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I've heard that there was a rectifier tube that was failure prone, with heater-cathode shorts. Think it was the 6X5, but I'm not sure if that was the one.
And how bright a tube heater looks to be doesn't really indicate the health of the tube. It depends partly on how much and how close the ends of the heater wires are to the end of the cathode pipe, some tubes the heater wires stick outside, some the ends are inside the pipe. Has no real impact on how hot the cathode inside where the grids and plate action is.
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