The thermionic valve, as the name suggests, is a device that controls the flow of something. In this case its a stream of electrons in a vacuum. 

A tungsten filament, similar to that found in an electric light bulb is heated by an electric current, in a glass envelope containing a vacuum. This produces a cloud of electrons around it, which are negatively charged. If a positively charged metal plate is positioned near the filament, it will attract electrons from the cloud and a current will flow in the circuit. If the plate is charged negatively the electrons will be repelled and no current will flow. This is the basis of the valve rectifier. This type of valve is called a diode, the filament is called the cathode and the plate is called the anode. The diode valve was developed by Sir John Ambrose Fleming in 1904, while working for the Marconi Company.

A diode with a filament as the cathode is called a directly heated valve, but most modern valves are indirectly heated. An indirectly heated cathode consists of a nickel tube that is coated with an oxide consisting of barium, strontium and calcium. Inside the tube is the heater which consists of an insulated tungsten filament. This type of cathode has two advantages. It produces more electrons and can be operated from an AC or DC voltage.

The diode is used as a rectifier to convert an AC voltage to DC. In the circuit opposite, the diode only conducts when the anode is positive with respect to the cathode, and so only conducts on each positive half cycle of the AC input. The voltage at the cathode consists of just the positive half cycles. The capacitor charges to the peak of these half cycles to produce a smooth DC voltage.

The Triode

The triode or three electrode valve, is a diode with the addition of a spiral wire in between the cathode and anode. This is called the control grid, and when a negative voltage is applied to it, the flow of electrons between the anode and cathode is reduced. As the negative voltage on the control grid is increased, the electron flow will be reduced even more, so allowing the current flow through the valve to be controlled.

If the supply voltage to the anode is fairly high and is connected via a large resistor, a large voltage will be developed across the resistor, due to the current flowing through the valve. If a small negative voltage is applied to the control grid, it will reduce the current flowing through the valve. This will increase the anode voltage, and a smaller voltage will be developed across the resistor, which is called the anode load resistor. The small control grid voltage will have made a large change to the anode voltage, so the valve has amplified this voltage.

Only the central part of the characteristic curve is flat, and so only this region will produce a linear change in the output voltage, from a linear change at the input. To operate the valve in this area, a negative voltage is applied to the grid, which is called the grid bias voltage.

In the triode amplifier, grid bias is applied by connecting the grid to earth via R1, and applying a positive voltage to the cathode. So making the grid negative with respect to the cathode. The current flowing between anode and cathode produces a voltage across R2, which is the bias voltage. C2 charges to this voltage, and ensures that the voltage does not change with the signal current. The input signal is connected to the grid via C1 and will vary the electron flow through the valve. The current flowing through the valve will produce a voltage across R3. This voltage will vary as the current in the valve varies, and the variation will be much greater than the variation of the input voltage. So the circuit has amplified the input signal.

Other types of valve include the tetrode which has a second grid inserted in between the control grid and anode. The second grid is called the screen grid and improves the high frequency operation of the valve, and increases the amount of amplification. It has a positive voltage applied to it. 

Another common type is the pentode which has yet another grid inserted in between the screen grid and the anode. This grid called the suppressor grid is used to prevent secondary emission. This occurs when electrons hit the anode with sufficient velocity to knock electrons from its surface. These would be attracted to the positive voltage on the screen grid, but the suppressor grid which is connected to the cathode, prevents this happening.

Variable-mu Valve

If the wires in the control grid are spaced closer together at the centre, the grid bias voltage will vary the valve's gain. This is often used in radio receivers to allow them to handle very strong and very weak signals.

T.R.F. Receivers

Many of the early valve receivers were of the tuned radio frequency type or TRF for short. These consisted of a number of valve amplifiers, which were each tuned to the incoming radio frequency, a detector and audio amplifier. Often the tuned stages, as in the circuit diagram below, were individually tuned, so there were several adjustments to make when tuning the radio to a station.

A simple tuned radio frequency receiver.

Circuit Description

The aerial is connected into the receiver via one of the aerial sockets; A1, A2 or A3. These are provided to give the best results from a variety of aerials and signal strengths. This is a Medium and Long Wave receiver, and the waveband is selected by S1, S2 and S3. The aerial tuned circuit consists of tuning capacitor VC1, and either medium wave coil L2, or Long wave coil L4. V1 is a tetrode amplifier that amplifies the tuned signal from the aerial. The output from the amplifier is coupled to V2; the detector, and audio amplifer, by L5, L6 and tuned circuit VC2, L7 and L8. L9 feeds the signal from the output of V2, back to its input and so the circuit can turn into an oscillator. The amount of feedback is controlled by VC3 which is the reaction control. In use the reaction control is adjusted so that the receiver can operate just below the point where oscillation starts. This gives the maximum gain and selectivity. On a strong signal the control can be set to give the minimum gain. In fact in this receiver the reaction control provides the only control of volume.

In an A.M. radio signal the sound varies or amplitude modulates the radio carrier wave, so that its amplitude varies with the sound. To detect a signal, the carrier is rectified, and filtered out to leave the original audio.

The leaky grid detector works as follows. The triode grid and cathode operate as a diode which rectifies the signal, so removing the negative going half cycles of the carrier. Coupling capacitor C2 charges negatively and suitably bias's the control grid. The remaining carrier is filtered out by C3 which is connected from the anode to earth.

H.F.C. (high frequency choke) L10 is used as a load for the reaction part of the circuit, and the audio signal is coupled to V3 via transformer L11 and L12. V3 is the main audio amplifier, which drives the loudspeaker via transformer L13 and L14. Grid bias is provided by the grid bias battery, the heaters are powered by the LT battery, and the radio is powered by the HT battery, which provides two voltages at HT +1 and HT +2. S4 is the On-Off switch. The circuit is typical of an early TRF receiver from the mid 1920's, to the early 1930's.

The Superhet

A superhet receiver is more complex than a TRF receiver but has many advantages. Its tuning is much simpler and its much more selective, so it’s a simple matter to tune in to a station and not hear adjacent stations in the background. Superhets are more sensitive than their TRF counterparts, as its an easier matter to amplify the radio signal before detection.

A superhet uses a mixer, or frequency changer as its also called, to convert any aerial signal to a fixed frequency. This is done by mixing the aerial signal with the signal from an oscillator. When the two are combined in the mixer, it produces the original signals plus their sum and difference frequencies. When a station is tuned in, the oscillator frequency is also adjusted so that the difference in frequency between it and the tuned frequency is always the same. This gives a constant difference frequency which can be easily amplified, and by using suitable tuned circuits its bandwidth can be precisely set so that the receiver is very selective. The difference frequency is called the intermediate frequency, or I.F. frequency for short. After amplification the I.F. goes to the detector to obtain the original speech or music. The detector also provides a negative voltage that is used to control the overall gain of the receiver. It is called the 'automatic gain control' or AGC for short. The stronger the signal, the greater the voltage will be, and so the gain will be reduced accordingly. This is done by using a variable-mu valve in the mixer. The audio output from the detector goes to the volume control, and to the loudspeaker via the audio amplifier. This type of receiver first appeared in the mid 1920’s and is what we still use today.

Circuit Diagram

The circuit diagram of a standard wartime austerity receiver.

The signal from the aerial is tuned by L1 and C1. V1a is the mixer, its output is tuned to the I.F. frequency by L2 and C3. V1b is the oscillator, which is tuned by L3 and C2. V2 is the I.F. amplifier which produces most of the receiver's gain. Its output is tuned by L4 and the two associated capacitors. D1 is the detector diode, and Rf and Cf forms the filter that removes the radio frequency carrier from the detected audio. D1 also provides the negative voltage for the receiver's AGC  which goes to V1a and V2. The audio signal from the detector goes to the volume control (Vol) and audio amplifier V3. The loudspeaker (LS) is driven by the audio amplifier via the output transformer T2. The mains power supply consists of mains transformer T1 and rectifier V4. The transformer has a secondary winding to supply an A.C. voltage to the heaters of the valves, which are indirectly heated, and a separate heater winding for the rectifier, V4. The third winding provides the A.C. voltage for the rectifier which supplies the receiver HT voltage. Grid bias for V1a and V2 is provided by the voltage that is developed across resistor Rgb, through which the HT current flows.

The descriptions in this section have been limited by space. It would require a lot of basic theory to be explained before anyone who is totally unfamiliar with electronics, could thoroughly understand the operation of the circuits. The information is provided here as a background to the products that are described, as its impossible to avoid some technical words in the descriptions. It also helps to put to developments in the industry into context.

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