High C (C5) string

Hi,
I've been playing about with doubled courses on my modded (tenor) guitalele, and having issues with the high C string. It is constantly snapping when being tightened.

I been using d'Addario rectified nylon in 0.022" gauge. It gets to the note initially, and the tension feels fine, but as it stretches and has to be tightened, it snaps.

I've also tried fluorocarbon fishing line (25 lb, 0.021" gauge), which lasts a bit longer if i'm careful, but doesn't match the rest of the nylon strings.

Does anyone have any experience getting workable (individual) strings for this?
AFAIK it is a standard on 6- and 8-string uke's, but I don't want a whole set for one string, especially one that needs replacing so often!

I don't have experience with a guitalele, but a little scientifically applied mathematics might help. You might try the next size smaller D'Addario, rectified nylon. (Obviously, the smaller the diameter, the lower the required tension to attain your pitch) See if it will work for you. If it breaks, go down another size. It's worth a shot. At some point, you won't like the sound, but the next size might be okay.

Of course, you have already taken your instrument to your luthier to be sure the string is the problem. A burr or too sharp an angle at the nut or saddle.... If, as you say, the tension is fine, the string should not be breaking.

I see you are in the UK. If you use an online tension calculator to work out exactly which gauge you need (in mm), then contact Bridgewood and Neitzert in London. They sell strings for period instruments such as lutes and will be able to sell you individual strings in fluorocarbon, nylon or nylgut in a wide variety of gauges. Good luck.

Thanks for the advice.

I've order the next couple of sizes down of the nylon strings to see if that will do the trick.

(I havn't taken it too a luthier, which i can't afford - i do all the saddle / nut / truss rod adjustments myself. But I don't think sharp or rough edge is the problem as it happens even when i remove the nut, and appears to be breaking near the tuning peg).