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RE: basic analysis of the S.E.X. amp

OK, great - you understand the "normal" parts of the design, and have only stumbled at the weird part. This is an old Western Electric trick from the twenties. The power supply current is split, with the main circuit from ground to the positive, and a resistor Rbias from ground to the negative of the power supply. This creates a voltage below ground, which biases the grid of the output triode to a negative voltage.

Weird, I agree. The reason is more complicated. I wanted to connect the output transformer to the cathode to keep the signal current loop small - Cpf to OPT to the triode. No signal current flows through the other components, including the power supply (which has electrolytic capcitors which are not especially linear). But the OPT is wired as an autoformer, so if it is not grounded the output will not be grounded - and the output goes to headphones which really must be grounded for safety. So we needed a way to get a bias voltage so that the triode's cathode could be grounded and thus connect to the OPT.

Another complication - every triode is slightly different due to manufacturing variability. That's why cathode bias is used - the bias depends on the triode's current in a way that stabilizes the operating point for that particular triode through negative feedback. The feedback only operates at DC and frequencies below audibility. For this to work, each triode needs its own feedback, so there must be dual independent power supplies.

It's my goal to simplify this eventually by using transformers instead of autoformers, and that are rated to have some DC on the primary winding relative to ground. Then I can use a more conventional power supply and will need only one supply for both channels.

Hope that provides food for thought! Let me know if I've been unclear anywhere - web posting is not the most efficient communication method...


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