As the title says, why do different tubes have different sound to it? do they change the frequency response some how?
Thanks for any explanations

When I think about a tube being a alive with electric current passing through the parts, and within a given vacuum, it is not surprising that they sound different. As L.S. indicates, differences can be from material quality (a surprising number of 50-60s tubes are extra good), but also construction quality, and design. Different material thicknesses, shapes, space in the tubes, getter shapes and locations...these all seem to effect the tube sound. If you have a look at a bunch of a single tube type, you will find similarities, but also many differences in design and construction. These all effect the measured values L.S. refers to. But two tubes of the same type, that measure the same, and of different construction will sound different. I think this is part of what makes tubes interesting to tube lovers....depending on their makeup, they are sort of alive and impart personality that is compelling.