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In parallel heater sets I'd reckon over %95 of all tube failures were NOT heater failure (usually worn out cathode, or shorts). On series string heater sets the rate of heater failure was higher than parallel sets, but it was typically only 1-3 tubes in the string and usually the ones with higher heater voltage. If the set isn't color and a series string there's fair odds that a standard color thermistor would work... could also install a series heater resistor (or a diode) and a time delay relay to short it out.
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