December 1936 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
|
This is the first of a two-part article about the strengths
and weaknesses, pros and cons, vices and virtues of vacuum tubes
with and without grids. Author Henri Dalpayrat offers a list
of no fewer than 17 drawbacks and limitations of gridded tubes.
While necessary to fully control the flow of electrons from
the cathode to the plate, their physical presence causes issues
with parasitic capacitance, thermal noise, electrical variability
due to physical differences, increased manufacturing cost, and
lower reliability. Eliminate the control grid(s) and most of
those problems go with it. Part 2 was printed in the January
issue, which I happen to own, so I'll try to get that posted
within the next couple days. Update -
Part 2 is now available!
Gridless vs. Grid Tubes
Henri F. Dalpayrat
In Part I, below, are listed some of the many disadvantages
and limitations of the "grids" in vacuum tubes. In following
parts, a revolutionary new "gridless" idea in tube design will
be described.

Fig. I - Schematic representation of electronic
attractions, repulsions, reflections and absorptions in screen-grid
and pentode vacuum tubes.
Part I
A grid, as used in radio vacuum tubes, is defined in radio
engineering textbooks, as "a slotted, or perforated metal sheet,
or a loosely-woven metal cloth, or a wire network, as in a solenoid
wire grid, forming a fixed and partial physical obstruction
to the flow of electrons emitted by a cathode, and offering
a variable electrical attraction or repulsion to these electrons,
as influenced by the voltage variations applied on this grid."
As is well known, the purpose of grids is to vary the number
of electrons (particles of negative electricity) passing through
its unobstructed portions, such as the spaces between the turns
of a solenoid wire grid, in order to vary the number of electrons
that are released from the emitter or cathode and reach the
anode (commonly called "plate") and thereby vary the flow of
current in this electrode.
Grids are also used to accelerate and propel electrons towards
an anode, as accomplished by a screen-grid; or to repel anode
secondary electrons, as in the case of a suppressor-grid; or
for electronic coupling in a frequency mixer tube, etc., by
means of an injector-grid; etc.
In every case, the process consists of passing electrons
through some sort of perforations in an electrode having a positive
potential attracting electrons; or a negative potential (or
bias) repelling electrons in various amounts according to the
input signal voltage variations.
It is evident that if a vacuum tube could be built in which
neither perforated electrodes, nor the process of passing electrons
between 2 or more conductors having a similar electrical operating
potential, were used, this new tube regardless of the electronic
principle involved, or the shape or position of one or more
modulating or accelerating electrodes, could never be defined
as a "grid tube."
The various uses and actions of grids in vacuum tubes are
well known today. They have been thoroughly analyzed from every
possible angle. Hundreds of books throughout the world have
been written on this subject. A multitude of carefully plotted
graphs and curves have been recorded, and backed by long and
complicated formulas, but while great efforts have been made
through circuit designs, to correct the inefficiency or objectionable
features of grids, very little has been done towards their complete
elimination.
A number of so-called gridless tubes lately described in
publications, have a number of perforated electrodes which are
really nothing but single-aperture grids, as compared to the
many openings between turns of solenoid wire grids.
Varying the number of openings in a grid does not correct
its disadvantages, while often new conditions are created which
present new objections.
The writer has catalogued, below, 17 undesirable electronic
actions resulting from the use of grids and for which there
seems to be no cure except perhaps the elimination of the grids
themselves.
(1) It is well known in the industry that in amplifying vacuum
tubes using concentric solenoid wire grids, or any slotted or
perforated controlling electrodes, through which electrons pass
in varying numbers, that the electrons emitted by the cathode
are reflected in various directions by the tube parts upon which
they impinge, and that these parts in turn, liberate secondary
electrons which produce so-called "high-amplification tube noises."
(Fig. 1A)
(2) Another disadvantage of solenoid wire grids, or slotted
or perforated metallic obstructions placed between the cathode
and the anode to vary the number of electrons transferred between
these 2 electrodes, is the repulsion of the cathode electrons
by secondary emissions and stray electronic reflections from
various tube parts, causing a constant limiting factor reducing
the amplification and efficiency of the tubes. (Fig. 1A)
(3) Also in grid tubes the absorption of electrons by tube
parts such as, signal input control-grid, acceleration - or
screen-grid, or other extra controlling grids, causing undue
voltage variations or excessive currents in these parts, interfering
with their normal purpose and functions, preventing the attainment
of higher degrees of amplification while also producing objectionable
noises and distortions of the signaling voltages. (Fig. 1B)
(4) Another well-known disadvantage of grid tubes, is the
relatively too-high electrostatic capacities existing between
the various electrodes, causing distortions of the higher modulation
frequencies due to their uneven amplification, and the well-known
inefficiency of these tubes for similar reasons, for the amplification
of very short waves. (Fig. 1C)
(5) Still another well-known disadvantage of grid tubes,
is due to the fact that near the surface of the output anode,
the electrostatic field density, per unit area of the anode,
is much greater than the negative electrostatic field density
of the electron streams reaching that surface, which prevents
the cathode electrons from repelling the secondary electrons
liberated by the anode when this latter receives primary cathode
electrons. (Fig. 1D)
(6) Also, in (screen-) grid tubes, the anode secondary emission
is attracted by the acceleration-grid which projects these interfering
electrons against and through the signal-input grid, thus varying
the effectiveness of this grid and also opposing the cathode-to-anode
electron stream. (Fig. 1E)
(7) Yet another fault of grid tubes, is that the anode secondary
emission increases with the increase in signaling amplitudes,
and that these varying electronic repulsions are responsible
for one type of distortion produced by non-linear amplification.
(Fig. 1E)
(8) A further disadvantage of grid tubes,
is that the so-called suppressor-grid, well-known in the art,
cannot be made sufficiently negative to repel all secondary
electrons, without also opposing the useful cathode electron
stream. (Fig. 1E)
(9) A difficulty encountered in grid tubes, is that when an
attempt is made to align the various grids, in order to reduce
the objectionable features mentioned in previous paragraphs,
it is found to be quite difficult to insure the uniform production
of these tubes on a large scale as is the case with the 6L6
tube.
(10) An important factor of grid tube construction with which
the radio Service Man is particularly well acquainted, is their
varying characteristics due to mechanical shocks and the internal
heat of the tubes which relaxes the tension of the grid wires
causing them to sag or bend in various directions, thus changing
the various interelectrode capacities.
(11) A disadvantage of grid tubes that is not so well-known,
is their total inability to provide self-limiting properties,
so very useful, per stage whether for R.F. amplification, detection,
or A. F. amplification, for the reduction of noises, or fading,
or to obtain individual A. V. C. per tube, or to prevent various
types of distortion, etc.
(12) The radio set design engineer is familiar with the following
disadvantage of grid tubes, which is their inefficiency when
they are used for multiple functions (such as mixer tube for
superheterodyne operation) due to unwanted capacity couplings
existing between the various electrodes.
(13) Another disadvantage of grid tubes of importance to
designers is that when it is attempted to obtain a greater isolation,
of different functions within one tube, either separate amplifying
units or complicated electrode arrangements are enclosed within
one envelope. This ordinarily renders the device more bulky,
more fragile, more expensive, and more complicated to manufacture.
(14) Also, grid tubes, of ordinary inexpensive types, exhibit
the inability to deliver full undistorted power output and especially
when a low positive voltage is applied on the anode.
(15) Another disadvantage of grid tubes, is the constant
variation in electronic field density existing between the cathode
and the signal input control-grid, rendering this grid temporarily
less effective to low-amplitude modulations or higher signal
frequencies, immediately after the liberation of a large percentage
of the cathode-to-control-grid space charge, by high-amplitude
signal modulations. (Fig. 1F)
(16) And a further disadvantage of grid tubes, including
the 6L6 beam tube, is that the various horizontal beams shaped
by grid-wire turns, constantly vary in cross-sectional area
according to the signal voltages impressed upon these grids,
while the anode potential is varied only by the quantity of
electrons flowing through its circuit; this discrepancy causes
a higher electronic velocity of the compressed beams when the
grid is more negative, thus directly opposing or counteracting
in various irregular amounts the full effectiveness of the signal
voltage variations received by the signal input control-grid.
(Fig. 1F)
(17) Finally, in grid tubes, the anode potential exerts a
variable attraction upon the electrons emitted by and in the
immediate vicinity of the cathode, and that this variable attraction
constantly counteracts, in various amounts the action of the
modulation control-grid. For example, when the modulation grid
turns more negative, fewer electrons reach the anode which then
draws less current, and whose voltage increases. This increasing
anode voltage, increases the attraction by the anode, of the
electrons which are near the cathode, thereby partially defeating
the purpose of the negative modulation grid which is thus prevented
from repelling a larger number of electrons from the emitting
cathode.
It is for these and other reasons that the writer experimented
with, and produced, truly grid-less tubes operating on an entirely
new basic principle.
These tubes will be described, for the first time, exclusively
in a future issue of Radio-Craft.
Revolutionary New "Grid-less" Tubes!
In Part II will be disclosed for the first time in any radio
magazine the details of a revolutionary new tube design in which
the grid element is eliminated!
Experiments to date indicate that this design is not only
applicable to every existing service of vacuum tubes, but also
in many instances, superior in performance in these respective
fields.
Furthermore the Grid-less construction makes possible numerous
results as an inherent characteristic of the design, that previously
were unattainable; or, that were attainable only through the
use of special, relatively complicated and expensive circuit
arrangements.
Posted December 2, 2015