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.
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