There are two things that necessitate changing of bass strings:
1. fret wear (the little indentations in the string wire, that finally cause breaking of the winding; mostly with the D-string). This can be delayed by shifting the string towards the bridge a bit from time to time. Prolongates the life of the D string so that you don't need to change the D string before the other bass strings.
2. more and more dirt coming into the spaces between the windings and baked together by finger sweat. It works very well to remove them in a 5% ammonia (you buy household ammonia and dilute it with water so that the resulting concoction is 5% Ammonia.) Opposite to light acids like vinegar or lemon ammonia solves effectively fat and organic substances. By leaving your bass strings for 15-30 minutes in that, and than well rinsing them in cold (!) clear water and wiping them dry in soft cloth you gain at least a second life for the bass strings. Of course be careful not to get the staff into your eyes and to do it in a well ventilated room. Especially the mixing of the 5% solution should be done in well ventilated room as ammonia is aggressive on respiratory organs. The limit of this is mechanical wear (#1). The advantage of this procedure over changing to new strings is that they settle very quickly and, if you are not specially keen for the sound of totally new strings, sound very nice - refreshed but not metallic like new strings.
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