What is fixed bias? What is cathode biased?
A fixed bias amp is amplifier in which the tube is biased with a DC voltage on the grid, usually a negative voltage with respect to the cathode. A good example is a Fender Twin Reverb and other higher powered amplifiers. There is usually a trim pot located somewhere on the circuit that controls the amount of Bias voltage from the bias supply. Cathode biased amps derive their bias voltage by means of a cathode resistor and grid resistor (usually a larger value 1M or more) referenced to ground along with a bypass capacitor. As the audio signal changes, the DC voltage on the cathode also changes. This causes a shift in the bias voltage which produces distortion and harmonic overtones which can be desirable to the ears. In order to keep the DC voltage on the cathode somewhat stable, a large capacitor is used to “bypass” the audio signal to ground. Capacitors pass AC current, and block DC voltage.
