March 1933 Radio-Craft
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles
If this article had appeared in an April edition of Radio-Craft,
I might have suspected it was a Fool's hoax, but it was the March
issue and, it turns out, it was serious. Obviously the concept did
not work out well since the overwhelming majority of vacuum tubes
sold up until the time semiconductors took over the electronic device
market had separate filaments (heaters).
It was a great idea, though, and the world is thankful for the pioneers
who take the figurative 'arrows' for the rest of us. We benefit
from their hard work and ingenuity, while they suffer the agony
of defeat, with an occasional taste of the thrill of victory
Wide World of Sports). It is too bad the
concept did not work because, as pointed out in the article, the
benefits of simpler, cheaper manufacturing and greatly extended
tube lifetime would have been a significant asset to the electronics
And Now - The Filamentless Tube
Tubes of various types and classes have been described in this and
other publications since the latter part of the nineteenth century;
but, in nearly every case, the tubes described employed a filament
as the primary source of electrons. Ionization of gas has been suggested
as a means of securing electron emission, and a great deal of work
has been done along this line in Germany. We herewith present the
first complete description of an American filamentless tube, recently
demonstrated in New York.
Fig. 1 - Illustration of one type of filamentless tube
along the lines suggested by Dr. Hund at his recent lecture
in New York. This tube is drawn approximately to scale,
and the location of all the elements is clearly indicated.
The type of tubes described in this article are not pipe
dreams, but actually have been constructed and successfully
demonstrated. While the total power required to work the
tube is slightly greater than a corresponding filament tube,
the extremely long life it enjoys more than compensates
for this slight increase in power. Then again, the necessity
for filament transformers is not present. Of considerable
manufacturing importance is the comparative leeway in gas
pressure allowed, and the tube may even function with about
10 mm. of air alone! The materials used as the elements
are not critical, both as to type and purity; a plate, for
instance, may be of iron, and this iron may be either clean
or rusty - the results are the same. It is expected that
commercial tubes may be available in about one year.
Radio-Craft takes pride in presenting a description of
one type of American filamentless tube.
Afuore was created at the January, 1933 meeting of the Institute
of Radio Engineers when Dr. August Hund, a member of the research
staff of Wired Radio, Inc., discussed the development and demonstrated
the operation of filamentless ("cold-cathode"), or ionized-gas,
tubes. (Based on the fundamental experiments of Dr. Lee DeForest,
nearly thirty years ago, as mentioned in the article, "Soon-The
Cold-Cathode Vacuum Tube," in the May, 1931, issue of Radio-Craft
Technical Editor.) Over 1,000 engineers listened to every word
of this well-known scientist and pioneer in the development of ionized-gas
discharge devices. In a short address, just before the discussion
of Dr. Hund's paper by members of the Institute, Mr. R. D. Duncan,
Jr., chief engineer of Wired Radio, Inc., stated that the primary
interest of his company in new tube developments was in long life,
because of the tremendous cost it would be for his company to service
burned-out tubes in the rented receiver system they plan to install
shortly in Cleveland, Ohio.
Uses of Filamentless Tubes
The experimental tubes demonstrated by the Doctor were put through
the paces of oscillation, detection (or demodulation, as the Doctor
chose to call it), voltage amplification, and power amplification.
The tubes of the power class operated as class B, push-push devices.
Oscillations were produced by feedback circuits.
A four-tube set (contained in a cabinet of conventional design)
demonstrated beyond a doubt, when music and speech of excellent
quality filled the packed auditorium, that the filamentless tube
can rival the filament tube in performance! (A beautiful lavender
glow; sufficiently strong to permit the reading of newspaper print
a short distance away, emanated from the ends of the tubes' cathodes.)
A one-tube set gave loudspeaker volume that would be sufficient
for any hotel room.
An experimental tube design shown and described is illustrated
in Fig. 1; a schematic circuit of a "1-tube loudspeaker set," designed
in accordance with the engineering data given verbally and via the
blackboard by Dr. Hund, is shown in Fig. 2; a theoretical amplifier
circuit is Fig. 3. It must be remembered that although experimental
work is still continuing, commercial tubes are not yet available,
and hard and fast figures cannot be given.
Two general types of tubes have been developed, one of which
is a five-electrode tube that makes use of the conduction of negative
ions, while the other is a two-electrode tube operating on the negative
resistance principle involved in the operation of the Poulsen arc.
Both types of tubes have been made to function as oscillators, amplifiers,
modulators and demodulators, and several forms of amplifier tubes
working on both the ionization and negative resistance principles
were described, but the design of greatest interest to the average
radio man is the former or "ionization" type.
The "Uniode" Filamentless Tube
In Fig. 4 we have the first blackboard illustration sketched
by the Doctor. In this elementary form of tube, we have the basis
of many already commercialized devices. A globe with about 10 or
20 mm. of some inert gas encloses two electrodes, a cathode A and
an anode B; a high-voltage battery and limiting resistor R complete
the circuit. This resistor limits the current through the tube,
which current otherwise would reach an excessive value due to the
low resistance of the ionized gas.
With the battery current adjusted to a value that is not critical,
we have a glow between the electrodes. The color is pink for neon,
and lavender or purple for helium. This glow is thought to be caused
by the collision of positive ions and electrons dissociated by the
highly charged electrodes A and B.
This "uniode" tube can be made to detect, oscillate and amplify;
also, relaxation oscillation has been produced from low audio frequencies
to 30,000 kc. (10 meters), according to Dr. Hund. However, these
two-element tubes have serious limitations when compared to the
orderly working thermionic class used in our present receivers,
and, therefore, it was found necessary to modify the design in order
to more closely approximate the performance of filament-type tubes.
At the same time, the feature of unlimited life was obtained. This
modification, Fig. 5, is the introduction of a third element marked
How the Diode Cold-Cathode Tube Operates
The dissociation of electrons and positive ions from the rare-gas
atom, as explained, makes it possible to pull great quantities of
negatively charged ions and electrons to the third electrode, which
is charged "plus plus" (the Doctor's terminology), or at a higher
voltage than electrode B. We now have one stream of electrons and
ions between A and B, and another to C. In a hot-cathode type diode
tube the filament may be likened to the path A-B, and the internal
plate circuit as the electron stream to C.
So far, the talk had only reviewed the work of previous investigators,
continuing with the remark that as soon as a grid was put between
the arc and the plate C, the grid becomes charged with positive
ions and causes it to become inoperative. This was the starting
point for the description of the structural changes which made the
filamentless triode practical.
Construction of the Filamentless Triode
The next blackboard sketch, Fig. 6, showed the introduction of
a perforated electrode in place of C in Fig. 5; the introduction
of a grid D and plate E completed the representation of a triode
which may be designed for any service.
The action was explained as follows: Electrons and positively
charged ions from the arc between A and B are accelerated through
the perforated cylinder; for purposes of explanation, only one of
the small holes is shown. (See Fig. 1.)
The shielding effect of this positively charged cylinder slows
the speed of the positively charged ions so that there is practically
no trace of this trouble maker in the electron stream between the
cylinder and plate. A grid in the electron stream now affords complete
control of the operation of the tube, similar to any triode.
In order to obtain power from the tube, the entire surface of
this cylinder, or "cathode," must be perforated, as shown in Fig.
3. (These holes measure about 40 mils in diam.) This gives us an
electron stream second to none; not even the best of filament or
hot-cathode emitters. Once the electrons are drawn through this
cathode, the operation of the tube is exactly analogous to the operation
of hot-cathode emitters, and the glass envelope, therefore, may
contain all the additional electrodes necessary to produce a diode,
triode, screen-grid quadrode, pentode, and, if the tube industry
finds need for such, a septode or heptode.
By placing the grid and plate all the way around the cylinder
it was found possible to take advantage of the electrons coming
through all the holes; this is the controlling factor which enabled
the Doctor to design almost any kind of a tube, be it for voltage
or power amplification, or detection or oscillation. The corrugated
appearance of the plate electrode (which very much resembles a biscuit-cutter)
is explained when it is recalled that if it were not corrugated
the electrons would tend to be drawn to that point on the surface
of the plate which is nearest the cathode - because of mechanical
asymmetry - and corrugating the plate tends to result in the electrons
being drawn through all the holes in the cathode and distributed
over the entire area of the plate (provided the elements are symmetrically
positioned with the usual commercial tolerances).
Fig. 2 - Schematic circuit of a one-tube receiver using
the filamentless tube described in this article.
Fig. 3 - A theoretical circuit illustrating the use of
the filamentless tube as an amplifier.
Fig. 4 - Simple circuit of an elementary gaseous two-element
Fig. 5 - Same as Fig. 4, with a third element, C, inserted
Fig. 6 - Simple schematic of the filamentless tube showing
the use of the holes.
Although it is regretted that curves of the static and dynamic
characteristics of the experimental tubes described by Dr. Hund
are not available at the present time, considerable information
may be obtained from the figures given and from known data regarding
the operation of gas discharge devices.
A potential of 100 volts is shown in Figs. 2 and 3 as the value
required to produce ionization between the two overlapping electrodes;
this figure, however, will depend entirely upon individual tube
design. The current required to produce sufficient ionization for
operation, in the desired service is another variable factor; in
one tube model this figure was 60 ma. The "power" consumption of
this ionized path (equivalent to the more usual "filament.") is,
therefore, 6 watts. In both Figs. 2 and 3 the cathode is shown placed
at a potential 50 volts above that required for normal ionization,
or 150 volts; the cathode current consumption will vary with tube
The grid and plate voltages, shown in Fig. 2, are those usually
employed for grid-leak detection using certain types of hot-cathode
triodes (zero grid bias, and 50 volts on the plate with equivalent
plate current drain). The grid and plate voltages, and plate current
of Fig. 3 parallel the figures for operation of a type 71A tube
used as an amplifier. As Fig. 2 indicates, it makes no difference
whether the operating potentials for a filamentless tube are obtained
from batteries or a "B" eliminator, provided correct bypassing is
secured in the latter instance.
Grid input impedances may run as low as 30,000 ohms and as high
as present tubes. Output impedances can be made to match present
tubes so that present transformers can be used. The power transformer
must be designed to accommodate the additional 150 volts required
by the ionizing and cathode circuits at the current drain of the
particular tube types.
At the conclusion of the reading of Dr. Hund's paper, it was
not necessary for the President of the Institute to ask for a rising
vote of thanks as the audience showed their appreciation of this
amazing development by long and loud applause. The discussion which
followed brought to light several interesting points.
For instance, it was brought to light that some heat is developed
by electron bombardment of the electrodes; however, this heat is
carried off through the stem of the tube by conduction, and does
not reach a temperature .high enough, for instance, to burn the
In answer to another question; it was explained that the material
used for the electrodes did not make much difference. In some tubes
the electrodes were made of iron, while in others aluminum and nickel.
The iron could be rusty, oily, and dirty; clean: electrodes were
no better than dirty ones.
The kind of gas used did not seem to make any differences; combinations
also did not exhibit any differences in operation. Chemically pure
gas reduced the ionizing potential from about 100 volts, which was
not considered too high, to as low as 35 volts. The amount of gas
was usually between 10 and 20 millimeters, but Dr. Hund said he
obtained quite satisfactory operation by using only ordinary air
exhausted to 10 mm., and without the addition of any other gases.
(Apparently occluded gases cause little trouble in a tube of this
type, and since high evacuation on expensive pumps is one of the
most costly procedures in the manufacture of present-day hot-cathode
tubes, it follows that a marked saving should be affected in the
procedure of manufacturing cold-cathode tubes.)
The doctor explained to another engineer that the insulation
within the tube was carried right up to the electrodes by extending
the glass to the point of contact with the electrodes. This is done
in order to prevent "sputtering" or uncontrollable sparking from
point to point which otherwise occurs in gas-filled tubes.
Although the inquiries of four engineers and the impromptu calculations
of another tended to indicate that the over-all power consumption
of a tube of the filamentless type exceeded that of the filament
type, it was pointed out that the practically "lifetime" longevity
of the tube, its low cost of production, its freedom, from variations
in characteristics with relatively large changes in gas pressure,
the absence of a filament winding on the power transformer, the
ease of reproducing tubes having a given characteristic, and the
simplification of receiver wiring, far more than offset the added
cost of the increase in high-voltage output necessary to supply
the ionization and cathode potentials and currents. Undoubtedly,
this talk by Dr. Hund has done more to stimulate nationwide interest
in the cold-cathode or filamentless type of tube than any other
publicity so far.
Posted January 22, 2015