THE FARNSWORTH MULTIPACTOR TUBE
Farnsworth's Cold-Cathode Electron
Multiplier Tube
Uses Neither Grid Nor Filament
By Arthur H. Halloran
From RADIO October, 1934
Think of a vacuum tube without filament or
grid, thus requiring neither an A nor a C battery, which
generates high-frequency oscillations! This is what Philo Taylor
Farnsworth II, the television genius, accomplishes with the
cold-cathode tube which he originally developed as a
current-amplifier for use with his cathode-ray pickup tube. It is
also an exceedingly efficient detector and modulator.
Its first public use as an oscillator was in a radio circuit
whereby communication was maintained between San Fransisco and
Honolulu and between San Fransisco and New York on September 13,
1934, over the Globe Wireless 35-meter channel. In this test,
with 30 milliamperes at 1100 volts on the anode, the tube drove a
pair of 150-watt tubes in the final amplifier of a transmitter at
the Heintz & Kaufmann factory in South San Fransisco. The
signals were received at both Honolulu and New York, and were
reported as R9 by a ship 500 miles west of Honolulu.
Wilkens of Dunedin, New Zealand, also heard the transmissions.
Previous laboratory tests proved that the tube is capable of
generating oscillations of any desired frequency throughout the
range from 200 kilocycles to 60 megacycles, these limits being
set only by the dimensions of the available tuned circuits. An
undistorted output of 25 watts was obtained from an input of 25
watts.
AS AN AMPLIFIER
The performance of this new type of tube depends upon
the emission of secondary electrons from two cathodes which are
bombarded with high-velocity primary electrons. The cathodes are
coated with caesium silver oxide to enhance secondary emission.
They, together with a central ring anode, are assembled in an
evacuated glass tube. The tube is placed within a solenoid which
is supplied with direct current so as to maintain an intense
magnetic field throughout the length of the tube. When used as an
amplifier, a high frequency voltage is applied to the cathode
terminals and a D.C. voltage is applied to the anode terminal to
hold it at a positive potential with respect to the cathodes,
which are shunted by a coil and variable condenser in parallel.
The shunt circuit is tuned so as to be in resonance for the
applied high frequency voltage.
When the D.C. voltage is applied to the anode terminal, any free
electrons in the inter-electrode space would immediately be drawn
to the anode were it not that the longitudinal magnetic field
neutralized the transverse electrostatic field from the anode and
were it not for the high-frequency electrostatic field which
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 in a zig-zag path between
the cathodes any desired number of times before it is finally
drawn out of circulation at the anode.
Each time that a high velocity electron strikes a cathode it
causes the emission of from 2 to 8 secondary electrons, the
number of secondaries depending upon the velocity of the
impacting electron, and thus upon the amplitude of the voltage
applied to the cathodes. Each emitted secondary likewise causes
the emission of more secondaries, so that the process is rapidly
cumulative and gives rise to a tremendous amplification of
current.
In the foregoing simplified account of the tubes operation as an
amplifier, one important factor has been omitted. The anode
attraction, which causes an electron to leave the vicinity of a
cathode and which accelerates 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 now
positively charged so as to attract it. Its resultant velocity
may therefore not be sufficient to cause emission from the second
cathode. To insure emission, additional energy must be imparted
to it, this energy being obtained from that stored in the
resonant circuit.
The high-frequency supply is of the order of 50 megacycles and is
loosely coupled to the tuned circuit so as to apply from 25 to 90
volts across the cathode terminals. The positive potential on the
anode may be 100 volts or more, depending upon the desired
current output.
The tube has a discontinuous voltage-current output
characteristic with a series of successively higher current peaks
as the voltage is increased. Maximum current output is obtained
when the anode voltage is just sufficient to allow an electron to
travel from one cathode to the other during 1/2 cycle of the high
frequency excitation. Other successively lower peaks occur at
anode voltages corresponding to transit times of 3/2, 5/2, 7/2,
9/2 cycles, the last being the least which has yet been measured.
The external magnetic field is unnecessary when the cathodes are
properly curved instead of being plane. Their curvature can be
calculated to focus the electrons automatically for specified
anode and cathode voltages. This eliminates the need of a D.C.
supply for magnetic focusing.
The tube's theoretical output is twice that of an equivalent
hot-cathode tube operated as a Class A amplifier. Its practical
output is limited by the ability of the cathodes to withstand the
high temperature to which they are subjected by bombardment from
a rapidly increasing number of electrons. One test of a small
tube showed an output of 100 watts of undistorted energy before
the cathodes were destroyed by heat. Such destruction is
prevented by means of resistors in the cathode leads. A small
tube can be safely operated so as to deliver 45 milliamperes with
200 volts on the anode. It is to be noted that the tube operates
as a current amplifier and that the amount of voltage
amplification is dependent upon the resistance in the output
circuit.
The theory of the tube's operation as a detector or modulator
should be evident from the non-linear voltage-current
characteristic and requires no elaboration here. It is especially
sensitive in the detection of ultra-high frequencies. [Editor's
Note: P.T. Farnsworth III told Eric Dollard that the multipactor
tube and the Tesla coil were a marriage made in heaven! TB]
It is a well known fact that any amplifier circuit generates
oscillations when arranged to furnish an input voltage of proper
magnitude and phase. Consideration of the manner in which
Farnsworth's tube functions as an amplifier shows that it
conforms to this requirement when connected to a resonant circuit
which is tuned to a frequency whose half-period is equal to an
electron's time of transit, as determined by the frequency of the
oscillations applied to the cathodes.
But the great value of the Farnsworth tube resides in the fact
that it is self-exciting, i.e., that it requires no external high
frequency voltage when used as an oscillator. Aside from the
energy which is required for the magnetic focusing field and
which may not be needed eventually, the only external source of
energy is that which maintains a positive potential on the anode.
It apparently represents a new discovery in vacuum tube
phenomena. Among engineers there is a difference of opinion as to
its cause.
One plausible explanation is based on the assumption that there
is no appreciable space charge effect in the tube when
oscillations start. There are always some free electrons present
in the inter-electrode space, if only those due to photoelectric
emission from the cathodes. These are attracted by the anode when
it becomes strongly positive but are prevented from immediately
going to it by the longitudinal magnetic field. Their
acceleration as they approach the plane of the anode causes a
current to flow, through half the inductance coil in the tuned
circuit, to one cathode. This provides an out-of-phase voltage
drop which accelerates the electrons toward the other cathode
with sufficient velocity to cause secondary emission therefrom.
The emitted electrons then establish a current flow through the
other half of the mid-tapped coil and cause a voltage drop in
opposite phase so as to accelerate the electrons toward the first
cathode which is thus caused to emit more secondaries. Repetition
of this process quickly builds the current up to a point where it
can be delivered to the output circuit without stopping the
internal oscillations. The oscillating frequency is that to which
the resonant circuit is tuned. This explanation has not been
confirmed by physicists, but is presented only as a means for
visualizing possible actions in the tube. When the engineers
disagree, the physicist must experiment.
Much work has yet to be done before standardized tubes will be
available for experimental use. Television Laboratories Ltd. has
licensed two factories for commercial production. But it will
probably be a matter of some months before tubes are available
for amateur use.
finis
Mrs. Elma "Pem" Farnsworth has recently lent Eric
Dollard and Borderland a Multipactor Tube for testing in our lab.
The Multipactor holds promise in the "free energy"
field as a truly workable electronic device that may produce over
100% efficiency. The test as noted in the above article claims
that "an undistorted output of 25 watts was obtained from an
input of 25 watts." This does not take into account radiated
heat and possible residual losses. The Multipactor that
Borderland is testing is a perfected model of the unit referred
to in this article. It appears as though it actually was used in
radio frequency broadcast equipment. Preliminary tests show the
tube to be good and further tests are being planned. More
information will be released through this Journal as we progress
on this project.
For those interested in obtaining more information on the
Multipactor Tube the related U.S. Patent numbers for P.T.
Farnsworth II are: #1,969,399 - Electron Multiplier, Aug. 7,
1934; #2,071,515 - Electron Multiplying Device, Feb. 23, 1937;
#2,071,517 - Multipactor Phase Control, Feb. 23, 1937; #2,135,615
- Multipactor, Nov. 8, 1938; #2,140,285 - Multiplier Coupling
System, Dec. 13, 1938; #2,140,832 - Means and Method of
Controlling Electron Multipliers, Dec. 20, 1938; #2,141,837 -
Multistage Multipactor, Dec. 27, 1938; #2,141,838 - Split Cathode
Multiplier Tube, Dec. 27, 1938; #2,143,262 - Means for Electron
Multiplication, Jan. 10, 1939; #2,161,620 - Two Stage Electron
Multiplier, June 6, 1939; #2,172,152 - Radio Frequency
Multipactor Amplifier, Sept. 5, 1939; #2,174,487 - Self-Energized
Alternating Current Multiplier, Sept. 26, 1939; #2,179,996 -
Electron Multiplier, Nov. 14, 1939; #2,203,048 - Shielded Anode
Electron Multiplier, June 4, 1940; #2,204,479 - Means and Method
for Producing Electron Multiplication, June 11, 1940; #2,217,860
- Split Cathode Multiplier, Oct. 15, 1940; #2,260,613 - Electron
Multiplier, Oct. 28, 1941; #2,274,194 - Apparatus For and Method
of Electron Discharge Control, Feb. 24, 1942; #2,286,076 -
Electron Control Device, June 9, 1942; #2,311,981 - Electron
Control Device, Feb. 23, 1943.
The work of Philo Farnsworth II is as important to energy
researchers as that of Nikola Tesla. All of us at Borderland wish
to thank the Farnsworth family for allowing Eric Dollard access
to the technology. There is much yet to be learned from the work
of P.T. Farnsworth II. Among his outstanding achievements he
developed television and also a way to produce nuclear fusion
electronically (see U.S. Patent #3,386,883 - June 4, 1968).
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