In an interesting discussion on Facebook's Vacuum tube amplifier group, the purpose of the 200k resistor from B+ to the cathode of the 12AX7 in the circuit shown below is discussed. The OP's question is "Hi All. Need help understanding the preamp biasing. Typical 470R on the cathode to ground, but also 220K from the B+ tied there as well. On a Magnatone M13 time machine, never worked on. Thanks!"
Soon dismissed are a voltage divider for bias, and local feedback. The winning answer seems to be feeding B+ noise antiphase to the cathode for cancellation - looks like a good idea for sensitive ccts like mic or phono preamps? I may play around in LTSpice to see.
Soon dismissed are a voltage divider for bias, and local feedback. The winning answer seems to be feeding B+ noise antiphase to the cathode for cancellation - looks like a good idea for sensitive ccts like mic or phono preamps? I may play around in LTSpice to see.
