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Questions about tubes and gear that glows. FAQ

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RE: I would guess it is how the cathode is heated and . .

Posted by Tre' on June 11, 2012 at 11:09:53:
6.3 volts AC RMS is another way to say how much AC voltage is required to get the same amount of heat into the cathode as you would with 6.3 volts DC.
RMS, as a way of measuring AC, was invented for that very reason.
A cathode will be heated to the same temp. using 6.3 volts AC RMS as it would for 6.3 volts DC. (BTW, 6.3 volts AC is 17.766 volts peak to peak)
With direct heated tubes (45, 2a3, 300b, etc.) the heater (filament) supply can be part of the signal path and there is good reason to think that the sound might be different between AC and DC heating.
This is where the idea that AC vs. DC makes a difference comes from, but not with in-direct heated tubes.
In-direct heated tubes have no way of telling the difference between AC and DC heating.
Tre'