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There's always a good amount of tolerance in heater voltage. Remember series sets directly powered heaters from the wall and transformer powered sets powered them through an unregulated stepdown transformer so tubes were always exposed to the same xx% line voltage tolerance/variance as the rest of the set, and the design engineers had to plan for it.
Generally the lower the heater voltage the lower the output and the longer the life. Unless a tube or it's circuit is marginal it should run fine at the bottom end of input voltage tolerance.
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