B string for 25-5/8" scale acoustic bass

[NinjaPyrate]

A while ago I put some Savarez Lower Octave guitar strings on an old beat up Alvarez acoustic 12-string and tuned it to EADG. For my use as a couch bass/beater bass it has served it's purpose surprisingly well - sounds ok with no buzzing, no adjustments needed, and stays in tune. I'm trying out 5 string right now so would like to try adding a B string - the 12-string neck is wide enough (and strong since it's a 12-string) and I'll resize the nut groove. I couldn't find any B strings from Savarez and do not know what would work best for this purpose. It does not need to sound awesome and I don't care if the B string sounds/feels different - just looking for the best working option to try out. Thanks.

 

[HaphAsSard]

A while ago I put some Savarez Lower Octave guitar strings on an old beat up Alvarez acoustic 12-string and tuned it to EADG. For my use as a couch bass/beater bass it has served it's purpose surprisingly well - sounds ok with no buzzing, no adjustments needed, and stays in tune. I'm trying out 5 string right now so would like to try adding a B string - the 12-string neck is wide enough (and strong since it's a 12-string) and I'll resize the nut groove. I couldn't find any B strings from Savarez and do not know what would work best for this purpose. It does not need to sound awesome and I don't care if the B string sounds/feels different - just looking for the best working option to try out. Thanks.

Try the low E from the La Bella/Gold Tone MBLNS Silverback set:
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Gold Tone Micro Bass MBLNS Silverback Strings by LaBella
Assuming identical, proportional (scaled-up) construction, a string has to have 4/3 the diameter of another string in order to have the same tension as the latter when tuned a perfect fourth lower. In other words, you'd need a .1186 for your B to match the tension of the Savarez E; the Silverback .115 is the closest I'm aware of.
I seem to remember it actually does not taper at the tuning post (the Gold Tone MBass tuners are huge, in order to fit rubber strings, so they can accommodate a full 115 string no prob), meaning you will probably have to find a way to fit it on the Alvarez. A piece of heat-shrink tubing between nut and post, and then a tedious unravelling process to bring down the gauge until it goes through the post hole, might suffice.