A very important feature of any thermionic tube circuit is known as ‘biasing’. It has already been mentioned that a negative voltage applied to the control grid of the valve will restrict or prevent the current flowing between anode and cathode. This is also known as a ‘bias’ signal, as it is used to control the amount of current which flows under ‘no-audio-signal’ conditions (quiescent conditions).It is common practice to operate tube amplifiers in a state known as ‘Class A-B’. This means that when no signal is present, a relatively small amount of D.C. current is allowed to flow in each tube to reduce distortion of the amplified signal to a low level. The amount of quiescent current allowed to flow is obviously very important to maintain optimum operating conditions. Too much current will cause harmonic distortion, increase heat dissipation in the anode, reduce audio output power and reduce the operational life of the tube. Too little current will cause severe harmonic distortion and, under extreme conditions, could cause a condition known as ‘cathode poisoning’, which also reduces the effective operation life of the tube.

Conventional Methods of Biasing Fall Mainly into Two Categories

- (1) Cathode (or ‘automatic’) Bias in an attempt to automatically control the quiescent current flow in the tube, a high power resistor is connected in the cathode circuit to signal ground.  The grid is connected via a high tube of resistance (usually 500k Ohms or greater) to earth or signal ground.  The principle of operation is that when current starts to flow through the tube (and the cathode resistor), a volt drop, proportional to the current will occur across the resistor.This will effectively make the cathode more positive than the signal ground (control grid potential). In effect, this is exactly the same as making the control grid more negative than the cathode. The grid will therefore begin to restrict the flow of current until equilibrium is obtained. The value of this current will be proportional to the value of cathode resistance. Any tendency for the current to increase or decrease due to aging or external conditions will be countered by the resultant change in control grid voltage.

Disadvantages of Cathode Biasing

Cathode biasing has been used in audio amplifier circuits for many years, and although fairly effective, it also has several serious drawbacks notably:

• In high power amplifiers, the value of the cathode resistor must be made relatively large, which results in them needing to dissipate several Watts of heat (adding to the general heat dissipation within the amplifier chassis).
• Reduced operating efficiency.
• A significant amount of audio signal power is ‘lost’ in the resistor, thereby reducing the audio output power for any given input power.
• It is necessary to bypass the cathode resistor with a capacitor in order to preserve the audio amplifying properties of the tube. This capacitor is in the audio signal path and therefore has a restricting effect on certain frequencies.

(2) Fixed Bias

Higher power amplifiers tend to use fixed bias due to the disadvantages of cathode biasing already mentioned. It has a tendency to be used in guitar amplifiers where distortion due to maladjustment comes second to higher output power. Fixed bias incorporates an external negative supply to the grid, which is controlled by a manually adjusted trimmer resistor. It is common practice to supply more than one of the tubes from the same adjustment.

Disadvantages of Fixed Biasing

Setting the bias conditions requires technical knowledge and skill; therefore this operation should be performed by an audio technician.

• Once set any tendency for the current to increase or decrease due to tube aging or external conditions cannot be catered for, therefore conditions can only be optimum immediately after adjustment.
• Frequent adjustments are necessary throughout the life of the valves to preserve optimum performance.
• If a single bias control is used, at best one, or possibly none of the output valves will operate under optimum conditions.
• Maladjustment of the bias control or slight leakage in grid coupling capacitors could result in severe (very costly) damage to the power supply, power output tubes or output transformer.

Heating Effect

It is a misnomer to assume that because tubes rely on thermionic emission for their normal operation, that they cannot overheat. Power output tubes are especially vulnerable to the effects of overheating due to their high anode dissipation under normal driven conditions. Any extra heat generated due to the effects of faulty coupling components, incorrect biasing or poor design can seriously shorten their operational life. Common conditions which cause overheating are:

• Faulty (high leakage) grid coupling capacitors – if cathode bias is used this can be offset to some degree by the automatic compensating effect of the cathode resistor, although overheating and destruction of the resistors may occur before audio distortion is noticed by the listener. In a fixed bias system, it is almost certain that severe damage will result.
• Failure of the cathode resistor by-pass capacitor in cathode bias systems.
• Bias voltage maladjustment in ‘fixed bias’ systems.
• Gas in the valve envelope (‘soft valve’)