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I once heard it explained that the lower heater voltage in the predicta tube was the result of 2 design limitations. They needed to make the cathodes shallower as part of making the gun shallower, and they had only one diameter of heater wire to work with. To get it shallower they couldn't use as much heater wire and thus the shorter wire had a lower voltage drop. The later 6.3V replacements they went to the engineering effort to get thinner heater wire that they could wrap to drop 6.3V and still fit in the cathode.
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