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In Reply to: RE: I have a different question about the circuit posted by Triode_Kingdom on April 25, 2015 at 08:23:24
Typically this circuit provides a response step function at subsonic frequencies for improved subsonic stability under closed loop conditions (controlled phase shift).
It may also reduce blocking under overdrive conditions but there are better means of achieving this.
I would think a truly revelatory simulation needs to include both real-world OPT characteristics AND the closed loop.
Follow Ups:
I simmed it again, this time down to 0.1 Hz. Phase is much better controlled below 10 Hz with the 470k resistors, and especially below 1 Hz. I also see that this would only be important in a system with significant NFB, where the amplitude of subsonic frequencies is greater. Otherwise, with no loop and no parallel resistor, signals below 1 Hz are so severely attenuated, it doesn't much matter what the phase is. In addition, without NFB, no instability can result from this.
OK, that tears my theory to shreds. I was thinking about what happens if the bias supply fails. The 470K resistors would bias the tubes on hard, instantly blowing the cathode fuses and protecting the outputs. I should have known the designer(s) weren't focused on that. The fuses would probably blow the instant the supply fails anyway. :)
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