US2032199A - Energizing circuits for space discharge devices - Google Patents

Description

Feb, 25, 11936. R. A. BRADEN 2,032,199

' ENERGIZING CIRCUITS FDR SPACE DISCHARGE DEVICES Filed June 19, 1933 A MHz/r;

I l l 70 700 1000 0.7 1 l0 f00 7000 CUR/WW7 7/?0U6HM147ZRMZ 01/0051 7/9006 A MTFRML .z an an TF A BRADEN BY ATTORNEY UNITED STATES PATENT OFFICE ENERGIZING CIRCUITS FOR SPACE DISCHARGE DEVICES Rene A. Braden, Merchantville, N. J., assignor to Radio Corporation of Delaware Application June 19,

4 Claims.

The present invention relates broadly to electrical distribution networks and more particularly to networks which include thermionic tubes.

It is well known to energize thermionic tubes from commercially available alternating current. For this purpose the available alternating current is usually rectified and filtered before it is used. Normally the rectified and filtered current is applied across a voltage divider circuit and the various desired voltages are derived from taps on this circuit.

In thermionic tubes of certain types it is customary to bias the control grid, in other words, it is customary to maintain the control grid at a negative potential with respect to the cathode of thetube. For this purpose there is usually provided an impedance or so called bias resistance which is included in both the control gridcathode path of the tube and in the anode-cathode path thereof. The space current or anode current passing through the bias resistor causes a voltage drop thereacross which due to the direction of current flow therein maintains the control grid negative with respect to the cathode of the tube. This type of bias is sometimes referred to as automatic bias because of the fact that it so to speak maintains itself as long as the tube is energized. Resistance materials heretofore employed for bias purposes obey Ohms law. that is to say, with these materials the rate of change of potential thereacross due to current flow is linearly related to the rate of change of the current flowing therethrough. It follows, therefore, that when a material obeying Ohms law is used for automatic bias purposes, the bias is varied with variations in the plate current. Changes in plate current are due to many causes the most important of which are changes in plate current due to uncontrolled voltage variations of the available power supply and changes due to signal modulations. Both of these changes may be considered as A. C. components superimposed upon the steady current component. These A. C. components when impressed across the bias resistor produce an'undesirable feed-back voltage which due to the normal amplifying characteristics of the thermionic tube may assume extremely annoying proportions.

In order to avoid this feedback action it has been customary to bypass the bias resistor with a suitable by-pass condenser. The capacity of the condenser being such as to form a low impedance path for the undesired frequencies so as to shunt the undesired A. C. components around the bias resistor.

America, a corporation of 1933, Serial No. 676,462

From what has preceded, it is apparent that the bias resistor may be considered as a portion of the voltage distribution or voltage divider circuit above referred to. As is well known with thermionic tubes of the screen-grid or multi-grid type wherein it is desired to maintain the screen or auxiliary electrode at a positive potential with respect to the cathode, the necessary positive potential is derived from a suitable tap on the voltage divider resistor. In such cases it is usually desirable to maintain the auxiliary electrode at a constant positive potential with respect to the cathode, however, it has been found that this is extremely difficult because of the fact that variations in the current drawn from the tap as when replacing tubes cause changes in the voltage drop across the voltage divider resistance due to the fact that the voltage divider material obeys Ohms law. By replacing tubes is meant substituting one tube for another and/or pulling d The object of the invention is attained by that 5 there is utilized for the bias resistor as well as for any desired portion of the voltage divider resistance, a material that does not obey Ohms law. For the purposes of this invention, one or more elements having such characteristics that the rate of change of potential thereacross is not linearly related to the'rate of change of current therethrough are interposed in the system. A material that very satisfactorily answers the purpose is commercially known as Thyrite, and is described in United States Patent No. 1,882,742 issued to McEachron. This material has a substantially hyperbolic resistance-ampere characteristic.

Thyrite--meaning gate or opening-is a material somewhat similar to dry process porcelain throughout the mass of which appear minute particles of conducting material. The substance is substantially an insulator at low potentials and becomes a continuously better conductor as the current through it is increased. No attempt will be made herein to discuss all the characteristics of Thyrite however, for a better understanding of the material itself and its characteristics reference may be made to the above identified U. S.

patent and to an article entiled Thyrite; A New Material for Lightning Arresters by K. B. Mc- Eachron, appearing on page 92 of the General Electric Review for February 1930.

It should be noted that the material Thyrite is usually utilized in the form of discs each face of which is provided with a sprayed metallic coating for contact purposes, however, the shape the material takes is not at all controlling and in fact it may be utilized in any mouldable shape. The resistance of Thyrite varies directly with its thickness but not inversely withits area as does specific embodiment when read in connection withthe accompanying drawing.

In said drawing:- 1

Figures 1 and 2 are representations of curve sheets showing certain characteristics. of Thyrite compared with corresponding characteristics of usual resistance materials; and; 1

Figure 3 illustrates an electrical network including a thermionic tube energized through a system utilizing Thyrite :in accordance with the present invention. Referring more particularly to Fig. 1, curve B, B shows the'voltage developed across a resistance material obeying Ohms law in terms of current through it, while curve A, A shows a curve for'Thyrite under similar conditions. It is obvious from the two curves that Thyrite has a much flatter characteristic than the other material. In other words compared to ordinary resistance materials the voltage across the Thyrite is maintained almost constant despite variations'in the flow of current through it.

The resistance characteristics of the two materials are illustrated in Fig. 2, wherein curve D, D depicts the resistance characteristic of a material obeying Ohms'law while curve C; C is a curve which illustrates graphically the resistance char acteristic of Thyrite. The diiference between the two is at once apparent since in the case of Thyrite the resistance. decreases as the current increases whereas for a material obeying Ohms law the resistance remains constant despite changes in the flow of currentthrough it. It should be noted that in both Figs. 1 and 2 the curves are plotted on a logarithmic scale for convenience. g 1 r The circuit'diagramshown in Fig; 3 is now referred to to illustrate the invention. In said figure, tubes T1, T2 and T3 are intended to illustrate generally a multi-stage amplifier. The output of tube T1 is coupled to the input'of tube T2 through a coupling condenser C1. The usualgrid leak resistance R1 is shown connected to the control'grid and cathode of tube T2 at points"! and 8. The lower end 8 of R1 is grounded as at H. I'Ihe output circuit of tube T2 is'cou'pled to the input of tube" T3 through a suitable coupling con denser C2. 7 f

For the sake of "simplicity, only the'energiza tion of tube T2 is here considered although it is to be understood that the principleshereinafter set forth may be applied'to any desired number 7 of tubes. The output of a socket power'unit is adaptedito be connected to terminalsl and 2 in any desired manner. The socket power unit may comprise a half wave rectifier, and filter or a full wave rectifier and filter or it may even comprise :tube or tubes. In the case at hand the anode of tube- T2 is connected to tap 3 through an output or load resistance R2 and a filter resistance R3 in series. A by-pass condenser C4 may be provided as shown connected from the lower end 6 of R2 to ground II. The auxiliary grid of tube T2 is connected to tap 4 through filter resistor R4 and a by-pass condenser C3 is connected between the upper end 5 of R4. and ground H. Since'it is usually desirableto'maintain the potential on the auxiliary grid of'tube T2 Tcon-f stant, resistor R6 is constructed of :Thyrite. "In" thisrway the voltage across Rs remains substan-' tially constant, despite changes incurrent drawn from the socket power 'unit, through Rsl "For biasing the grid of tube T2 there is provided a; resistor R2 which is included in both the input and output circuits of tube T2; In this way the space current of the tube flows through'Rv with the result that the drop across R7 maintainsthe control grid negative with respect to the cathode of T2. Since as previouslypointed'out it is some= 7 times desirable to provide a substantially con-E stant bias, resistor R'zjis in accordance with the present invention made of Thyrite, hence; the

signal, or control grid of tube T2. is maintained at a substantially constant negative potential with respect to the cathode thereof despite variations in'the anode current. j

It is obvious that if a resistance materialobeye ing Ohms law were substituted for the Thyrite.

resistor R7 it' would be necessary'to 'provid'e" 1a by-p'ass condenser of such a capaoitylthat'a'low impedance path is formed for the A. doomponents due'to signals'so as to prevent undesired feedback voltagesbeing developed across the bias resistor. It is to be understood that the present inven tion is notlirnited to the use of Thyrite Tor'aii' equivalent material since many modifications will 'at onceibe apparentto those skilled 'in'the art to which the invention pertains. I claim:

1. In combination with a thermionic tube having input and output circuits, a source of 'uni-' between points" in the outp'utand input circuits of said tube and said source for energizing said tube, at least one or said resistance devices come prising a resistance materialincluding :amas's of silicon carbide crystals and a binder holding adjacent crystals in contact, said material having a hyperbolic resistance.characteristicwhich V is unaffected by variations in'therate of change of voltage applied to the material to produce a flow of current therethrough, the resistance or the material being always substantially the same for a given densityof current flowing through it whether the value of the'said current density is attained by increasing or decreasing the current flow. y V 2 a 2. In combination with a thermionic tube hav ing'input and output circuits, a source ofspace current for said tube and a resistance element 75 including granular material having a hyperbolic resistance-ampere characteristic and having free carbon as an ingredient and means for holding the grains together under pressure, said characteristic being unafiected by variations in the rate of change of voltage applied to the element to produce a flow of current therethrough, the resistance of the element being always substantially the same for a given density of current flowing through it whether the value of the said current density is attained by increasing or decreasing the current flow, said element being common to both the input circuit and the output circuit of said electronic tube for generating a bias potential for said tube of substantially constant value despite variations in the flow of space current through the tube.

3. In combination with a load requiring difierent potentials for the energization thereof, a source of current, a distribution circuit connected across the source, connections between the load and points in said distribution circuit, said distribution circuit comprising a plurality of resistors in series, at least one of said resistors comprising a resistance material including a mass of silicon carbide crystals and a binder holding adjacent crystals in contact, said material having a hyperbolic resistance characteristic whereby the potential drop developed across it by the flow'of current therethrough remains substantially constant despite variations in said current.

4. An amplifier system including at least one electronic tube having input and output circuits, a source of uni-directional fluctuating current, a distribution circuit connected across said source, said distribution circuit including a voltage divider resistor and independent resistance elements interposed in series between points in the output circuit and input circuit of said tube and points in said distribution circuit for energizing the electronic tube while preventing interference in said tube circuits from the fluctuating component of the uni-directional current, at least one of said resistance elements comprising a granular resistance material having a hyperbolic resistanceampere characteristic and having free carbon as an ingredient and means for holding the grains together under pressure.

RENE A. BRADEN.

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