January 1935 Short Wave Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics. See articles
from Short Wave Craft,
published 1930 - 1936. All copyrights hereby acknowledged.
Cold cathode tubes are distinguished from hot
cathode tubes in that they do not use a separate heated element in order to generate free electrons
(thermionic heating). Rather, a 'starter' type process is used to initiate the electron generation and
then a cascade multiplication keeps the process running. Although this article reports on a cold cathode
oscillator vacuum tube - designed by none other than television pioneer
Philo Farnsworth - some
more familiar examples are neon and fluorescent bulbs and even the veritable
Cold Cathode Tube Demonstrated!
Has No Filament or Grid
Ralph M. Heintz (center) explains the operation of the Farnsworth Cold Cathode Tube
to Bernard H. Linden (left), U. S. Radio Inspector, and Donald Lippincott (right), director of Television
The Electron Multiplier as a high-frequency self-excited oscillator.
Hook-up of "Cold Cathode" Tube.
Mr. P. T. Farnsworth, of television fame, has displayed his genius by inventing a cold cathode tube.
The new tube has no filament or grid and is one of the outstanding tube developments so far to take
place in the radio industry. The tube consists of two cathodes and a ring-anode sealed in an evacuated
glass envelope. It can be used as a detector, modulator, or oscillator, and has tremendous possibilities.
It can be made to generate oscillations over a frequency range from 2000 kc. to 60 mc., the limits of
which only depend on the dimensions of the tuned circuits and it has a power output of approximately
25 watts with 35 watts input. At a recent demonstration, one of these new tubes was used to maintain
communication between San Francisco and Honolulu, and between New York and San Francisco, on approximately
35 meters. On this test, the cold cathode tube was used to drive a pair of 150 watt tubes in the final
amplifier of a transmitter. With 1100 volts, at 30 milliamperes, on the anode, ample excitation for
the two 150-watt tubes was obtained. The cathodes of these new tubes are coated with Caesium silver
oxide to facilitate secondary emission. A large solenoid is placed around the tube and supplied with
direct current in order to maintain an intense magnetic field which envelopes the tube.
When used as an amplifier, a high frequency voltage should be applied to the cathode terminals and
a DC voltage should be applied to the anode, to hold it at a positive potential with respect to the
cathodes. In this case, the cathodes are shunted with a coil and variable condenser in parallel. This
tuned circuit, of course, should resonate at the frequency of the applied high frequency voltage. The
longitudinal magnetic field prevents any flow of free electrons in the inter-electrode space from being
drawn to the anode. The high frequency electrostatic field draws them to the alternately, positively
charged cathodes. The strength of these several fields can be adjusted to allow an electron to be shuttled
back and forth between the cathodes any desired number of times before it is finally drawn out of circulation
at the anode. The high velocity electron striking the cathode causes the emission of from 2 to 8 secondary
electrons, the number of secondary units depending upon the velocity of the impact electron and thus
upon the amplitude of the voltage, which is applied to the cathode. Each emitted secondary also causes
the emission of more secondary electrons, the process being rapidly cumulative and gives rise to a tremendous
amplification of current.
Condition for Maximum Output
The anode attraction which causes electrons to leave the vicinity of the cathode and which increases
its velocity as it approaches the plane of the anode also decelerates its velocity as it leaves the
anode plane, and approaches the second cathode which is charged positively so as to attract it. The
resultant velocity may not be sufficient to cause emission from the second cathode but in order to insure
this emission, additional energy must be imparted to it. This energy is obtained from that stored in
the resonant circuit shown in the diagram. The high frequency supply is of the order of 50 megacycles
and should be loosely coupled to the tuned circuit in order to apply from 25 to 90 volts across the
cathode terminals. One hundred volts or more can be applied to the anode depending upon the desired
Maximum current output is obtained when the anode voltage is just sufficient to allow an electron
to travel from one cathode to another during one half of the high frequency excitation cycle. The external
magnetic field can be then done away with, if the cathodes are properly curved instead of being plane.
This curvature can be calculated so as to focus the electrons automatically for specified anode and
cathode voltages. This would eliminate the D.C. supply for the magnetic focusing.
Posted January 18, 2017