This is because a tube is a "normally on" device that is, it allows current to flow from the cathode to the plate when the grid is at zero volts with respect to the cathode. As the bias voltage is adjusted less negative, towards zero volts DC, the bias current becomes larger, and the tube is biased "hotter". In general, as the bias voltage becomes more negative, the bias current becomes smaller, and the tube is biased "colder". As the negative grid voltage is adjusted, the bias current will increase or decrease, depending upon the direction the bias voltage is going. It is no wonder the subject is confusing to people!įixed biasing means the tube is biased by means of a DC voltage, which is usually a negative voltage applied to the grid of the tube with respect to the cathode. Cathode biasing is usually fixed, and not adjustable, and fixed biasing is usually adjustable with a small trimmer potentiometer, or "trimpot". Fixed biasing does not mean the bias is not adjustable, in fact, it usually means the opposite. There are two main types of biasing: fixed biasing and cathode biasing. The proper bias point is a tradeoff between all of these factors, and selecting the optimum bias point can sometimes be difficult, and it will vary depending on the amplification stage requirements. DC input power), gain of the stage (the magnitude of the output signal for a given input signal), noise of the stage, and class of operation (class A, AB, etc.). It determines the power output, amount of distortion, headroom (the size of input signal that can be applied before the output signal clips), efficiency of the stage (the amount of output signal power vs. The bias point determines several things about a tube amplification stage. This DC bias current can be set in a number of ways. A tube is biased by setting the amount of DC current that flows in the tube when there is no signal present at the tube's grid with respect to it's cathode. Tubes have to be properly biased in order to function as amplification stages. All tubes must be biased, both preamp and output tubes, but it is not always clear whether or not the bias needs to be adjusted when changing tubes. When people talk about "biasing" an amplifier, they are referring to setting the "idle", or quiescent, current in the power output tubes.
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