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RE: 12AX7A contact-potential biasing

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The input stage here is not contact-potential biased, if connected to a moving magnet cartridge. These devices have DC resistance of roughly 1000 ohms, and there will be almost zero net bias voltage on the 12AX7 grid with < 1 uA current through such a low value. For contact-potential grid bias, the source must be capacitor-blocked, and the grid-to-ground resistor is typically much higher (around 1 megohm), to develop a small negative bias.

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