US2135034A - Reduction of tube noises - Google Patents

Description

Nov. 1, 1938. c. w. HANSELL. 2,135,034

REDUCTION OF TUBE NOISES I Filed Jan. 28, 1935 AUD/O OUTPUT INVENTOR CLARENCE W. HA A/JELL BYWggWM/ ATTORN EY Patented Nov. 1, 1938 UNITED STATES REDUCTION OF TUBE Noises Clarence W. Hansell, Port Jefferson, N..Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 28, 1935, Serial No. 3,692

6 Claims.

This invention relates to a novel method of and circuit for operating thermionic tubes including a method of and means for obtaining more quiet operation of thermionic tubes in relay or amplifier circuits than has been possible heretofore.

If in any tube there is a grid between the oath-- ode and anode which is operated at positive potential then there will be a flow of current to this grid in which should appear fluctuations corresponding to the fluctuations in current between the cathode and anode. If the circuit between this positively charged grid and the filament contains a high series impedance adjacent the grid or such an impedance is in the filament return or a circuit tuned to the frequency at'which the tube is being operated is placed in either of said positions, the fluctuations in current flowing from the filament will be smoothed .out. by the choking action and prevented, or reduced to a low value, and there will be instead fluctuations in potential between the filament and said grid. This fluctuation in potential will tend to prevent variations in the current flowing to and through the grid. That is, it will reduce the effect ofalternating cathode heating current fluctuations in cathode emission and initial electron velocities and shot effect.

' If the wanted variations, e. g., variations caused by or productive of signalling currents, in anode current also cause variations in electron current leaving the cathode then the means described above acting on the grid will tend to cause degeneration by opposing or reducingthis useful flow as described above. This may reduce the useful output from the tube in the same ratio as the reduction in tube noise. However, I propose to so construct and use the tube that there is a storage of electrons or space charge outside the grid which encloses the filament and the useful anode current variation will flow from' this storage reservoir. In other words, the volume of space surrounding the space charge grid will act as a virtual cathode, spaced from the heated surface of the real cathode, insofar asthe useful functions of the tube are concerned. Now if the variations in cathode emission are prevented or reduced as described in the preceding paragraph there will be no degenerative effect on the useful output which depends on the stored electrons rather than on rate of emission; This space will be fed with a constant flow of electrons from the filament equal to the average plate current while,

at the same time, it acts as a reservoir to accommodate the useful variations in plate current.

diminution of the charge during current peaks.

The novel features of my method and circuit have been set forth with particularity in the claims appended hereto.

The nature of my invention and the mode of operation of thesame will be best understood from the following. detailed description thereof when read in connection with the drawing, throughout which like reference characters indicate like parts, and in which:

Figures 1 to '3 inclusive sho-W "various circuit arrangements for carrying out the principles of my invention.

The invention is applicable to relays, amplifiers, modulators or demodulators, and the potentials involved may be of any frequency.

Referring to the drawing, in Figure 1, I have shown a circuit by means of which quiet and'effi- 'cient amplification of audio frequencies may be obtained. 1

'In Figure 1, 2 is a thermionic tube having a control grid 4, an inner-grid 6, an anode 8, and a cathode 5. Qrdinarily these electrodes will be constructed concentrically and in said structure the space charge grid 5 will surround the cathode. The control grid 4 is coupled, as shown, by way of a transformer H! to any source of low frequency alternating current which :it is desired to amplify. The control grid 4 is maintained at the desired negative potential by a battery H connected, as shown, with its negative terminal towards the control grid 4 and its positive terminal to ground.

The inner-grid 6 may be maintained at positive potential with respect to the cathode or may be connected to ground, as shown, while the cathode 5 is maintained negative with respect to ground by means of the source It connected, as shown. The cathode 5 is energized by a source of energy such as an accumulator or rectifier, or as shown, by output from an alternating current transformer T, and the cathode energizing circuit includes the audio frequency choke M which is of high impedanoetothe oscillations-from ID. If desired the cathode may be heated by an insulated heater, inside a coated cathode, to separate the heating and cathode circuits.- In this case a choke may be included only in the connection to the cathode. This choke has high impedance at the frequency to be relayed, that is, at the frequency of the input oscillations from l0 and prevents fluctuations in the electron current flowing from the cathode. Consequently the audio frequency choke coils M' prevent the noises which would normally result from the fluctuating cathode emission. At the same time the direct current potential difference between the cathode 5 and the inner-grid 6 insures a constant flow of electron current to and through the grid 6 and, consequently, a space charge or store of electrons is built up between the inner grid 8 and outer grid 4. So long as the outer grid 4 remains at a constant negative potential there will be a constant flow of current to the anode.

When the audio frequency potentials to be relayed are impressed on theouter grid, the current to the anodeelectrode will vary also and there will be a useful output from the tube which will appear in the transformer I2. This output will be relatively free from noises produced by alternating current heating energy, or direct current on which alternating current components are superposed, thermal agitation and varying emission from the cathode. In this case the fluctuations in anode current are obtained primarily from fluctuations in the current flowing to the space charge grid but not to any appreciable extent from fluctuations in electron current from the cathode. At very high frequencies the fluctuations will come primarily from the electron cloud or space charge outside the. space charge grid. Thus, at low frequencies, the space charge grid becomes a virtual cathode while at very high frequencies the electron cloud or space charge near the space charge grid absorbs the current. fluctuations and becomes the virtual cathode.

Of course, the present invention maybe applied to a wide number of circuits and the circuit of Fig. 1 may be modified considerably without departing from the spirit of the present invention. For example, a circuit as shown in Fig. 2 including a neutralized amplifier tube, may be used.

In Fig. 2 it is assumed that radio frequency potentials are to be relayed or amplified. In Fig. 2 the potentials to be amplified are applied, as shown, by transformer 20 tothe control grid 4, and the radio frequency oscillations may be derived from the output of transformer 22, connected with the anode of tube2. The control grid 4 is connected by way of the secondary winding of the transformer 20 'to ground, while the space charge grid is connected directly to ground. The input and output circuits are, as shown, tuned to the radio frequency to be amplified. The cathode heating circuit includes radio frequency impedances 24 connected, as shown, and tuned to the frequency of the oscillations to be amplified by a tuning capacity 26.

In this modification the operation is the same as in the modification shown in Fig. 1. The tuned impedances 24, prevent fluctuations in the cathode emission at the frequency of the oscillations to be amplified. The potential of the electrode 6 with respect to the cathode 5 builds up a space charge and stores up electrons in the space around the electrode 6 which insure the desired useful stream of electrons to the anode to amplify or relay the potential variations. The capacity between the anode and grid electrodes may be balanced tothe desired degree by the capacitor NC. V

The modification shown in Fig. 3 is similar in many respects to the modification shown in Fig. 2. Here, however, an additional screening grid electrode SG is interposed between the control grid 4 and the anode to further insure quiet operation by preventing anode potential variations at the signal frequency to be amplified, from reaching the control grid, space charge grid, or

cathode. The shielding grid SG is maintained at the desired positive potential by connecting the same to the positive terminal of the source by way of a resistance 21, as shown. Radio frequency oscillations are by-passed around the resistance 27 and the source by means of by-passing condenser C connected as shown.

Here, as in Fig.2, the inductances 24 in the cathode heating circuit are tuned by the capacitor 26 to the frequency of the oscillations to be amplified. Here, as in Fig. 2, it is assumed that radio frequency oscillations are to be amplified, and the input and output circuits are tuned to the frequency of said oscillations.

Having thus described my invention and the operation thereof, what I claim is:

1. In a thermionic relay, an evacuated envelope enclosing a control grid electrode, a cathode, an anode, and an auxiliary electrode located adjacent said cathode, a circuit for applying alternating currentpotentials between the control grid and auxiliary electrode, a load circuit connected between the anode and cathode, a circuit connecting said auxiliary electrode to said cathode, said circuit including means for maintaining said auxiliary electrode at a potential relative to said cathode such that a large flow of thermionic current from said cathode to said auxiliary electrode is produced, and means for preventing fluctuations in the current flowing from said cathode including an inductive reactance in said auxiliary electrode-cathode circuit of high impedance to current changes at the frequency of the alternating current to be relayed, said means also producing potential variations between said auxiliary electrode and cathode which tend to prevent fluctuations in current flow from said cathode to said auxiliary grid.

2. In a thermionic relay, an evacuated envelope enclosing a control grid electrode, a cathode, an anode, and an auxiliary electrode adjacent said cathode, a tuned circuit for applying alternating current potentials between the control grid and said auxiliary electrode, a tuned circuit connected between the anode and cathode, said auxiliary electrode being connected with said cathode by means for maintaining said auxiliary electrode at a potential relative to said cathode at which there is a large flow of electrons to said auxiliary electrode, and means for preventing fluctuations in the flow of electrons from said cathode tosaid auxiliary electrode, including reactances in said auxiliary electrode-cathode circuit, said reactances being tuned to the frequency of the alternating current to be relayed, said means also producing potential variations between said auxiliary electrode and cathode which tend to prevent fluctuations in current flow from said cathode to said auxiliary grid.

.3. In a signalling system, an electron discharge tube having an anode, a control electrode, an auxiliary electrode, and a cathode adjacent said auxiliary electrode, means for applying signalling potentials between the control electrode and the auxiliary electrode, acathode heating source, means including a bias potential source in a circuit connecting said cathode heating source to said auxiliary electrode for maintaining said auxiliary electrode at a potential such that a large flow of current to said auxiliary electrode from said cathode takes place, whereby said auxiliary electrode forms a virtual cathode, and means including an inductive impedance for pre venting the effect of, said' signalling potentials upon the emissiveness of said cathode, said impedance being connected at one end to said cathode and at the other end to said circuit which interconnects the cathode heating source and the auxiliary electrode biasing means.

4. In a signalling system, a tube having an output electrode, an electron emission electrode, an auxiliary electrode adjacent said electron emission electrode and a control electrode,gmeans for applying alternating current potentials to said control electrode, means for supplying alternating current potentials from said output electrode a heating circuit for said electron emission electrode, means in said heating circuit for converting fluctuations of the currenttherein into potential variations, and means for maintaining said auxiliary electrode at a potential relative to said emission element such that a large concentrationof electrons takes place on and around said auxiliary electrode whereby the same acts as a virtual cathode.

5. A system as recited in claim 4 wherein said means in said heating circuit for converting fluctuations in current therein into potential variations is an impedance the value of which is high with respect to the alternating currents applied to said control grid. 7

6. A system as recited in claim 4 wherein said means in said heating current circuit for con pressed upon said grid circuit.

' CLARENCE w. HANSELL.

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