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Have to Disagree

Posted by Triode_Kingdom on May 15, 2023 at 11:17:16:
"Some people say that only AC heating of direct heated triodes sounds good and that DC heating does not sound good. I think the people that say that have listened to amplifiers where the direct heated triodes are heated with poorly filtered DC."

The prototype for my 211 SETs initially used 60Hz AC to heat the filament. The sound was wonderful, but I couldn't reduce the hum to a reasonable level. I even tried a reverse-phase injection scheme to cancel it, but the waveform included energy that was complex and non-repetitive. Eventually, I came to the conclusion that the long filament structure was actually vibrating and creating AC anomalies that could not be predicted or cancelled.

At that point, I switched to DC. The first attempt was the usual rectifier/filter approach. It was quiet, but the life seemed to be gone from the music. Thinking this might be due to residual AC energy or harmonics, I added a regulator. That produced pure DC, but it didn't help sound quality. Finally, I used a switching supply to isolate the filament from the supply for common mode effects. Still not good.

In the end, I resolved the issue by heating the filament with ultrasonic AC. That brought the life back to the music. This is all very subjective, of course. I also can't claim it applies to all DHTs, but other builders have reported similar AC VS DC results with other tubes.

Here are the filament supplies in their final form. The output is a quasi-square wave at roughly 80 kHz: