. The thermionic vacuum tube and its applications . 10,000 placed over the grid and filament. The grid is in the form of ahelix enclosing the filament which is Hkewise helical and consistsof tungsten wire. In one t>T3e of tube of this construction the normal operatingfilament current is amperes and the filament voltage amphfication produced by the tube is equivalent to about20 miles of standard cable. Tubes that are used as amplifiers and radio detectors and in- 242 THERMIONIC VACUUM TUBE tended to deliver power directly to a telephone receiver need notbe capable of handli
. The thermionic vacuum tube and its applications . 10,000 placed over the grid and filament. The grid is in the form of ahelix enclosing the filament which is Hkewise helical and consistsof tungsten wire. In one t>T3e of tube of this construction the normal operatingfilament current is amperes and the filament voltage amphfication produced by the tube is equivalent to about20 miles of standard cable. Tubes that are used as amplifiers and radio detectors and in- 242 THERMIONIC VACUUM TUBE tended to deliver power directly to a telephone receiver need notbe capable of handling more power than is necessary to operatethe receiver and in most cases this corresponds to a very smallplate current. This makes it unnecessary and undesirable to. 20 30 40 Anode Volts Fig. 129. dissipate more than a fraction of a watt in heating the desirabihty of reducing the power necessary for heating thefilament is apparent. A tube in which this has been done is shownin Fig. 131. The filament is of the oxide-coated type, 2X10~^inch in diameter and is designed to operate on one dry cell, the THE THERMIONIC AMPLIFIER 243 operating voltage ranging from to volts with a normalfilament current of about ampere. The normal power dis-sipated in the filament is therefore about one-quarter of a wattThis tube is a good detector and gives about 20 miles amplificationwith a plate voltage of about 30 volts. It was designed not onlyfor the purpose of reducing asmuch as possible the power con-sumption in the filament, butalso to facilitate the construc-tion. The four stout wireswhich connect to the electrodes,are moulded in a glass bead 11,or other insulating forms a unit on which therest of the structure is
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