To those of us who have been bought up with transistors, valves can seem unnecessarily complex. On this page, I will attempt to explain the workings of the valve in a clear simple manner - without the atomic theory and the maths!
A Brief History Lesson
In 1883, Thomas Edison was experimenting with electric lamps. In his early experiments, the glass bulb was becoming dull, and he wondered if this was due to particles being given off by the filament. He fitted a metal plate inside the bulb to attract these particles, and found that if the plate was at a positive potential a current would flow from the filament.
Later Professor Flemming found that current only flowed when the plate was positive, and that the arrangement could be used to rectify an alternating voltage. He patented this in 1904.
Lee de Frost then discovered that, by placing a wire between the filament and plate, the current could be controlled. Thus he invented the Triode (or Audion as he called it) - the first ever electrical amplifying device.
When a metal is heated to a sufficiently high temperature in a vacuum, it will give off electrons. These will be attracted to any electrode that is at a more positive potential.
Most metals will melt by the time they are hot enough to emit a significant amount of electrons. Tungsten is an exception, which gives good emission at 2300 to 2500 degrees Centigrade, and melts at 3380 degrees Centigrade. This would glow almost as bright as an electric lamp, which was a characteristic of early Bright Emitter valves. In later valves, the tungsten was coated with an oxide (such as barium or strontium) which gives good emission at around 700 degrees Centigrade.
In most valves, the emitting conductor is a separate component to the heating filament. The emitting conductor is known as the cathode, and is normally in the form of a thin tube. The heater passes inside the cathode and is electrically insulated from it. This is known as an indirectly heated cathode. Some valves have directly heated cathodes, where the heater and cathode are the same component. These were frequently used in battery sets.
Electron Flow vs Conventional Current Flow
We are now used to thinking of current flowing from positive to negative. However current is actually a flow of electrons in the opposite direction. This anomaly is the result of an incorrect assumption by early scientists, which has become established - hence we have the separate terms Electron Flow and Conventional Current Flow.
To avoid confusion (hopefully!), think in terms of electron flow when considering the actual workings of the valve, and current flow when thinking about the circuit.
The electron collecting plate is known as the anode. It normally consists of a cylinder of metal around the cathode, a few millimetres away.
When the anode is at a positive potential relative to the cathode, current will flow. This is useful for detection and rectification, but is obviously incapable of amplification.
A rectifier valve has larger, more substantial electrodes than a detector diode, to cope with the much greater currents involved. This diagram shows a rectifier valve circuit with an AC input and a half-wave rectified DC output.
A smoothing capacitor would normally be connected across the load (RL) to give a relatively steady DC supply. The load would normally be the remainder of the circuit rather than a single resistor.
The valve electrodes are indicated by the normal abbreviations - a for anode, k for cathode and h for the heater connections. A heater supply is not shown in the diagram for simplicity.
By adding a spiral of wire between the cathode and the anode, it is possible to control the current flowing between them. This spiral of wire is known as the control grid.
Referring to this diagram, if a varying signal is applied to the control grid (g1) via C1, the anode current will vary in sympathy. By placing a resistor (Ra) between the anode and the positive supply, the varying current will be converted to a varying voltage on the anode.
In normal use the control grid will not be at a positive potential relative to the cathode, otherwise it will act as another anode and draw current (known as grid current). It is normally biased a few volts negative (although some triodes are designed to be biased at 0V). In very early radio sets, a separate grid bias battery was used, often having several tappings to give different bias levels - but this was quickly superseded.
Usually cathode biasing will be used. Instead of connecting the cathode directly to ground (0V), it is connected via a low value resistor (Rk). This will drop a few volts, so the cathode will be a few volts positive. The control grid is at high impedance and draws virtually no current. It is normally connected to ground via a high resistance (Rg), and the signal is coupled via a capacitor (C1).
If Ck is omitted, the voltage at the cathode will vary with the anode current. This causes negative feedback which gives a reduction in gain (and also reduces distortion). Ck is fitted to obtain the maximum gain from the stage, and has a low impedance over the signal frequency range.
Triode valves are mainly used for low level audio amplification. Their use is limited at radio frequencies because of the capacitance between the control grid and the anode. Although this is only a few pF, the "effective capacitance" is approximately equal to this value multiplied by the stage gain. This effective capacitance becomes the input capacitance of the stage, and has a drastic shunting and detuning effect on a radio frequency signal.
The tetrode was a development of the triode, designed to overcome this problem. A second grid is placed between the control grid and the anode. It is known as the screen grid, and acts as an electrostatic screen, the purpose being to minimise the capacitance between the control grid and anode.
For this to work it must be connected to ground at signal frequencies. If it were connected directly to 0V it would act as another control grid and greatly reduce the anode current. It is therefore often connected to the HT rail via a resistor to drop some voltage, and decoupled to 0V with a suitable capacitor.
The tetrode solves the capacitance problem allowing operation at high frequencies, and also gives greater gain. However, it introduces another problem - distortion. This is caused by secondary emission, which is too involved to describe in this brief article. Consequently the tetrode is seldom used, but it is included here because it is an important stage in the development of a better solution.
As its name implies, the pentode has five electrons. Four of them are the same as those in the tetrode, namely the cathode, control grid, screen grid and anode.
To suppress the secondary emission a further grid, known as the suppresser grid, is added. This is normally connected to the cathode, sometimes internally within the valve envelope, otherwise a separate connection is provided.
The result is a valve that retains the advantages of the tetrode - high gain and operation at high frequencies - without the distortion. Pentodes are commonly encountered in RF and IF amplifier stages, and in amplifier power output stages.
This diagram shows a basic pentode amplifier
stage. This is fairly similar to the triode circuit discussed previously, with
the addition of the connections to the screen and suppresser grids (g2 and g3).
It is often necessary to be able to control the amplification (gain) of a valve either manually or automatically. This is commonly required in the AGC (Automatic Gain Control) circuits in radio receivers.
To achieve this the spacing of the wires that make up the control grid are varied, being closer together at the centre and wider apart at the ends. By varying the negative voltage on this grid, the gain can be adjusted.
Pentode Power Amplification
This diagram shows a typical Class-A pentode output stage. The anode load resistor is replaced with the primary of the output transformer (T1), which drives the loudspeaker (LS1). The purpose of the transformer is to convert the relatively high anode impedance of the valve to the low impedance of the speaker.
Since the output transformer is inductive, its impedance varies with frequency giving an uneven frequency response. A capacitor (Ca) is often connected in parallel with the transformer primary, which corrects this to a great extent (this is sometimes referred to as tone correction). In some cases more than one capacitor is used, together with series resistors to give correction that is more accurate.
The screen grid (g2) is shown connected to the HT supply after a decoupling resistor (Rd). This is a common arrangement in valve radio receivers.
A resistor is placed in series with the control grid (g1). This works in conjunction with the input capacitance of the valve to attenuate the high frequencies (above the audio range) to ensure stability.
Many hi-fi amplifiers and some more expensive valve receivers use a Class-B push-pull output stage. This is an involved subject in its own right and will not be covered in this brief article. A higher quality output transformer is normally used in conjunction with negative feedback, which makes impedance correction capacitors (such as Ca) unnecessary.
Other Valve Types
A number of special-purpose valves have been produced with a greater number of electrodes. For example, Hexodes, heptodes and octodes (containing six, seven and eight electrodes respectively) are sometimes used in mixer-oscillator stages. The operation of these valves is rather complex and I will not attempt to describe them here!
Often more than one valve section is contained in a single glass envelope. These sections normally share the same heater connections and are sometimes interconnected.
For example, the mixer-oscillator valve in radio receivers often consists of a hexode (or similar) and triode sections in the same envelope. The triode is used as the oscillator section and the hexode acts as the mixer and amplifier. The two sections may be connected internally within the valve, or externally.
Those requiring a more detailed discussion of valve operation are advised to refer to the book by Chas E. Miller entitled Valve Radio & Audio Repair Handbook.