Thermionic valves pdf

Such signals, therefore, need to be amplified before they are fed into the indicating device. Amplification, which is the most common of all electronic operations, cannot be performed by using networks of resistors, capacitors and inductors. It is necessary to energize such networks by using some active device. The two major groups of active devices are a thermionic valves and b semiconductor devices.

This chapter deals with thermionic valves. James, G. Polgreen and G. Methods of measuring very small values of current and power are described. Although the uniformity existing between the characteristics of valves of the same type is not quite perfect, the modern valve gives a very stable performance during its life, more particularly under the operating conditions imposed in measuring devices.

Of all valve-measuring devices, the valve voltmeter probably is the most widely used instrument, especially for high-frequency measurements. It is sensitive and it can be designed to absorb negligible power from the source. In normal instrument practice, current is measured either by its magnetic or by its heating effect, and voltage is measured by measuring the current through a known resistance. In the case of instruments employing valves, the converse procedure is adopted.

Voltage is measured by its controlling action on the electron current in the valve, while current may be determined by measuring the voltage produced across a known resistance. Edison's work preceded the discovery of electrons by Joseph J. Thomson, who proposed their theoretical existence and in demonstrated that they could be attracted or repelled by an electric field. The concept of a flow of charged electrons could not therefore have been easily envisaged by Edison in But as Sir John Ambrose Fleming once said, "The little things of today may develop into the great things of tomorrow".

In , Fleming became a consultant to the Marconi Company, designing the equipment which Guglielmo Marconi used to make the first transatlantic radio transmission in It was because of his involvement with Marconi radio, that Fleming realised that the Edison Effect might be capable of providing a better detector of radio waves by rectifying AC radio signals, the pulses of Morse Code or audio applied to the radio signals by amplitude modulation may be detected.

Using one of the Edison effect experimental lamps from the s, he found that it worked, and applied for a patent for the first practical vacuum-tube rectifier on November 16th, although the first solid-state detector was to follow only 6 years later- the cat's whisker crystal device invented by B. Miessner in The effect is easily understood as the negatively charged electrons emitted by the cathode are the only current carriers, and given that the cold anode does not emit electrons, when the anode is negative relative to the cathode, the electrons are repelled from the anode.

P includes an inverse dependence on the square of the cathode-anode spacing d, and is proportional to cathode area A, i. At higher anode voltages, the current eventually levels out as there is a limit to the number of electrons available from the cathode, and when this limit is reached, the valve is said to be in saturation.

At lower currents, electrons tend to collect in a "space charge" around the cathode, a cloud of electrons whose mutual repulsion electric charges of the same sign repel suppresses further emission from the cathode unless a positively charged electrode starts to remove them. The filament cathode, as in the Edison lamp, emits electrons from a directly heated wire surface, as soon as the thermal energy added to the electrons exceeds an energy level characteristic of the material called the work function.

A tungsten filament has a work function of 4. A good example of a valve with a directly heated filament cathode is the B triode, manufactured since the s and still in use today we will discuss the triode in more detail later. The following illustrations show the structure of a directly heated thoriated filament valve of the ss similar to a Mullard ORA , compared with a picture of a "modern" indirectly heated cathode valve for comparison. Illustration of the construction of a directly heated thoriated filament valve of the ss.

Picture of a PCF, which includes a triode and a pentode designed for television line oscillator applications, a model introduced no later than , and in common use in the ss.

The post-WW2 style B9A "Noval" base from , when introduced with the 12AU7, 12AX7 and 12AT7 is made with Kovar alloy pins, matched for thermal expansion to the glass in which they are sealed without the need for a separate holder, and the glass envelope is 20 mm in diameter and 46 mm tall excluding pins. Although this illustration describes "Amplified impulses passing to loud-speaker", in practice a transformer is required between the valve anode circuit and a loudspeaker, as the valve anode circuit is high impedance high voltage, low current , whereas a loudspeaker is low impedance, requiring high current at low voltage.

Most modern valves use an indirectly heated cathode, which consists of a metal cylinder containing a wire heater element that is separated from the cathode by a high-temperature insulation material such as aluminium oxide, so that the heater can be independently powered for example from a low-voltage AC winding on the mains transformer while the cathode is connected to its own DC bias potential e.

Their major disadvantage is that they are more sensitive to "cathode poisoning", which is a loss of electron emission caused by absorbing impurity ions present inside the valve. Secondly, it is sometimes possible for the insulation to break down, causing a "heater-cathode short", both well known causes of failure of domestic valve-based radio and television- which usually required the valve to be replaced.

The cathode cylinder typically of nickel is coated with a special material that does not need to be heated as much as a carbon or tungsten lamp filament in order to emit electrons.

For example, barium and strontium oxide gives a work function of 1. The oxide layer is formed by first coating the nickel tube with a mixture of barium and strontium carbonates, which is then baked to remove binder material and reduce the carbonates to oxides. The effectiveness of an oxide cathode for electron emission is related to an excess of the metal atoms in the crystal structure. Barium is an excellent donor of electrons, which results in the cathode coating acting rather like an n-type semiconductor.

Directly heated cathodes can also be coated with oxide, but it is a must for indirectly heated ones because otherwise it would be hard to maintain sufficiently high temperatures on their outside in order to emit electrons.

The photo shows the valve with heaters energised. The heater voltage is 5V and current 2A, unlike the more common 6. As RCA's competitors introduced 6. With The EY51 is an EHT rectifier valve rated at 17kV peak inverse voltage, average current 3mA, that was commonly used in the high voltage supplies of CRT based televisions and oscilloscopes. Although similar in structure to an indirectly heated cathode, the EY51 cathode is directly connected to the heater wires, the heater usually being driven from an isolated winding on a transformer.

In the American engineer, Lee De Forest was looking for improvements in radio signal detectors, and added a third electrode, the control grid, which was a zig-zag of fine wire placed close to the cathode in a modified Fleming diode.

This was found to give control of the anode voltage using a much lower voltage on the grid, i. However, its performance was poor as he mistakenly believed that some gas was necessary, instead of a good vacuum, for its operation. De Forest applied for a patent for the new device in , which he called an "Audion", which later become the "De Forest valve", and eventually the triode valve as it has three principal electrodes. He was awarded a special Oscar in for pioneering inventions which brought sound to the motion picture.

The conventional method of grid construction that is used in most valves up to the present day is illustrated below. As you can see if you look closely at a typical glass valve from an audio amplifier, the control grid is formed by wrapping a thin wire around two metal support wires of approximately 0.

It must be rigid and heat-resistant, and may be plated to minimise secondary electron emission. The anode commonly takes the form of a cylindrical or oval tube structure, or a box with rounded corners, open at the ends, which surrounds the cathode and grid. It is normally welded or crimped to support wires. The support wires of anode and grid, as well as the cathode cylinder, are normally located in matching holes in insulating washers made of a material such as mica, which is rigid, has good insulating properties, and does not release ions when in a vacuum.

The mica washer is normally made a close fit to the valve envelope to act as a support. Anodes are often surface treated to make them a dull grey or dark colour to radiate heat more effectively, and may be formed with fins on the outside. If the grid voltage is the same as the cathode, i. If we apply a negative voltage to the grid, the electrons being negatively charged will be repelled from the grid wires.

As the voltage is increased more negative , eventually the repelling electric field between the grid wires becomes sufficient to cancel out the attractive field from the anode and to cut off the anode current completely. If we apply a positive voltage to the grid, electrons are attracted to the grid, most passing through to the anode and resulting in increased anode current, but some of them can also now enter the grid wire and flow as a current in the grid circuit.

The ability of the triode valve to amplify signals meant that not only could weak signals be amplified as much as necessary to drive a loudspeaker, but if the output of the valve was fed back to its input, it could be made to generate a signal by means of positive feedback, which was called reaction or regeneration.

This allowed generation of pure sinusoidal RF signals instead of the ill-defined noisy frequency bands generated by spark-gap transmitters, and enabled voice and music radio transmission. Radio receivers could use a regenerative valve circuit to boost the received signal at the same time as demodulating the signal to audio frequency to drive headphones or a loudspeaker.

The self-oscillation when combined with a morse code transmission allowed its reception as a tone generated by the beat frequency between the signal and the regeneration. In this example circuit, which can generate a kHz signal into a 50 ohm load, or be used as a regenerative receiver with headphones inserted in series with the anode power supply , the output of V1 is fed back to the grid circuit via a magnetic coupling between the anode inductance L1 which was sometimes called a "tickler" and the tuned circuit inductance L2.

L2 is resonated at kHz as a tuned circuit with C1. The tuned circuit is coupled to the grid via C2, and R1 is present to provide grid bias. The negative bias is created because the positive peaks of the AC signal at the grid drive the grid positive with respect to the cathode. This causes current to flow which drops a negative voltage across R1, and C2 serves as a reservoir capacitance to maintain the negative DC bias throughout the RF cycle The time constant of R1 and C2 is 22 microseconds, and the duration of a kHz cycle is 5 microseconds.

Such circuits are rarely used any more for radio reception, because of the potential of the regeneration oscillations to radiate from an antenna and cause interference to other users of the radio spectrum. The electrical characteristics of the triode depend on the geometry of the electrodes and properties of the materials used. Data sheets normally list characteristics in the form of tabulated numbers e.

With valve ratings, as the rating systems used have changed over time, care must be taken to understand what kind of rating method is being used. There are a wide variety of possible characteristic plots, but the most commonly useful ones are:. Since the output voltage swing is likely to be large, pentode type valves are employed instead of triodes which are shown in the arrangement of Figure 1, in. The oper'- ation of thearrangement is in substance similar to that already described with reference ure 1.

This application of the invention permits great economy in anode current, as high currents are only drawn when sudden changes of input signal voltages occur necessitating sudden bursts of current to charge or discharge the capacity 5. Although the invention has been described with reference to output stages delivering television signals to low impedance loads, it will be understood that other kinds of signals may be used and the invention is particularly useful when the signals consist largely of sharp pulses.

In an amplifier system of the cathode follower type comprising an electron discharge tube provided with signal input terminals and having a load impedance element in the cathode circuit thereof; the improvement which comprises an electron discharge device having at least a control electrode, cathode and anode, the anode to cathode path of said device and a current source being connected.

In an amplifier system, a pair of electron discharge devices each having at least a cathode, control electrode and output electrode, a closed circuit including the space current paths of said devices in series relation, means establishing the output electrode of one device at a positive potential relative to the cathode of the second device, an input circuit connected between the control electrode of said one device and a point of reference potential, nected between the cathode of said one-device and said point of reference potential, means for to man output circuit con- 7 trode oi the second device to the cathode-oi the first device.

In an amplifier system, a. In combination withla signal amplifier tube having input and output electrodes, a resistive load element in the tube space current path common to the input electrodes and output electrodes of the tube whereby signal voltage developed across the load element is applied between the tube input electrodes, means for applying signals to said input electrodes, a second tube provided with input signal connections to the output electrodes of said amplifier tube, and connections including said load element between the output electrodes of said second tubewhereby said load element is arranged in push-pull relation to the space current paths of said two tubes.

In combination with a signal amplifier tube having input and output electrodes, a resistive load element in the tube space current path common to the input electrodes and output electrodes of the tube whereby signal voltage developed across the load element is applied between the tube input electrodes, means for applying signals to said input electrodes, a second tube provided with input signal connections to the output electrodes of said amplifier tube, and connections including said load element between the output electrodes of said second tube whereby said load element is arranged in push-pull relation to the space current paths of said two tubes, and adirect current connection from the anode of the first tube to the cathode of the second tube including a direct current source and a resistor in.

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