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still permitting an interchange of energy between them. 20. When heated or subjected
to strong electric charges, the cathode emits large quantities of electrons. 21. Being a
good insulator, rubber is often used in cables. 22. Having conducted many experi-
ments scientists proved that electricity, like matter, had an atomic character. 23.
When focused, the beam from the gun produces only a small spot on the screen. 25.
The fluorescent screen materials used have varying characteristics according to the
type of work for which the tube is intended. 24. A magnetic substance is any material
which acquires this property when placed near a magnet, or near a conductor carrying
an electric current. 25. The space surrounding a charged body is called an electric
field offer. 26. No matter how small the pieces are into which we cut the magnet,
each piece is a magnet in itself having two poles. 27. Having already made remark-
able progress, radio and television continue to develop and to find wider and wider
application in science, industry and agriculture
15 Texts for supplementary reading
15.1 The power amplifier
The final tube in an audio amplifier which is feeding audio frequencies into a
loudspeaker must be essentially a power amplifier. Its task is to deliver undistorted
power to the loudspeaker, and not to develop any great amount of voltage amplifica-
tion. The task of the previous amplifier stages is to build up the small output voltages
of the detector, 11 that the large voltages necessary to swing the grid of the rower
tube may be obtained. The A-C power in the plate circuits of tubes previous to the
power stage is small; what is required is that each previous stage shall give a maxi-
mum voltage amplification without distortion, and the fact that maximum power may
not be developed in these plate circuits is not important. These tubes work at very
high impedances in which it is not possible to generate much power although 11 is
possible to build up considerable voltages across them.
The a-c plate current of the last tube then must be rather large and this means
that the grid a-c voltages must be large, which in turn means that the Eg— Ip curve of
this tube must have a long and straight line part. The 171 tube, for example, which
can deliver about 700 milliwatts without much distortion must have an r.m.s. (root
mean square) grid voltage applied to it of about 27 volts; there must be a portion of
the Eg — Ip curve which is straight over at least twice this characteristic must be
straight from minus 78,5 to minus 2,5 volts. The next preceding tube has much
smaller voltage swings to handle and so its characteristic need not be straight over
such a long part.
Because of the high cost of power apparatus when power tubes requiring high
voltages are used the final tube in the average radio receiver is a low R, low resis-
tance tube which will deliver considerable power to the loudspeaker without demand-
ing excessive plate voltages. Such tubes require large input a-c voltages and therefore
require greater amplification before them than tubes of higher values of.
43
still permitting an interchange of energy between them. 20. When heated or subjected
to strong electric charges, the cathode emits large quantities of electrons. 21. Being a
good insulator, rubber is often used in cables. 22. Having conducted many experi-
ments scientists proved that electricity, like matter, had an atomic character. 23.
When focused, the beam from the gun produces only a small spot on the screen. 25.
The fluorescent screen materials used have varying characteristics according to the
type of work for which the tube is intended. 24. A magnetic substance is any material
which acquires this property when placed near a magnet, or near a conductor carrying
an electric current. 25. The space surrounding a charged body is called an electric
field offer. 26. No matter how small the pieces are into which we cut the magnet,
each piece is a magnet in itself having two poles. 27. Having already made remark-
able progress, radio and television continue to develop and to find wider and wider
application in science, industry and agriculture
15 Texts for supplementary reading
15.1 The power amplifier
The final tube in an audio amplifier which is feeding audio frequencies into a
loudspeaker must be essentially a power amplifier. Its task is to deliver undistorted
power to the loudspeaker, and not to develop any great amount of voltage amplifica-
tion. The task of the previous amplifier stages is to build up the small output voltages
of the detector, 11 that the large voltages necessary to swing the grid of the rower
tube may be obtained. The A-C power in the plate circuits of tubes previous to the
power stage is small; what is required is that each previous stage shall give a maxi-
mum voltage amplification without distortion, and the fact that maximum power may
not be developed in these plate circuits is not important. These tubes work at very
high impedances in which it is not possible to generate much power although 11 is
possible to build up considerable voltages across them.
The a-c plate current of the last tube then must be rather large and this means
that the grid a-c voltages must be large, which in turn means that the Eg— Ip curve of
this tube must have a long and straight line part. The 171 tube, for example, which
can deliver about 700 milliwatts without much distortion must have an r.m.s. (root
mean square) grid voltage applied to it of about 27 volts; there must be a portion of
the Eg — Ip curve which is straight over at least twice this characteristic must be
straight from minus 78,5 to minus 2,5 volts. The next preceding tube has much
smaller voltage swings to handle and so its characteristic need not be straight over
such a long part.
Because of the high cost of power apparatus when power tubes requiring high
voltages are used the final tube in the average radio receiver is a low R, low resis-
tance tube which will deliver considerable power to the loudspeaker without demand-
ing excessive plate voltages. Such tubes require large input a-c voltages and therefore
require greater amplification before them than tubes of higher values of.
43
