| If you do some modeling you're running about 15.5 watts of dissipation on the plate and pushing over 4.5 watts output. Running the higher B+ voltage (350 volts) with a 1K cathode bias resistor and the 2.5K load results in higher distortion than the text-book operating point, which is B+ of 300 volts and a 750 ohm cathode biasing resistor and the 2.5K load. From my findings with the (NOS) 2A3 and higher voltages, going to a higher plate load impedance makes a large difference by lowering distortion. My basic 2A3 SET design uses a 3.5K load with a 866 ohm cathode bias resistor and a 350 volt B+ supply. This was far more optimal and provided much better (clean) low-frequency response and a noted reduction in measured distortion and better wave-form symmetry. |
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