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Thermionic Tubes

Thermionic tubes are very tolerant if their specified operating conditions are temporarily exceeded; unlike most semiconductors which tend to suffer from ‘catastrophic’ failure (defined as a failure mode which causes instantaneous and irreversible damage).Unlike semiconductors, tubes do however, ‘wear out’, which means that they have a finite operating life. Operating them under unfavourable conditions or beyond the manufacturers working specification, usually results in a reduced lifespan, although this may not be audibly apparent in their operation. In audio applications, the majority of seriously damaging problems lie in the power output stages.

Introduction to Tube Amplifier Operation

The majority of amplifiers use at least two output tubes in a circuit configuration known as ‘Push-Pull’. Basically, this means that the audio signal is split into two halves by preceding circuits. Each of the two ‘Split’ signals now drives one output tube. The signals then undergo power amplification by the output tubes and are re-combined by a special audio grade transformer which also drives the loudspeaker.  Please note that, although there are many different types of tube for various applications, the vast majority operate on the same basic principles.

Common Internal Tube Construction:

Cathode - this is the ‘power house’ of the tube as it is coated with a material which, when heated, releases electrons. The entire operation of the tube relies on controlling their movement (hence the term ‘Thermionic’).

Heater - this is an electrically heated filament which provides thermal stimulus to the cathode.

Grid - depending on the type of tube, their may be one or more grids present, each with a specific function, however all audio tubes contain at least one control grid. The control grid is made negatively charged with respect to the cathode in order to restrict or prevent the flow of electrons in the valve.

Anode - this electrode is made positive with respect to the cathode (usually by a few hundred volts), and serves to collect the electrons which are allowed to pass through the grid. Vacuum - audio grade tubes will only operate satisfactorily if there are virtually no air or gas molecules in the envelope. To achieve this, the glass envelope is evacuated to a very high degree during manufacture, which is known as a ‘hard’ vacuum. If small amounts of gas are present, their molecules will ‘ionise’ due to the high voltages present and can give rise to electrical noise or overheating in worse cases. Under these conditions, the valve is said to have ‘gone soft’. Each valve contains a small amount of a reactive metal such as barium (known as the ‘getter’), which is burnt inside the envelope after evacuation in order to ‘mop-up’ the last remaining gas molecules. If the envelope is cracked or broken, air will enter and cause the heater to oxidise and ‘burn out’; it will also disrupt the electron flow and the tube will cease to conduct current.It will be seen from the description of the control grid that if an alternating potential audio signal is applied to the grid, it will modulate the flow of electrons to the anode. Electrical current is defined as electron flow; therefore the signal applied to the grid will produce a modulated current in the anode circuit. Under ideal conditions, this current will be an exact replica of the input signal, but of much greater amplitude. Power valves are optimised to produce large current swings at the anode for relatively small control grid signals. This large output current is used to drive a loudspeaker via a matching transformer.

Biasing

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’)

Tube Matching

Thermionic tubes rely on extremely fine mechanical positioning of the electrodes for their operating characteristics. These are obviously subject to manufacturing tolerances which affect various parameters. Unlike semiconductors, valve operating characteristics are highly dependent on the particular circuit in which they are used. Manufacturers print a large amount of tables, graphs and data on each tube to allow Design Engineers to use and exploit this trait.One of the most important parameters for audio power output tubes used in ‘Class A-B Push-Pull’ which is the ability of both valves to amplify the individual halves of the split signal by exactly the same amount. This has a huge bearing on the quality of the final re-constructed signal after amplification. Equally important is the current flow in each tube should be precisely the same. Any imbalance causes a net D.C. current to flow in the output transformer primary windings giving rise to distortion of the output signal to the loudspeaker and a reduction in audio output power due to transformer core saturation. Manufacturers of both High Fidelity and Guitar valve amplifiers need to carefully ‘match’ pairs or multiple pairs of tubes to achieve good results. It will be appreciated that this is both a time consuming and costly exercise.

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