Valve amplifier biasing

What is valve amplifier biasing?

Amplifier repairs on Marshall 9200 power amplifierChecking/adjustment of the bias setting (Valve grid control voltage) is necessary when installing new power valves on many valve amplifiers. This small crucial adjustment has a direct influence on the valves gain structure, function & lifespan.

The bias setting controls the idle current to the valve grid, ensuring the valves run smoothly within their intended operating conditions (applied DC voltage & current). Not all valve amps require bias adjustment so it’s always advisable to refer to the amplifier manufacturers guidelines when changing valves.

WARNING! Please do not attempt any of the procedures listed below unless you are qualified to do so! We have various safety measures in place when handling this type of equipment. Certain components on valve circuits can store around 450 VDC for several hours after disconnection from the mains supply!

Valve amplifier biasing methods:

  • Fixed bias with no adjustment resistor:
    Amplifier repairs on valve amplifier checking bias on a Marshall JCM800 headAmplifiers with this design use a fixed resistor to control the bias voltage rather than an adjustment potentiometer. This method of biasing relies on the output valves being rated & set for a specific operating range (plate current & voltage) during manufacture. Mesa Boogie use this system, hence their own branded (colour coded) tube selection. Generally speaking whilst replacing valves on this type of amp is fairly easy, it is still advisable to check with the amplifier manufacturer to ensure correct valve specifications.
  • Adjustable (fixed) bias using potentiometer:
    The bias setting pot is normally easy to access & does not take long to set (if you know what you’re doing). The variable bias resistor (normally a small pot mounted inside or to rear of amp chassis) is adjusted with the power on & load (speaker) connected.
    We recommend the installation of all amplifier valves & biasing be carried out by a qualified technician, mainly for safety reasons. High voltages are present inside valve amps even after disconnection from the mains supply. The replacement of amplifier valves should ideally be combined with a service. See amplifier servicing.
  • Self (automatic) biasing:
    These amplifiers require no bias adjustment & can handle a wide variation of valve plate currents as the circuit design is self adjusting. The auto bias design relies on a large resistor placed between the valves cathode & earth, along with additional large value shunt capacitors for impedance reduction. This type of amplifier is often referred to as ‘plug n play’ due to the ease of replacing output valves.