Vacuum Tubes Objectives Part 02

21 . Oxide coated Emitter material usually needs less than 1000 V as plate voltage

22 . In a vacuum tube diode, the rate of collection of electrons at plate is equal to the rate at which electrons are emitted by the cathode, only when the tube is working in temperature limited conditions and space charge limited conditions

23 . Oxide coated cathodes are used for the tubes designed to handle small power

24 . In case of oxide coated electrodes the sleeve is coated with a layer of barium oxide and strontium oxide

25 . The emission efficiency of a cathode is emission current/W

26 . Thoriated tungsten is used as emitter material in tubes upto1000 W

27 . An oxide coated cathode cannot be used if the voltage at the anode is 500V

28 . The ratio of small change in plate current to small change in grid voltage at constant plate voltage is known as transconductance (gm)

29 . A directly heated cathode may be made of1. Tungsten 2. Thoriated tungsten 3. Tungsten coated with strontium oxide 4. Any of the above

30 . In case of indirectly heated tubes the heater filament is usually made of tungsten

31 . In case of indirectly heated cathode , the cathode is usually in the shape of cylinder

32 . The anode material is usually nickel

33 . Fins are often provided on anode

34 . Fin are provided on anode to dissipate heat produced

35 . The surface of anode is usually blacked and roughened to dissipate heat produced at the anode

36 . The reciprocal of the characteristic curve of the diode is the output resistance of the diode

37 . The characteristic of a vacuum diode is similar to that of a semi-conductor diode except for one difference that when a vacuum diode is forward biased ,saturation of plate current occurs at high voltages

38 . In a vacuum diode , the plate current can be varied by1.changing the plate voltage2.changing the heater voltage 3. Any of the above

39 . In triodes , the grid is made in the form of mesh

40 . A control grid is provided in a pentode tube to control the number of electrons moving from cathode to plate