#4. What Is Biasing?

In regard to basic tube amplifiers we can define “Biasing” as setting the idle plate current flowing through the tube. Less current means lower dissipation in the plate (less heat in the bottle), more current means increased dissipation in the plate (more heat in the bottle). We can also create a broader definition for biasing which we would define as: Establishing a DC operating point by which the component can operate or swing from. With vacuum tubes this is generally a DC voltage that is applied between two electrodes which sets the DC current flowing through the tube.

Traditionally setting the bias point was a purely academic process. The ideal bias point is calculated/chosen to provide the desired compromise between signal swing and distortion. With guitar amps (and even some HIFI applications) we can manipulate the bias point within an acceptable window to provide the coloration we desire. 

Both pre and poweramp tubes are biased, however in terms of user adjustment typically only power output tubes are of concern. There are three biasing topologies we most commonly use in the audio industry.

Cathode biasing is one of the oldest and most common arrangements. It is sometimes referred to as auto-bias or self-bias. With this topology a resistor is inserted between the tube’s cathode and ground which puts it “in-series” with the tube relative to the power supply. This means that any current flowing through the tube also flows through the “cathode resistor”. Any time current flows through a resistance it creates a voltage drop. This voltage drop effectively becomes the bias voltage for the tube. This arrangement is mostly self compensating because the more current the tube tries to pull the more bias voltage the cathode resistor generates. This keeps the idle current reasonably consistent from tube to tube. 

Most all pre amp tube stages are cathode biased. Cathode biasing is also commonly used for output tubes in low to moderate output applications.  

Fixed or Grid biasing (as we prefer to call it) is most commonly used for power tubes. In this arrangement the cathodes of the tubes are effectively grounded and a bias voltage from a separate supply rail is applied to the grid of the tubes. This topology was originally called fixed-bias as the bias voltage does not vary with the current flowing through the tube like it does with a cathode biased arrangement. This nomenclature can be a bit confusing because most fixed or grid bias amps have an adjustment trimmer that is used to vary the bias voltage. Today we normally call this an adjustable bias amp. However back in the day Fixed Bias was commonly used to describe any grid bias amp whether it had an adjustment trimmer or a fixed resistor. 

Fixed/Grid Biasing is used where maximum output and efficiency is desired. 

With a fixed bias non-adjustable amp (i.e. Mesa Boogie, Peavey 5150’s, Peavey Classic series) it’s critical that the correct grade of power tubes are selected for optimum performance. We can select the correct grades by request or by default if you are ordering a Full Retube Kit. In these amps we can also select hotter or cooler grades of tubes to create subtle changes in the way an amp overdrives and sounds.