Read the text again and complete the sentences.
1. In a vacuum-tube triode there are…
2. When heated…
3. Electrons flow…
4. The grid is placed…
5. … the flow of electrons.
6. The closer the grid is to the cathode …
7. … this results in a large plate current.
8. If a large negative voltage is applied to the grid …
9. … it can be used for amplification.
Match the terms in Table A with their definitions in Table B
Table A
1. cathode
2. triode
3. amplification
4. electrode
5. plate
Table B
a. A vacuum tube having three electrodes or a semiconductor rectifier having three connections.
b. The negatively charged electrode by which electrons enter an electrical device. The opposite of anode.
c. The positively charged electrode by which the electrons leave a device. The opposite of cathode.
d. An increase in strength or intensity, especially of sound.
e. A conductor through which electricity enters or leaves an object, substance, or region.
Speaking
Work in pairs. Draw a circuit of a triode and ask your partner some questions based on it. Make sure all the questions and answers are in Present Continuous.
For example:
Student A - In what direction are the electrons shown in the scheme moving?
Student B – They are moving towards the plate.
17. Summarize the text “Triodes” in 150 words using the following plan:
1. The structure ofa vacuum-tube triode.
2. The principles of its work.
3. The function of a grid.
4. The influence of the distance between the cathode and the grid on the electron flow.
5. The influence of the voltage on the current flow.
6. The grid application.
18. Compare a vacuum-tube triode with a diode using appropriate degrees of comparison and the following words of comparison and contrast: both, like, whereas, unlike, but, however.
For example: 1. When heated in both tubes cathodes emit electrons.
2. A triode can be used for amplification unlike a diode which is used as …
Act as an interpreter. Translate the description of the operation of a semiconductor triode given by your group mates from Russian into English.
20. Divide into 2 groups. Group 1 translates Extract A and group 2 – extract B of the text “Heat Transfer and Appearance of Tubes” with a dictionary in writing.
Highlighted words are international. Try to guess their meaning without a dictionary.
Extract A
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Any types of vacuum tubes can be recognized from their appearance. A considerable amount of heat is produced when tubes operate. In most circuits the tube is about 30-60% efficient depending on the class of operation (classes A, B, or C), which means that 40-70 % of input power is lost as heat. The requirements for heat removal significantly change the appearance of high-power vacuum tubes.
Most tubes contain two sources of heat when operating. The first one is the filament or heater. Some types contain a directly heated cathode. This is a filament similar to an incandescent electric lamp and some types glow brightly like a lamp, but most glow dimly. (The "bright emitter" type possesses a tungsten filament alloyed with 1-3 % thorium which reduces the work function of the metal, giving it the ability to emit sufficient number of electrons at about 2000 degrees Celsius. The "dull emitter" types also possess a tungsten filament but it is coated by a mixture of calcium, strontium and barium oxides, which emit electrons easily at much lower temperatures due to a monolayer of mixed alkali earth metals coating the tungsten when the cathode is heated to about 800-1000 degrees Celsius.)
The second form of cathode is the indirectly heated form which usually consists of a nickel tube, coated on the outside with the same strontium, calcium, barium oxide mix used in the "dull emitter" directly heated types, and fitted with a tungsten filament inside the tube to heat it. This tungsten filament is usually uncoiled and coated by a layer of alumina, (aluminium oxide), to insulate it from the nickel tube of the actual cathode. This form of construction allows for a much greater electron emitting area and, because the heater is insulated from the cathode, the cathode can be positioned in a circuit at up to 150 volts more positive than the heater or 50 volts more negative than the heater for most common types. It also allows all the heaters to be simply wired in series or paralleled rather than some requiring special isolated power supplies such as specially insulated windings on power transformers or separate batteries.
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Extract B.
For small-signal tubes such as used in radio receivers, heaters are rated from 50 mW to 5 watts, (directly heated), and about 500 mW to 8 watts for indirectly heated types. Once filament/heater power is included in total power consumption, small tubes have very poor efficiencies. A 6BM8/ECL82 audio stage consumes a total power of some 15 watts for 3.5 watts of useful audio power, giving an efficiency of around 23%. Some signal amplifiers, particularly high-frequency amplifiers such as the 6BA6, consume some 5.9 watts of power in normal operation and deliver only 1.1 watts of power at the plate.
The second source of heat is generated at the anode, when electrons, accelerated by the voltage applied to the anode, strike the anode and impart a considerable fraction of their energy to it, raising its temperature. In tubes used in power amplifying or transmitting circuits, this source of heat will exceed the power dissipated in the cathode heater. (The plates or anodes of 6L6 devices used in guitar amplifiers can sometimes be seen to reach red heat if the bias is set too high, they should not emit any visible radiation when driven at maximum ratings.) No tubes in domestic, music, or studio equipment should operate with glowing anodes.
This heat usually escapes the device by (black body) radiation from the anode/plate as infra red light. Some is conducted away through the connecting wires going to the base but none is convected in most types of tube because of the vacuum and the absence of any gas inside the bulb to convect.
For devices required to radiate more than 500 mW or so, usually indirectly heated cathode types, the anode or plate is often treated to make its surface less shiny, (see black body radiator), and to make it darker, either gray or black. This helps it radiate the generated heat and maintain the anode or plate at a temperature significantly lower than the cathode, a requirement for proper operation.
Other internal elements of high-power tubes, such as control grids and screen grids, may also dissipate heat if carrying large currents. Limits to grid dissipation are listed for such devices to prevent distortion and failure of the grids.
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