This page explains some basics of biasing and why we use Fixed Bias as our default bias setting for tube testing. If you want Auto-Bias for your tube tests, check that box on the order form.
Bias is a negative voltage applied to the control grid of a triode, pentode, or other design that uses control grids to set the amount of idle current the tube draws at rest. It allows a way to repel or attract the electrons being emitted from the cathode and thus control the electron flow between the cathode and anode. Generally, a correctly biased tube when idle has little or no current flow between the cathode and anode. At that point, the tube is said to be at idle.
The automobile engine idle analogy is helpful when discussing tube bias. On a car, you want the engine idling fast enough to keep the engine running smoothly, but not any faster. On a vacuum tube, having no negative voltage applied to the control grid causes the current between the cathode and anode to flow wide open (full throttle), and the tube would quickly get very hot and self-destruct. Tubes biased too hot wear out faster than normal. Tubes biased too cool last longer but can produce distortion and otherwise don’t sound very good. As one respected audio writer put it: “A typical bias setting is a reasonable compromise between good sound and acceptable tube life.”
Fixed-Bias Test Method
In the Fixed-Bias test method, the bias voltage is set to the manufacturer’s negative voltage value specification (bias voltage).
The resulting plate current–and the emissions and Gm readings–will be whatever the tube is capable of at this fixed grid voltage.
Auto-Bias Test Method
In this method, bias voltage adjusts to achieve the manufacturer’s specified plate current value (if possible). If initial emissions are lower than normal for a given tube, the grid voltage drops, sometimes lower than the normal bias voltage specification, to produce the specified plate current. Plate current is pushed toward normal, but the grid voltage often drops below normal to get it there. In worn-out or defective tubes with very low emissions, grid voltage can drop to zero and the rate plate current never reached.
Why We Use the Fixed-Bias Test Method as Default
Forgetting for now what kind of device the tube will be used in (self-biased/cathode-biased or fixed bias), even if there is a potentiometer to adjust grid voltage, the Fixed-Bias test method produces, in a sense, more-honest results. It also makes for easier comparisons of results between the Amplitrex AT-1000 we use and vintage testers, most of which use fixed bias.
The Auto-Bias method tells you what the tube would be capable of if it were allowed optimal conditions (lower grid voltage to coax higher current). So, it can be somewhat misleading.
However, some amplifier circuits operate in a self-biasing way, where the grid voltage is held constant, and so the tube bias is largely self-adjusting. Normally, this is achieved through the use of resistance in series between the cathode and ground and resistance that holds the grid voltage at a fixed level. Therefore, the Auto-Bias test mode results could be more representative of how your tubes will perform in those circuits.