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Single ended tube audio output stages
Here's a "philosophical" question that I could answer at least part of in a
minute with an empirical test, but I want to see how people think about it. In the conventional single ended tube audio output stage, having a conventional audio output transformer the ends of whose primary is wired to B+ and the output tube plate, and the ends of whose secondary is wired to the speaker (don't throw in any complications like additional circuitry on either side of the transformer; keep the tube in conduction at all times, i.e. class A operation; and provide an input such that the output stays within the rated frequency and power range of the speaker and transformer), WILL THE INSTANTANEOUS PLATE VOLTAGE EVER RISE ABOVE B+? Or will it remain in the range somewhere between B+ and zero volts, with a midpoint approximately halfway between the extremes, a midpoint that depends on the amplitude of the output signal? But if it rises above B+, why? And then why don't tube manuals reflect the maximum instantaneous permissible plate voltage, but rather what looks like maximum quiescent voltage, when they give "absolute maximum" figures for plate voltage? (Yes, I know that the data for television horizontal output tubes generally give such instantaneous maximum plate voltage information, but these tubes cut off conduction in normal operation and they normally operate into an underdamped circuit which rings, i.e. flyback, when the tube cuts off.) |
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