What the Bias Point Controls
Every valve power stage runs its output tubes at a quiescent current — a baseline idle draw — that determines where on the transfer curve the tube amplifies. Set that current too low, and the tube runs cold: crossover distortion appears at the zero-crossing point where the two halves of a push-pull pair hand the signal off to each other. Set it too high, and the tube runs hot: it draws excess current, dissipates too much anode power, and shortens its own life, sometimes dramatically. The quiescent operating point (the bias) is a deliberate balance between those two failure modes, and the service specification for a given amplifier — EL34, 6L6, KT88, or otherwise — names the milliamp or milliwatt target a technician confirms at the test point.
Fixed-Bias, Cathode-Bias, and the Difference at the Bench
The trade divides power stages into two camps. In a fixed-bias design, a separate negative supply voltage is applied to the grid of the output valve, and an adjustment potentiometer on the PCB or chassis allows a technician to trim that voltage against a meter reading at a cathode resistor test point. It is not "fixed" in the sense that it never changes — it is fixed in the sense that it is set externally, and it drifts as the tube ages. Any EL34-equipped amplifier of this type requires a bias check after fitting new output valves, and again at service intervals.
Cathode-bias designs take a different approach. A resistor sits in the cathode circuit, and the tube biases itself: as cathode current rises, the voltage across the resistor rises with it, driving the grid-to-cathode voltage in the corrective direction. These amplifiers are genuinely self-correcting within limits — a classic Vox AC15 runs this way — but the cathode resistor itself dissipates real power and adds a degree of negative feedback that influences the amplifier's character. Technicians replacing output tubes in a cathode-bias stage still check the resistor value and condition; the component wears too.
Drift in a fixed-bias stage is normal and predictable. New tubes are not matched to the exact transconductance of the set they replaced, ambient temperature changes the operating point, and resistors in the bias supply age. A six-monthly check in a working amplifier — or an immediate check after any tube swap — is standard practice. A stage left biased too cold produces crossover artifacts in the signal well before a fault shows on any test instrument.