Help - Directly heated AC valves

[Simon Buxton]

Hi Gang

I notice that in the 1930s quite a number of directly heated valves were
specified as having AC filaments. Examples include the F443 or 45. Why
was hum not a problem with these? Was it too low a frequency for the
speaker or was the filament heavy enough to have sufficient thermal
inertia for the vairaition to have no effect?

Just wondered.

Simon
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