• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

RCA SP-20 circuit question.

donnyab

Member
2011-05-30 10:49 am
I think this question came up in another thread a few months ago. Did you search?

The cathode resistor, presumably used for degeneration, generates more voltage than is required for valve bias so the grid is attached to a potential divider formed by the 330k and 1M.

Most circuits either ground the grid (for DC) or pull it up to a significant positive value. This circuit does not, it pulls it up just a small amount, so it looks strange at first sight.