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 in c1936 Radio-Craft magazine 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,
extolling the virtues of gridless, was
printed in the January 1937 issue, which I happen to own, so
here it is.
Gridless vs. Grid Tubes
Fig. 1 - Schematic representation of electronic attractions,
repulsions, reflections and absorptions in screen-grid and pentode vacuum 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.
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;
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
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.
(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
(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
(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
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
Posted January 18, 2023
(updated from original post on 12/2/2015)