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Back to Basics...
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We live in an exciting age where anything that is
conceivable is possible. Perhaps the most important
segment in this magazine was the “proof” that Tesla did
have a car powered by cosmic rays. In going through the “proof”
and developing the arguments pro and con, I must admit I went
from skeptic to true believer. Now, firmly convinced it did
exist, we can move into the next phase of actually recreating
the device, or developing a derivative.
To do so we must have a true working knowledge of Tesla’s
applicable patents, the various components and circuits used
in such devices, and information on similar devices that may
have worked. For this, we must have a basic understanding of
what each component in the circuit did... many which are no
longer in use today. In short, we need to be able to read
Tesla’s material in the same context that Tesla wrote it
in.
Quite frankly, an entire book can ... and has... been
written on each of the various components. With that in
mind, in this column, we will be running a series of
articles covering the basics and reviewing some
“obsolete” methods of signal processing technologies,
including power circuits. This is intended to be a brief
overview, so often, we will include a recommended reading list
for additional more-in-depth material.
Our primary focus is on receivers, not transmitters.
Cosmic rays, our ultimate target, are naturally generated.
The receiver is what we use to capture them and convert
them into useful power.
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Hertz Resonator
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Developed and used by Heinrich Hertz in his famous
“radio” experiment, the first and simplest radio wave
detector was the Hertz resonator. It consisted of an
open metal ring with a small metal sphere attached to
both ends. Whenever a spark was generated in the
transmitter, a spark jumped across the small gap between
the spheres.
Ultimately, the Hertz resonator was a “proof of concept” device.
It was quickly replaced with an improved detector... the
coherer.
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Coherers...
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You most likely will not find this term in any modern
circuit textbook today. I went through four years of
standard electrical engineering courses, including both
graduate and undergraduate level classes, and was stumped
the first time I came across this term reading
Nikola Tesla’s Colorado Springs Notes!
A coherer makes use of what today would be called a “dry
joint” which is formed by an imperfect solder connection.
This type of contact between two conductors results in a large
resistance between the two. However, the application of AC or
DC voltages (starting from a few tenths of a Volt) can cause a
large decrease in resistance!
Typical Coherer
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The phenomenon of coherence, first observed in 1835,
found its first application by 1852 as a lightning
protector for telegraph lines. Under stress of a high
voltage lightning strike, the coherer would short to
ground, thus diverting destructive currents away from
the rest of the system. Later, the coherer played an
important role in making the development of radio
possible.
In 1884, Professor Edward Branley’s studies of nerve
impulses led him to develop the ‘coherer’ as a device
for detecting radio signals. His device was a glass tube
filled with metal filings and two electrodes. The device
decreased in resistance in the presence of electrical energy,
as the filings stuck together–or ‘cohered’.
By the early 1900’s Marconi had greatly improved the
performance of coherers. In his design, they contained
nickel filings (95%) and silver (5%). The filings lay
between two silver plugs which had been amalgamated with
mercury. The tube was evacuated to prevent any further
oxidation.
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Tesla’s Rotating Coherer...
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Many coherers utilized a small hammer-like device to tap
the tube after each signal, breaking up the filings and
increasing the resistance in preparation for the next signal.
A number of ingenious mechanisms such as shakers were developed
for decoherence.
The Colorado Springs experiments led Tesla to the
discovery of cosmic rays. For his Colorado Springs
experiments, Tesla developed a rotating coherer.
A clockwork drive mechanism was used to continuously
rotate the small glass cylinders thus decohering the
chips after each received impulse from lightning strikes.
It is now believed by Dr. James Corum that coherers are
uniquely suited to detect the particular type of natural
radio phenomenon that Tesla observed in 1899.
Thermionic Valve
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Magnetic Detectors...
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The widespread use Marconi made of the coherer exposed
its poor efficiency. In response, Marconi developed the
magnetic detector which employed clockwork mechanical
power to magnify the weak incoming pulses from a spark
transmitter so they were strong enough to drive a pair
of headphones.
An intricate device, it was a significant improvement
over a coherer. Patented in 1902, it was widely used for
about 20 years, despite the introduction of the thermionic
tube circa 1904.
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Cat Whiskers...
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The popular cat whisker used in crystal sets emerged at
this time as well. Employing a fine wire electrode whose
pointed tip was pressed against the crystal of a detector (
often galena), it was simply connected to a properly tuned
circuit, antenna, ground and headphones to make a useful
receiver for local stations.
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Thermionic Valves...
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Often referred to as a self-restoring coherer, the
thermionic valve was the forerunner of the triode vacuum
tubes. Conceived by J.A. Fleming in 1904, it was the first
practical application of the Edison Effect (The emission of
electrons by a heated element) and used as a vacuum tube
rectifier and detector.
Due to the fact that it consists of two active elements
(a cathode and an anode) it was referred to as a diode. Today,
the most basic semiconductor device is a diode. It also has two
active elements, and performs the same operation. The
difference is in construction and materials used which
we will not get into here. Do not be confused by the
terminology. Keep in mind the time frame when confronted
with a word with dual meanings!
The early 1900s diode consisted of a glass bulb containing
a carbon filament (the cathode) surrounded by a cylinder of
thin metal (the anode or plate). When the cathode was heated,
it would emit electrons. If the anode plate was positive (+) it
would attract the electrons and a current would flow through
the filament-plate circuit. If it became negative no current
would flow.
Technical Evolution
of the Vacuum Tube
Fleming Device - 1904
J.A. Fleming
Heated cathode emits electrons. Replaced coherers in radios.
Audion (Triode) - 1906
Lee de Forest
Control grid increased stability.
Replaced Fleming Valves in radios.
Tetrode - 1919
Walter Schottky
Screen grid reduced capacitance. Enhanced performance
at the higher frequencies coming into use.
Pentode - 1926
Jobst/Telegen
Suppression grid eliminated the discontinuities in
operating characteristics caused by screen grid.
Low Power Pentode
1929
Separating cathode from heater reduced power requirements.
Enabled the use of a separate power source.
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It was a major advance in signal processing and set off a race
by major researchers to find a way to utilize the “Edison
effect” without infringing on Fleming’s patent.
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Audions...
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By 1906, Lee de Forest had discovered that by placing a
third electrode (the control grid), one could control
the flow of electrons. When the grid was negative with
respect to the filament (cathode), the electron stream
was repelled so that the anode current was reduced.
When grid potential was positive, the anode current
increased with an amplifying effect!
Initially, the triode proved to be a sensitive detector,
but it wasn’t until 1912 that de Forest brought the triode
or Audion) into its true glory as an amplifier. In the
interim, a number of improvements were developed for optimal
performance.
The first issue was the vacuum itself. At first, the small
amount of gas in the Audion seemed to enhance its function.
However, both the Audion and Fleming tubes suffered
irregularities stemming from the gas within them. As they
were used, the amount of gas within them actually increased,
leading to the breakdown of the tube, due to the increased
bombardment of positive ions upon the filament.
It was discovered by Langmuir that the reduced vacuum in the
tube was caused by gas atoms contained in the molecules of the
filament, as well as those in the metal and glass parts of the
tube. In addition to constructing new pumps to create a hard
vacuum in the tube, he devised new procedures to expel gas
atoms from the glass and metal parts of the tube, and sealed
filaments inside the bulb at high temperatures.
Meanwhile, Lee de Forest led the way to the introduction of
metal filaments in place of carbon elements found in earlier
versions. Efficiencies were further improved by coating
filaments with alkaline oxides. The basics out of the way,
further improvement in tube operation was to come directly
from new design innovations concerning the very internal
structure of the tube itself.
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Tetrodes...
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The structure of the triode (filament-grid-plate) was
inherently an “active” capacitor.... all tubes are to
some degree. As a result of the high grid-plate
capacitance, the triodes would burst into oscillation
when they were used at frequencies above a few hundred
kilohertz. This form of instability is referred to as
anode-grid feedback.
In 1919, it was discovered by Walter Schottky that by
placing a grid between the anode plate and the control
grid, the grid-plate capacitance was reduced to almost
one-hundreth of that in a triode. The second grid acted
as a screen to prevent the anode voltages from affecting
the control grid.
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Pentodes...
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However, in solving one problem another was introduced...
a discontinuity in its operating characteristics caused by
electrons bouncing off the anode. The solution was the addition
of a third grid called the suppressor grid.
One of the main problems remaining with tubes was their
inherently high power requirements. Circuit configurations
were limited because the heater and cathode were one and the
same. It was discovered that the cathode could be indirectly
heated and this meant that the heaters could be electrically
isolated from the cathode. This had the advantage that the
heaters did not need to be run from a battery supply supplying
DC. Instead an AC supply derived from the mains could be used.
This was a major improvement because it meant that size of
radios could be considerably reduced as could their running
costs.
Structure of beam power tube
showing beam-confining action.
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Beam Power and other Tubes...
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This type of tube is particularly important to our study
as the 70-L-7 tube is a beam tube. In the beam power tube, the
basic four-element structure of the tetrode was maintained, but
the grid and screen wires were carefully arranged along with a
pair of auxiliary plates to create an interesting effect:
focused beams or “sheets” of electrons traveling from cathode
to plate. The new arrangement resulted in lower screen current
(and more plate current!) than an ordinary tetrode tube, with
little added expense to the construction of the tube.
The introduction of the tetrode and pentode brought
revolutionary improvements in performance. As a result the use
of tubes rose dramatically. In addition to radios, they were
used in our earliest computers and other electronic
applications. By the late 1930s many thousands of different
types of tubes were being manufactured... but Bell Labs
introduction of the transistor doomed the vacuum tube as
semiconductors supplanted them.
Although semiconductors found widespread use in low power
applications, tubes are still valuable in high power
applications. Besides, to a tube aficionado, nothing can
replace that warm glow and distinctive hum of a tube-powered
device!__SRE
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