Tung-Sol.com:: FAQs WWhat is Amplifier Bias?

FAQs

Question:
What is Amplifier Bias?
Answer:
Setting the bias of an amplifier involves adjusting the voltage to the control grids of the power tubes so the tubes draw the correct amount of operating current. This is much like setting the idle on a car when doing a tune up. On amplifiers that have adjustable bias, it is a potentiometer that is provided for the technician to set the bias voltage to the control grids while monitoring the plate or cathode current of the output tubes. If the operating current is set too low, the amplifier will sound weak and have a gritty non-musical sound caused by crossover distortion. If the operating current is set too high, the tube will draw excessive current, which will shorten the life of the tubes. Setting the bias correctly will give the best sound and longest tube life. Due to the dangerous high voltages encountered in tube amplifiers in addition to the knowledge and equipment required to set the proper amount of operating current, this adjustment is best left to be done by an experienced technician. Many technicians will also perform minor preventive maintenance, such as cleaning the controls and soldering loose connections, while they have the amplifier open to adjust the bias. If you use matched power tubes, make note of the Ip (plate current) numbers on the tube boxes. Once the bias is set with this set of tubes, you can order replacement sets with the same or close Ip (plate current) numbers and install them without needing to reset the bias.

Some amplifiers provide a bias voltage to the grids of the power tubes, but do not have a way to adjust the voltage. Most Mesa Boogie amplifiers do not have a bias adjustment control. Many of the ‘70s master volume Fender amplifiers provided a balance control instead of a bias voltage adjustment. For amplifiers that do not have a way to adjust the bias voltage, it is best to use matched tubes that have middle Ip (plate current) numbers. This will ensure that the amplifier is operating at or close to the optimum desired plate current.

Cathode-biased amplifiers use a resistor and bypass capacitor in the cathode circuit of the output tubes to provide a positive voltage to the cathode. This sets the grid voltage negative in relation to the cathode and establishes the proper operating current. Fender Champ, Fender Tweed Deluxe, and VOX AC-30 are common examples of cathode-biased amplifiers. There is no bias adjustment on these amplifiers.

Preamp tubes are cathode biased, so there is no need to check or set bias on any preamp tube.

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