This argument about DC effects on filament has been around for more than 30 years - You take some and leave some. Some talk about "cathode poisoning" or some ion migration in heaters of IDH tubes and cathodes of DH tubes. It doesn't discredit the high frequency AC filament heating method. I have tried regulated DC before embarking on the HFAC and did find it was a bit "muffled", but I'm not endorsing the "need to switch current" mumbo jumbo. Interaction with stray heater/cathode/grid capacitances maybe?Why should one bother after reading nonsense like this (in the first link)?
At any rate HFAC is certainly more rational than sticking a class D amp operating at HF for heater duty IMO.
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