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Funny coincidence...

"I would suggest the 211 if you want to do A2, much easier and something I have done. A simple reactor follower is what I did with a 100H/20ma inductor to ground and a 6BL7 driving the grid though a grid stop into the 211. Yeah no top end considering the output transformer was a 10K direct feed. Parallel feed would have been better for that tube. Just a bit before it's time."

That's essentially what I built (schematic below). With the standing current at 70mA (80mA possible with this transformer), the onset of visible distortion occurs at 25W at 1kHz. Low frequency output is as follows:

100Hz - 24W
60Hz - 18W
40Hz - 15W
20Hz - 6.5W

The amps measure flat from 20Hz to 20kHz at low power (5W or less) with no NFB. Adding 5dB of NFB improves the 20Hz maximum output to 10W. These measurements created a somewhat lengthy discussion on this topic over at HiFi Haven. I estimate the core of these already-large transformers would need to be about five times as large for flat 20Hz output at full power.



Schematic Edits: There is no hum pot on the filament. It is powered from a toroidal transformer with the center tap grounded through a current meter and fuse. Filament power is approximately 80kHz.



Edits: 05/07/24
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