A self-biased stage you compute in 3 ways:
- either as self-bias phase inverter (Ra ~ Rk)
- or as a cathode follower (Ra = 0)
- and if you null Rk, you end up with the normal basic amplifier stage
Additionally, you may use it as a high impedance input stage for piezo pickups, by connecting the grid leak resistor to the mid-point of the cathode resistors, while decoupling Rgk. This bootstrap effect multiplies the apparent grid leak resistor as the voltage on the mid-point tends to mirror the input voltage. More details in “Designing Valve Preamps for Guitar and Bass” of Merlin Blencowe, page 89.