Cathode Bias & Bypass

In a cathode-biased (self-biased) stage the grid sits at 0 V through the grid-leak Rg, while the plate current flowing through Rk lifts the cathode above ground. Relative to its cathode, the grid is therefore negative by exactly Vk; the valve biases itself.

N is the number of valves sharing one resistor (a push-pull pair often shares one). Choose a resistor rated at least twice the dissipation; it runs cooler and drifts less.

The bypass capacitor

Without Ck the signal current also develops a voltage across Rk, opposing the input (local negative feedback). Gain drops, distortion and output impedance improve. Fitting Ck shorts Rk at audio frequencies, but only above a shelf. The cathode looks back into the valve with an impedance of roughly:

so the response is a shelving filter with two corners:

Guitar amps deliberately use small bypass caps (0.68–2.2 µF) to cut bass and tighten distortion, while hi-fi designs choose Ck so that f₂ sits well below 20 Hz. The capacitor's voltage rating only needs to exceed Vk with margin.