Im about to order some flats to try for the first time. I want to make sure that the gauge is right. If I like medium gauge rounds should I order medium gauge flats or is there some trick involved where flatwounds have a higher tension at the same gauge or something. i.e if i like medium rounds, order light-gauge flats?
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Most strings, even in the same gauges, will have different tension. If you like 45-105 rounds, I'd get the same gauge flats. But, be prepared to need a little truss rod tweak. You just never know until after you change strings.
What gauge rounds do you have on there now? You could go to the D'Addario website and find the tension (approximately) of each string that's on the bass now, then order a set of Chromes flats that comes closest. It would be close.
Medium rounds, light flats will also get you close.
Tension wouldn't be the only concern... core wire thickness, core and winding wire alloy/composition and winding type can affect stiffness. A .090 string can feel stiffer than a .105 even if it's tension is less.
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Flatwound gauge 
