US2029354A - Electrical amplifier - Google Patents

Description

Feb. 4, 1936. P. N. BOSSART ELECTRICAL AMPLIFIER Filed Dec. 50, 1932 INVENTOR Paul N. Bossa z M QK. ,N w W M Q 7 QN @fi k wwhw HIS A TTORNEY Q Q RQ wSQw GRQQ QQQQ MN mass .SQ 5

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I NN N S aw m kammwwmq Patented Feb. 4, 1936 UNITED STAT ES PATENT OFFICE ELECTRICAL ADIPLIFIER Application December 30, 1932, Serial No. 649,571

9 Claims. (01. ate- 20) My invention relates to electrical amplifiers, and particularly to amplifiers for modulated alternating currents.

I will describe one form of amplifier embodying my invention, and will then point out the novel features thereof in claims.

One object of my invention is to provide an amplifier of the type described having'a high degree of sensitivity and highly efiective'in reducmg the influences created by shock excitation fromrandom currents. Another object of my invention is to provide an amplifier of the type describedwhich is effective to reduce the influences of unwanted currents without introduc ing harmful distortion to the wave form of the signaling current. A further object of my invention is to provide an amplifier of the type de-' scribed in which the influences of stray currents are minimized with but a slight loss of ampli- 20 fication to the signaling current. Other objects and advantages of my invention will appear as the specification progresses.

In-the accompanying drawing, Fig. 1 is a diagrammatic view of one form of amplifier embodyingmy invention, and which amplifier is particularly adapted to signal systems for railway trains. Fig. 2 is a diagram illustrating a characteristic curve of a rectifier used with my invention. Fig. 3 is a detail view illustrating another arrange- 30 ment for connecting the rectifier with the transformer between stages.

Signaling-systems have been proposed for signaling between spaced locations on a railway train, or between two trains, or between a train and-a wayside station, in which modulated carrier" current is transmitted from one location to the other through the medium of the trafic rails. Where telephone communication is provided by such systems, the carrier current is modulated by the voice frequencies; and the signaling device is preferably a loud-speaker. In such systems it is'usually necessary to interpose an electrical amplifier of relatively high gain between the receiving means and the signaling device operated thereby, in order to obtain suffic'int energy to reliably actuate a signaling device of desired ruggedness. In these train telephone systems the receiving means at each location remains normally in circuit at all times ready to 5 receive a message, except at such an interval as a'message is to be transmitted from that location;

This-normal receiving condition of the apparatus results in the amplification of minute random currents flowing in thetrafiic' rails, and of stray 5'5 magnetic fields: Such amplification of random currents and stray fields produce a noise in the loud-speaker that is quite objectionable to the listener, as well as to impair the efiectiveness of the system. These unwanted influences effect sudden overloads on the amplifier, and occur so rapidly that the usual volume control provided for such amplifiers does not have time to act. For the purpose of materially reducing the effects of such unwanted influences I provide the electrical amplifier illustrated in the accompanying drawing. Although I am here disclosing an amplifier embodying my invention which is particularly adapted to a telephone system for railway trains, it will be understood that I do not limit myself to this one form of amplifier; and my 1 invention is equally useful for amplifiers for other signal systems requiring amplification of the signaling current. The amplifier here disclosed will serve to illustrate the different forms of electrical amplifiers embodying my invention.

In 'Fig. 1 the electrical amplifier comprises four stages designated in general by the reference characters I, 2, 3 and 4. Stages l and 2 each include an electron tube designated by the reference characters 5 and 6, respectively, and these 5.

tubes are preferably of the well-known screen grid heater type, although other types of tubes may be used if found desirable. Stage 3 includes an electron tube I, and stage 4 includes two tubes 8 and 9 in parallel. preferably of the well-known pentode type, although other types of tubes could equally as well be used for these stages if it seems desirable to do so. The filaments in of the tubes 5, 6, 1, 8

and 9 are connected in series with a suitable battery A by a simple circuit easily traced and which includes a resistor 68. As shown at theleft-hand end of Fig. 1, this filament circuit is grounded by a ground connection I2. Plate current is sup-' plied to each of the tubes 5, B, 1, 8 and 9'fr'om a 40' suitable current source such as a battery B. Connecte'd across the opposite terminals of battery B is a voltage control circuit comprising a resistor 25, reactor 26, and resistors 31 and 38 in series,

the negative terminal of the battery B being for' the plate circuit of these tubes, the reactor 5;

These tubes 1, 8 and 9 are 26 preventing any high frequency in the output circuit of tubes 8 and 9 from feeding back to the tubes of stages I, Z and 3. The screen grids 65 of tubes 5 and 6, and the space control grids Bl of tubes 1, 8 and 9 are connected with different points in the control circuits to give the proper voltage to these grids. The manner of connectingthese screen and space control grids, however, forms no part of my invention and it is deemed unnecessary to describe it in this specification other than to say that it is in accordance with the usual practice for multiple stage amplifiers employing these types of electron tubes.

Associated with the first stage is a filter II, the output of which is connected across the grid M and the cathode E5 of the tube 5, a biasing element comprising a resistor l6 and. a condenser H in parallel being inserted in the connection to the cathode l5; and a condenser l3 mitted through the medium of the trafiic rails,

' the input of tube 5 may be supplied with signalingcurrent by means of train carried pick-up coils 2 i and 22 mounted in inductive relation with battery B. This plate circuit for tube 5 is coupled.

to the grid circuit of tube 6 of stage 2 through v the winding 28 of the autotransformer 39, one

terminal of which is connected to the grid 3! of tube 6 through a condenser 32;. and the other terminal of which is connected with the cathode 33 of tube 6 through a condenser 34 and a biasing element 35. It follows that the signaling current picked up from the traffic rails is supplied to the input of stage I, and thence is applied to the grid circuit of tube 6 of the stage 2 at greatly increased amplitude. I have found in amplifiers ofthe type here involved that voltages applied to the grid of the tube of stage 2 may be as much as 100 times greater than the voltage applied to the input of the stage i. Furthermore, I have found that modulated carrier current, voltages of the order of one volt after having been amplified by the stage i will produce satisfactory reception in telephone systems for railway trains. In the following description I shall assume that the magnitude of the voltage of the telephone or signaling current as itis applied to the input of stage 2 of the amplifier is of the order of one volt in order to clarify the specification.

Tube 6 preferably operates with a negative grid biasof the order of the signaling voltage that is,

in this instance, a bias voltage of not more than voltage larger than that of the usual signaling current cited above, the resistance grid to cathode,

will become relatively low and the'amplification of the tube will become relatively small, although relatively large amplification will be obtained for voltages of the usual signaling current.

In place of the usual grid leak resistor for tube.

6, I introduce a rectifier 33. The rectifier 36 may take different forms but it preferably is a copper oxide rectifier of the type described in the United States Letters Patent No. 1,861,083, issued May 3 1, 1932, to P. H. Geiger and C. K. Strobe] for Electrical rectifiers. Such rectifiers have in the forward or pass direction a volt-resistance characteristic as shown in Fig. 2. A disk of a rectifier of this type will have in its forward'direction a resistance of several thousand ohms at a relatively low voltage, and a resistance of only a few hundred ohms when the vpltage is doubled. For example, a disk of a rectifier of this type in commercial use has at two-tenths volts a resistance of the order of 30,000 ohms, but at a voltage of four-tenths volt its resistance is reduced to the order of 1600 ohms, and atone volt the resistance is reduced to only about I00 ohms. If a pile consisting of severalv disks in series, say,-five disks, is used, the rectifier 36 will have at onevolt (twotenths volt-per disk) a resistance of the order of 150,000 ohms, and. at a voltage of two volts, it will have a resistance of the order of 8000 ohms or only about one-sixteenth as large as at one volt. If five volts are applied, the resistance would be about 3500 ohms. In the high resistance direction this rectifier would have a resistance of two megohms or more up to 30 volts. Hence, if rectifier 36 consists of, say, five disks of a copper oxide rectifier in series, its resistance to the voltage produced by the usual signaling current, that is to the ordinary speech current sufiicient for satisfactory reception, is of the same order as the usual grid leak resistor it replaces. It will be noted that the pass direction of rectifier 30 is such as to discharge the grid 3| as it swings negative with respect to the cathode 33.

.It follows that if the ordinary signal voltage after being amplified at stage I is of theorder of one volt, the leak through the rectifier 3B is negligible, and the negative bias provided for the tube 6 is sufficient to prevent the grid 3! ever swinging positive with respect to the cathode. If a relatively large voltage appears in the grid circuit of tube 6, however, the resistance through the rectifier 33 becomes only a small fraction of its value for the signaling voltage, and the rectifier will discharge the half cycle that the grid is rendered negative with respect to the cathode.

tery B. From the foregoing, it is clear that voltages of theorder of the usual signaling current when applied to the input of tube 6 willbe reproduced in the plate circuit just traced greatly amplified, the biasing of the tube 0 and the relatively high resistance ofrectifier 36beingsufficient to permit full amplification. If, however,

1 a voltage somewhat greater than that of the signaling current appears. .the grid circuit of c5 30, and thence to the negative terminal of bat tube 6,:the low gridto cathode resistance of the tube itself will greatly reduce the amplification during the positive swing of the voltage, and the relatively low resistance ofiered by the rectifier 36 .to'this higher voltage will discharge the negative swingof the voltage. Consequently, the signaling current will be amplified at stage 2 at substantially full amplification of tube 6 in the usual manner, but abnormal voltages will have their influence on the platecircuit greatly attenuated. -In other words, a signaling voltage of the orderof one volt appearing in the grid circuit of tube 6 will be amplified to the order of, say, ten volts in the plate circuit, but voltages above the one volt range will be passed to the plate circuit at a much smaller ratio of amplification.

The output of stage 2 is coupled to the input of stage 3 by means of the coupling transformer 42, the primary winding 4| of which is connected in the plate circuit of tube 6 as described above, and the secondary winding 45 of which has one terminal connected to the grid 46 of tube 1; and its-other terminal connected to the negative terminal of filament battery A. The negative bias of grid 46 will be determined by the voltage drop of .the filaments I5 and the resistor 68, and. will be in this instance of the order of ten volts. A rectifier 41 is connected across the secondary winding 45 of the coupling transformer 42. This rectifier 41 is preferably of the copper oxide type similar to rectifier 36, and is positioned to pass current in its forward direction when the terminal of secondary 45 connected to the negative side of battery A is positive. The voltage of the usual signaling current after being amplified at stage .2 will be of the order of ten volts, and rectifier 4'! will include a sumcient number of disks in series toestablish a relatively high resistance in its forwarddirection for voltages of this range, and will thusconstitute but a small load on the coupling transformer 42 at voltages of the signalingcurrent. For voltages appearing at the terminals of the secondary winding 45 above that of the usual signaling current,'the resistance of the rectifier 4] falls rapidly, and it will then constitute a relatively large load on the transformer 42 during the half cycle that the lower terminal of "secondary winding 45 is positive. As stated above the negative bias provided for the grid 46 of tube 1 is of the order of ten volts, and hence it is suificient to prevent thegrid from being made positive by the signaling current; but for larger voltages the grid 46 will swing positive and the resistance of the grid to filament of tube 1 will at this time become low and the amplification of the tube 1 will be greatly reduced.

The plate circuit for tube 1 can be traced from the positive terminal of battery B through resistor 25, reactor 26, wires and 48, primary winding v49 of a coupling transformer 50, plate 5 I, filament ID of tube 1, filament circuit through : tubes 6 and 5 to the ground connection l2, ground connection 36, and thence back to the battery B.

(It follows that voltages of the amplitude of the usual signaling current when applied to the grid circuit of tube 1 will be reproduced in the plate circuit just traced greatly amplified. Voltages larger in amplitude than those of the usual signal- 1 ing current when applied to the grid circuit of tube 1 will, however, influence the plate circuit of tube .1 at a much lower ratio, the rectifier 4'5 functioning to reduce their influence during onehalfcycle; .andthe tube ,1 itself functioning at a greatly reduced ratio of amplification during the other half cycle of such voltages.

0n the same basis as used for rectifier '36 of stage 2 the rectifier 41 should have about fifty disks in series, so that it will have a resistance of theorder of 150,000 ohmsat ten volts and only about 8,000 ohms at twenty volts. The rectifier 41 is shown connected across the secondary winding of transformer 42, however, it might equally well be used across the primary 4 l providing proper resistance for the rectifier is selected. and proper'phase relations are preserved. The asymmetry in the signaling current produced by the leak through the rectifier 41 during the negative swing of grid 46 changing at a different rate from that for the leak through the tube .1 during the positive swing of grid 46, will not noticeably affect-the quality of the telephone current, since the envelope of the carrier will be little affected.

The amplified signaling current flowing in the plate circuit for tube 1 is supplied by the secondary winding 52 of the coupling transformer to the input terminals of a detector D, the output terminals of which are connected with primary winding 53 of a transformer 54 through a low pass filter 55. The positive terminal of the detector D is connected to ground as indicated by the reference character 66, and its negative terminal is connected to the grid circuits of the tubes 5 and 6 through resistors 56, 51 and 58 toprovide the usual volume control.

After the signaling current has been detected and the carrier frequency filtered out by the .device D and filter 55, variations corresponding'to the modulation frequency of the signaling current are induced in the secondary winding 59 of transformer 54. One terminal of the secondary 59 is connected to the grids 60 and 6| of tubes 8 and 9 in parallel, and the other terminal of the secondary 59 is connected directly to the negative terminal of battery A as will be readily understood by an inspection of the drawing. Across the secondary 59 are connected rectifiers 62 and 63 in parallel and arranged to have their forward direction reverse to each other. The plate circuit for tubes 8 and 9 can be traced from the positive terminal of battery B through primary winding 2! of the output transformer '1, plates l9 and 20 of tubes 8 and 9 in parallel, filaments I0 of these tubes 8 and 9, the filament circuit to the ground connection l2, and thence by ground connection 30 to the negative terminal of battery B. It follows that the modulation frequencies of the signaling current delivered by the secondary 59 of transformer 54 are amplified at stage 4 and caused to appear in the secondary winding 64 of the output transformer T to thereby operate a loud-speaker or any other suitable signaling device connected therewith.

.Rectifiers 62 and 63 are preferably copper oxide rectifiers similar to rectifiers 46 and 41. These rectifiers 62 and 63 should have a sufficient number of disks in series as to maintain a fairly high resistance for all voltages up tothose obtained by the usual signaling current. For voltages having an amplitude greater than those of the usual signaling current the resistance of the rectifiers 62 and 63 will fall rapidly and these rectifiers will serve as a low resistance path for these higher voltages. In the case of stage 4, which follows detection, the asymmetry of the half waves of the signaling current caused by the unequal rate of change in the resistance through the rectifier and the resistance through the tubes may be .noticeable, and it is desirable to use thetwo re'ctifiers 62 and 63, one for each half-wave, so that the two halves of the wave will be retained uniform. Again, it is to be noted that the rectifiers 62 and 63 might be connected across the primary winding .53 of transformer 54 instead of across the secondary winding, providing proper resistance is selected and proper phase relations are preserved. In addition it should be pointed out that if it is desired to use fewer discs in rectifier 41 or in rectifiers G2 and 83, they might be connected across only a portion of windings 45 or 59 as shown in Fig. 3, reducing the turns used in the same ratio as the number of discs. 7

In summing up the operation of the amplifier of Fig.. 1, the telephone or signaling current is picked up from the traffic rails and amplified at stage I, the amplitude of the signaling voltage appearing in the output of stage I being of the order of one volt. Random currents and stray fields, both of which produce noise, may also be picked up by the coils2-l and 22 and these unwanted influences will be amplified at stage i the same as the signaling current. It is to be observed that in this instance the noise voltages are, however, quite small as applied to tube 5, and there is only a small change in voltages acrossthe grid leak resistor 88 produced there from. Consequently, there would be practically no change in the resistance properties of a copper oxide rectifier of suitable resistance connected in place of the grid leak resistor l8, and little would be gained by replacing this straight ohmic resistance l8 by a copper oxide rectifier. It will be understood, however, if the unwanted currents were of large magnitude it would be advantageous to replace the grid leak resistor E83 by a copper oxide rectifier similar to the rectifier '33.

The signaling voltages being of the order of one volt at the grid of tube 6, the, tube 6 of stage 2 is provided with a low negative grid bias and tube 6 operates at full ratio of amplification for the signaling current, the bias preventing the grid 3! from swinging positive and the rectifier 36 offering a relatively high resistance for the negative swing of grid 3i. In the event amplification of the noise currents by stage i results in' a relatively large voltage, a voltage somewhat higher than that of the usual signaling current and which would produce undesirable noises, the grid of tube 6 is caused to swing positive with respect to its cathode and the amplification power of tube 6 made very small during one-half of the cycle, and the resistance of the rectifier 36 is relatively low when the grid of tube fiswings negative during the other half of the cycle. Thus, stage 2 of the amplifier provides normal amplification for the signaling voltages but will function at a greatly reducedamplifying power for voltage variations of higher amplitude than that ofzthe usual signaling current, and consequently noise currents of unusual magnitude will have their influence on the output of stage 2 largely attenuated.

-At-stage 3 of the amplifier the'negative bias of the tube 7 is sufiicient to prevent grid $6 from swingingpositive with respect to the filament of tube 1' for allvoltages of the magnitude of the usual signaling current, and the rectifier it! operates at a relatively high resistance for voltages of the magnitude of the signaling current. If the voltages of the noise-current are of such magnitude as to be greater than those of l the usual signaling current, the grid 45 will swing positive during one-half of the wave and tube 1 will operate at greatly reduced amplifying power; and the rectifier 41 will operate at a greatly reduced resistance during the half wave that the grid swings negative to discharge the grid.

7 Consequently, at stage 3 of the amplifier normal amplification of the signaling current is accomplished, but noise currents will be passed with little amplification.

After detection, the stage 4 will function to amplify the modulating or voice frequencies of the signaling current at substantially full amplifying power of tubes 8 and 9, and rectifiers 62 and 63 will serve as a low resistance path for all voltages above those of the usual signaling cur rent. Consequently, the signaling current will be passed to the output of transformer T at high gain, but influences of unwanted current that cause objectionable noise will not be amplified in the same proportion, and will thus have their in-.

voltages of the signaling current at the, different stages, it will be understood that an amplifier embodying my invention is not limited to these values, and that the several parts will be readily adjusted and proportioned 'to other voltages should the incoming signaling current require. An electrical amplifier such as here disclosed provides a high gain for the incoming signaling current, and will greatly reduce the noises produced by random currents and stray fields. It is particularly'useful for sudden sharp noises, which occur before the customary automatic volume control has time to act. Although I have herein shown and described only one form of electrical amplifier embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my inven, tion. I

its operative element a series of copper oxide rectifier disks only, said disks arranged with their forward direction toward the grid and said series characterized by consisting of 'a su'flioient" Having thus described my invention, what I number of disks so as tosubstantiallypass rio current for voltage below said maximum voltage andreadily pass current at voltages above said maximum voltage. f

2. An electrical amplifier including an electron tube having a plate, a grid and a cathode; a grid circuit adapted to receive alternating voltages, a plate circuit for said tube in which said alternating voltages are to be amplified, means including a direct current source to establish a negative bias of; a given voltage for the grid, and a grid leak consisting of a series of Copper oxide rectifier disks having one terminal connected rectly with the gridand its other terminal connected at a point effectively the cathodasaid disks all arranged with their forward direction toward the grid and said serieschar'acterized by having a sufiicient number of disks toprovide a relatively high'resistance at 'said 'given' l'v'oltage and a rapidly falling resistance for voltages above said given value, whereby the ratio of amplification of the two half waves respectively of alternating voltages greater than said given voltage is reduced.

3. In combination; an electron tube having a plate, a. grid and a cathode; a grid circuit for said tube including the secondary winding of an input transformer, a plate circuit for said tube, means to supply the primary winding of said transformer with a signaling current for producing in the plate circuit variations corresponding to said signaling current amplified at substantially full amplifying power of the tube, and a shunt path connected directly across said secondary winding consisting of a pile of copper oxide rectifier disks only and all of which disks are arranged with their forward direction toward the terminal of the secondary winding connected with the grid and said pile characterized by having a sufiicient number of disks so as to substantially pass no current for voltages of the signaling current and to readily pass current when the voltages exceed those of the signaling current.

4. In combination; an electron tube having a plate, a grid and a cathode; a grid circuit for said tube including the secondary winding of an input transformer, a plate circuit for said tube, means to supply the primary winding of said transformer with a signaling current for producing in the plate circuit variations corresponding to said signaling current amplified at substantially full amplifying power of the electron tube, and a shunt path connected directly across a portion of the secondary winding consisting of a pile of copper oxide rectifier disks only and all of which disks are arranged with their forward direction toward the terminal of the secondary winding connected with the grid and said pile characterized by having a sufficient number of disks so as to substantially pass no current at voltages corresponding to the signaling current voltage across said portion of the secondary winding and to readily pass current when the voltages across said portion of the secondary winding are higher than those of the signaling current.

5. In combination, a first amplifier including an electron tube having a plate, a grid and a cathode for amplifying in its plate circuita carrier telephone current supplied to its grid circuit; a detector, means to couple the input of said detector to the plate circuit of the first amplifier for producing in its output a voice frequency telephone current corresponding to the modulation of the carrier telephone current; a second amplifier including an electron tube having a plate, a grid and a cathode, means to couple the grid circuit of said second amplifier to the output of the detector for amplifying in its plate circuit the voice frequency telephone current; a .first copper oxide rectifier characterized by having a rapidly falling resistance for voltages greater than that of the carrier telephone current, means for connecting one terminal of said first rectifier with the grid of the first amplifier tube and its other terminal at a point effectively the cathode of said tube, a second copper oxide rectifier characterized by having a rapidly falling resistance for voltages greater than that of the voice frequency telephone current passed by the detector, and means for connecting one terminal of said second rectifier with the grid of the second amplifier tube and its other terminal at a point effectively the cathode of said tube, whereby the telephone voltages are amplified at substantially full amplifying power of said first and second amplifiers and unusual random voltages are amplified at a much reduced ratio.

6. In combination, a first amplifier including an electron tube having a plate, a grid and a cathode for amplifying in its plate circuit a modulated carrier current supplied to its grid circuit, an electrical detector, means to couple the input of said detector with the plate circuit of the tube of said first amplifier to establish in its output voltages corresponding to the modulation of the carrier; a second amplifier including an electron tube having a plate, a grid and a cathode; means to couple the output of said detector with the grid circuit of the last mentioned tube for amplifying in its plate circuit said modulation voltages, a first grid leak connected between the grid and a point effectively the cathode of the tube of said first amplifier including a copper oxide rectifier characterized by having in its forward direction a rapidly falling resistance for voltages greater than that of said modulated carrier current, said rectifier arranged with its forward direction toward the grid, a second grid leak connected between the grid and a point effectively the cathode of the tube of said second amplifier and including a copper oxide rectifier characterized by having in its forward direction a rapidly falling resistance for voltages greater than that of said modulation voltages, and said rectifier arranged with its forward direction toward the grid, whereby the effect of random voltages having an amplitude greater than that of the modulated carrier current is reduced.

7. An electrical amplifier including an electron tube having a plate, a grid, and a cathode; a grid circuit adapted to receive alternating voltages, a plate circuit for said tube in which said alternating voltages are to be amplified, and a grid leak for said tube connected directly with the grid and a point effectively the cathode and which comprises as its operative element a pile of copper oxide rectifier disks only, said disks arranged with their forward direction toward the grid and said pile characterized by having in its forward direction a relatively high resistance for voltages below a given value and a relatively low resistance for voltages above said given value, whereby said grid leak also functions as a power limiting device for the half wave that renders the grid negative with respect to the cathode of alternating voltages greater than said given value.

8. In combination; an electron tube having a plate, a grid and a cathode; a grid circuit for the tube including means for receiving an incoming periodic signaling current, a plate circuit for the tube including means responsive to variations corresponding to such signaling current, and a grid leak path connected directly at the grid and at a point eifectively the cathode of the tube and consisting of copper oxide rectifier disks only and all of which disks are positioned with their forward direction toward the grid, the parts being so proportioned that during the half cycle in which the grid is negative with respect to the cathode said grid leak path is efiective to shunt away from the tube all potential above a predetermined value and that during the half cycle in which the grid is positive with respect to the cathode all potential above said predetermined value is amplified by the tube at a reduced ratio.

9. In combination; an electron tube having a plate, a grid and a cathode; a grid circuit for the "tube including means for receiving an incoming periodic signaling current, a plate circuit for the tube including means responsive to variations corresponding to such signaling current, and a grid leak connected directly with the grid and a point efiectively the cathode and including as its operative element a pile of copper oxide rectifier disks only which are positioned with their for- .410 ward direction toward the grid, the parts being so proportioned that during the half cycle in which the grid is negative with respect to the cathode said pile of rectifier disks is efiective to shunt away from the grid all potentials in excess of a predetermined value and during the half cycle in which the grid is positive with respect to the cathode all potentials above said predetermined value are amplified at a reduced ratio.

PAUL N. BOSSART. .10

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