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Tubes are not CRTs
CRTs are picture tubes with a phosphor coated screen. They are not used in audio at all. Also, you can't use a slide rule for adding up. They're used for multiplication, division, trigonometric functions and possibly more esoteric functions. I still have a cylindrical slide rule that's equivalent to a linear slide rule 5 feet long and consequently much more accurate than the conventional foot long linear slide rule.