we as a community would benefit if we spoke with at least some generally agreed-upon terms.

LONG VS SHORT STRINGS: If I'm using nylon (just as an example) for a string at a certain note, I can figure its vibrating length at any percentage of that string's breaking point I want. So I put it at around 60%. When I pluck it firmly, I'm approaching 80%. If I go much higher than that, I risk breakage. Also, all materials vary slightly so I want to be safely within that fudge factor (I know of one harp maker who claims his strings are accurate to within ±.002, which is the tolerance given by the material's manufacturer and has nothing to do with the string making) . If I go below about 35% of the breaking point the string starts to act like a slack rubber band.

All else being equal, a longer string is stiffer, and a shorter string is slacker. Longer string lengths exert more energy on the soundboard creating a more responsive harp; shorter strings less (talking about monofilaments here). But much longer lengths may break more, shorter less. It's a balancing act. And it's why you've probably never broken a string on a Harpsicle–they're short string lengths.

Oh, also, all harps start in their upper range at the maker's chosen target % and slowly fall away from that as it heads towards the bass. So my harps might go from, say 62% in the treble to 35% in the bass. And this is for a particular material - nylon, carbon, bronze, etc. This is because that theoretical 60% length in the bass wouldn't fit in your living room, it's so long.

Also, it's important to know this when changing string materials, say from nylon to carbon. A string at 62% nylon becomes 94% of breaking point with carbon on it. You're definitely going to be breaking strings (but this explains why so many harps with short flubby strings get a new lease on life with carbon strings).

THICK VS THIN STRINGS: This is what guitarists are talking about if they put on "heavy", "medium", or "light" strings. What most folks don't get is that if I've got my nice 60%-of-breaking-point-string, I can put a FAT length of nylon (or whatever) on it or a THIN length of nylon on and it's STILL at 60% of its breaking point! A thicker string will feel "stiffer" and a thin on "slacker" but for different reasons (and in a subtly different way) than in the Long/Short String case. A thicker string also increases the tension (For example, a 60% nylon G, 10 1/2" long, with a .028" diameter string exerts about 16 lbs pressure on the harp. Go to .045" and it jumps to 43 lbs!).

We can use the diameter of the string to fine tune the tone and feel of an instrument to some degree. For example, if your Harpsicle feels slack, a slightly thicker string will stiffen it somewhat. (I''m not necessarily recommending this since one needs to know that a harp's construction can handle the slightly increased tension, but really, a few strings different here and there won't affect any harp that much. A whole set of heavier ones, yes!). Or if your big harp has a dull upper midrange, you can decrease the diameter of a few strings in that area and brighten the sound up a bit - but at the expense of a slightly slacker feel.

So just saying "medium light" or "stiff" stringing doesn't really tell us much. How about we come up with something like "60 to 30 carbon" or "55 to 25 nylon" etc, to indicate the string lengths' range of % of breaking point and which material it's designed for. Then we can say if we're putting light or heavy strings on it.

In fact I'm inspired by this post to do this with my harps. I'll start with the SweetHarp: it's a 62-35 nylon/carbon (it goes from 62% breaking point to 35%, designed for nylon in the top and carbon in the bass).

A good system for quantifying "heavy", "medium", or "light" strings I'll need to think about. Any ideas?


PS: FWIW I think Tynex is still a great stringing material, even though carbon is the string du jour (and definitely has a place in our panoply of stringing materials). Tynex is a "type 60" nylon - the original one. It's stiff, tough, and it's breaking point is very near its yield point meaning it will stretch slightly but not much, and not continually except under certain conditions. Other nylons (and carbons) can be self-damping (a BAD thing for musical instrument strings) and stretch sooner and forever.