Gridless vs. Grid Vacuum Tubes
January 1937 Radio-Craft

January 1937 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.

Assuming the 10 enumerated advantages of a gridless vacuum tube may be added to the 17 enumerated disadvantages of a gridded vacuum tube, there are 27 reasons, per author Henri Dalpayrat why one should consider abandoning the 'old style' tubes for his revolutionary concept. Part 1 of this 2-part series discussed the unavoidably negative features of a gridded vacuum tube. Part 2, presently, extolls the wonders of a gridless tube. Chief among the features is the use of 'compressor bar' elements that are situated parallel to the electron flow rather than in series with it. Another major difference is the cathode element running vertically up the center of the tube and the cathode wraps around it. If I had to draw a comparison of gridless versus gridded vacuum tubes with semiconductor devices, the former more closely relates to a field effect transistor (FET) and the latter to a bipolar junction transistor (BJT). I say that because the compressor bars' action on the electron flow is to change the concentration (density), effectively having the ability to cut off the current. Although not related functionally to the BJT, the gridless tube uses 'collectors' to help control the transconductance.

Gridless vs. Grid Vacuum Tubes

Henri F. Dalpayrat

Ever since the grid element was first incorporated in a vacuum tube it has been a dogma that all subsequent tubes be similarly constructed. The author points the way to a new era, in electron tube designs, in which the previously-discussed disadvantages of a grid are eliminated by means of a "compressor."

Fig. A - Phantom view of basic gridless tube; note the "compressors."

By the simple expedient of arranging "compressor" elements to control by electronic means the electron emission from cathode to anode, a radically new design in electron tubes is effected. Coincidentally, many new and important functions are made available; one of the most outstanding of these is automatic noise suppression; in this design, both the above- and below-signal-level interference voltages are counteracted.

Part II

In the preceding issue of Radio-Craft, an enumeration of several disadvantages inherent in all grid tubes, were discussed, to point the way to much-needed improvements in electronic amplifying devices.

The new electronic principle described in the present article, and discussed in connection with gridless tubes, indicates the possibility of designing greatly-improved radio vacuum tubes without adhering to the conventional arrangement of electrodes.

"Grid" Tubes Fundamentally Unsound

Contrary to popular belief, the standard "grid" tubes now available in the industry, although highly perfected, still have many objectionable features which cannot be corrected as long as grids are used, and which cannot be overcome by electrical circuit designs.

The source of most of the difficulties now found in thermionic amplifiers is in the tubes themselves, and it seems more logical to produce new and better tubes, rather than to improve an old and inefficient electronic technique; even though it is now generally conceded that much is to be gained by attempts to "modernize" it.

The average radio engineer, accustomed to think in terms of "grids," is inclined to take grids for granted, or as almost indispensable; though a little thought on this subject, along new theoretical principles, will disclose innumerable and important advantages derived by the total elimination of all grids!

The great number of new electrical circuits possible with the new type of "gridless" tubes (here illustrated in theory and practice) open up new fields for experimentation and invention, independent of the patent restrictions or technical limitations usually allied with well-known, well-developed standard tubes.

The Design of Gridless Tubes

One important feature of this invention is the complete elimination of the grids and their disadvantages.

Another object of the principle disclosed in these illustrations and data that follow, and the novel combinations of new electrodes in the devices, is to provide new types of vacuum-tube designs for a wider range of applications and, generally, greater usefulness than the now well-known vacuum tubes using a number of solenoid wire grids, or their perforated equivalent, concentrically positioned around a heated cathode emitting electrons.

Referring to the drawings, Fig. A shows a new type of amplifying vacuum tube, or thermionic relay, with an evacuated envelope made of glass or metal (shown in this figure as glass) on a base having contact prongs connected within the tube to separate or similar electrodes. The centrally-located cathode sleeve is coated externally with rare-earth oxides and heated internally by a filament. Two plates in close proximity to the cathode are shown as "collectors," and two wires (rods or hollow metal cylinders) placed in spaced parallel relations with the cathode and "collectors" are the "compressor bars," within the anodes or plates, as shown sectionally in Fig. 1.

The anodes may be separate plates, either electrically connected or insulated from each other, or formed of a single tubular plate having its facing surface plates stamped out, to decrease capacity coupling between the anode and. collectors.

Figure 1 (A, B and C) shows the arrangement of the tube electrodes mentioned, as seen endwise. The filament-heated, coated cathode is emitting electrons which are repelled by negatively-charged electrodes or compressor bars; thus forming 2 electron beams which are each attracted and collected by the positively-charged collectors; these have their positive potential regulated to be much lower than that of the anode plates. The voltage on the "collectors" is made small enough to allow the compression of the stream of electrons by the "compressors"; but also high enough to establish a constant electronic emission to the "collectors" or absorption electrodes.

Figure 1A shows how a high negative charge, applied on the compressors, directs all electrons towards the central portion of the surfaces of the collectors. The combined positive charge of the collectors and negative charge of the compressors completely prevents any electrons from reaching the anodes. Note the width of the electron beams, spreading over the surfaces of the collectors; in Fig. 1A; while Fig. 1B shows how these electron beams spread in cross-sectional areas, and impinge upon a larger portion of the collector surfaces when the negative charge of the "compressors" is decreased.

Figure 1C shows how a further decrease in negative potential of the compressors, or even the application of a positive charge on them, reduces the compression of the beams of cathode electrons, and allows them to spread over and beyond

Fig. 1 - Approximate representation of gridless tube operation.

the surfaces of the collectors; thus allowing electrons to reach the anode plates in the form of 4 electron beams. These beams vary in electronic density according to the voltage variations applied on the compressors, which control the direction of the cathode electrons towards the collector plates, and vary the number of electrons either absorbed by these collectors or passing between the collectors and compressors to be received by the anodes.

Gridless-Tube Circuits

Figure 2 shows how an individual A.V.C., per stage, can be obtained with the electron- beam compressor tube described above. In this I.F. (the same principle is applicable at R.F. or A.F.) amplifier, the signals are applied on the compressors, varying the electrons passing through to the anode. The D.C. potential applied on the collectors is obtained through a potentiometer R and a signal-load ohmic impedance high-resistance) R1. When the signal voltage variations, or excessive noise impulses, rise above the A.V.C. signal level, the positive potential of the collectors is increased through capacity C. The positive voltage, increasing in these collectors, absorbs a greater number of electrons from the cathode; thus diverting electrons from the cathode to the anode streams, and decreasing automatically the amplification of the tube for only the excessive amplitudes, as adjusted by the positive absorption potential obtained through R.

The addition of regeneration to this circuit through a tickler coil T, connected in the anode circuit at point (X) can be advantageously utilized for greater sensitivity, improved selectivity, or larger power output, or accentuation of certain modulation frequencies; while the voltage-controlling functions, previously described, enable this circuit to provide stable and constant automatic regeneration, a feature unusually useful in short-wave reception.

The foregoing described only a few possible applications, such as individual (per stage) automatic volume control, automatic regeneration control, and a combination diode and power output amplifier tube. Other circuits, such as automatic volume control and noise suppression controls, incorporated in each stage of a receiver, in order to produce a practical "noiseless" or "static proof" receiver, will be described, exclusively and for the first time, in a future issue of Radio-Craft.

Referring again to the advantages of "gridless" tubes, and more particularly to the type invented by the writer and described in this article, a number of advantages will be given below.

Fig. 2 - Experimental circuit in analysis of gridless tube operation.

Advantages of Gridless Tubes

It is one purpose of this invention to eliminate solenoid wire grids, and the method of passing electrons through spaces between wires, or slots, or small perforations in any controlling electrodes. Another purpose is to simplify the mechanical construction and assembly of parts in tubes, while providing stronger, more rigid structures, and insuring the manufacturing production of tubes having uniform characteristics.

Still another purpose is to greatly decrease interelectrode capacities, and to render practical these tubes for the faithful amplification of audible frequencies, or the amplification of very short waves, or the generation of constant oscillations, etc.

Fig. B - Actual gridless tube in an experimental receiver.

The "gridless" design also provides a new vacuum tube, having inherent self-limiting amplification properties, and/or self-selecting amplitude-selection properties, or automatic current or voltage stabilizing properties, in addition to their normally intended functions such as detection, rectification, and/or amplification.

There are many other important advantages to be obtained by applying the "gridless" principle of construction to tubes of all types. Some of these features are listed, numerically, as follows.

(1) Combine, in a simple manner, several functions in one electronic device, without one function interfering with another.

(2) Permit the manufacture of simple, inexpensive, small tubes, practical for short-wave operation, and capable of delivering (as working models have proven) stable, constant, undistorted and highly amplified current.

(3) Provide new tube designs, especially applicable to power output-tube operation and capable of delivering relatively high undistorted power output, with a minimum of positive voltage applied on the main output anode.

(4) Reduce or eliminate internal vacuum-tube noises, which are due to electronic bombardments upon electrodes, or to unwanted electronic reflections or uncontrollable electronic emissions.

(5) The reduction, or the elimination, of currents in certain electrodes, in order to minimize a type of output current distortions caused by these electrode currents.

(6) Inherent ability to limit either their own output current automatically, within any fixed range of variations, or unwanted signal or noise amplitudes, either below or above a given intensity level, as determined by adjustable electrical circuit constants-thus producing an "anti-noise" tube!

(7) Possibility of modulating several electron beams similarly and simultaneously, or else differently and independently, as desired to utilize their various outputs for a common purpose or for individual and different purposes.

(8) Production of (relatively) very short electron beams of the full and complete cathode electronic emission, without resorting to the use of so-called "electron gun" or other electrical means well-known to the art (which. however, are unable to cause the utilization of the full cathode emission).

(9) Possibility to cause purposely the liberation of secondary electrons, by coating certain electrodes with appropriate rare earth oxides; and to utilize the secondary emission produced by primary electron impacts thereon to obtain a higher voltage and/or current amplification.

(10) Another new and advantageous feature (available in one design) is a novel type of controlling electrode, shaped and positioned in such a manner as to divide an electronic emission, radiating uniformly around a cathode, into a number of electron beams; while exerting electrostatic pressure upon all these beams simultaneously or independently.

There are many more features and advantages to be found in the radically-new "gridless" pri­ciple of operation, and tubes that, by means of "compressor" electrodes, utilize this principle of operation; but space does not permit further elucidation at the moment. However, the writer will be glad to answer any inquiries, concerning the gridless tube, if these inquiries (addressed in care of Radio-Craft) are accompanied by a stamped and return-addressed envelope.

 

 

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