Re: When did Valve heaters change from 4v to 6.3v
I can understand the change from 4V to 6.3V. I guess it's largely coincidence that it happened at the same time as the general adoption of the International Octal base.
But why the move from 4V to 5V for rectifiers, eg 5Z4 (IO based)? And yet there's the AZ31 (also IO based, but 4V)? Whereas the EZ35 is 6.3V - this at least makes sense as its heater is insulated well enough to share the same LT as other valves, so it has to have the same voltage.
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