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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps
Post Subject: The sound of the 2A3 however?Posted by drdna on: 2/13/2009
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 floobydust wrote:
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. I also tested with a 2.5K load and a 1K cathode bias resistor.... so at least with NOS 2A3 tubes, performance suffered.
Perfectly reasonable, however I would be very interested to know the way in which the sound changed with the adjustments, beyond oscilloscope measurements with the different ways you tried it. This would be useful information to me.

Also what is interesting is that the general observations I have had are similar to those of others who have used these tubes in different circuits. That is the sonic differences remain consistent. So I wonder how profound would be the sonic impact of the optimization of the circuit you speak of. No doubt a tube must be operated correctly to do its best, but the question to me is how big is the reward and in what respect it affects the sound.

Adrian

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