Another fun fact about the loctal types, though rather useless for TV, is that they have a ruggedized heater-cathode assembly to allow overvoltages of up to 7 volts. This is the approximate voltage from a fully charged 6.3 volt car battery with the alternator running as well.
I have a Williamson I built with 7A4s in the first four stages and Sylvania 807Ws for the output tubes, with a regulated 6.3 volt heater supply. I expect the loctals out front to last until Armageddon...
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