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