US1517058A - Ing co - Google Patents

Description

Nov. 25, 2 1,517,058 ,D. GRIMES INVERSE DUPLEX VACUUM TUBE CIRCUIT Filed Dec. 1, 1923 5 Sheets-Sheet l Vjfbofl.

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D. GRIMES INVERSE DUPLEX VACUUM TUBE CIRCUIT Filed Dec. 1 1923 5 Sheets-Sheet 2 ATTORNEYS Nov. 25,1924. 1,517,058

D. GRIMES INVERSE- DUPLEX VACUUM TUBE CIRCUIT Filed Dec. 1, 1923 5 Sheets-Sheet TI M I I I I Q INVEI'QTOR Jami firzmes A'ITQRNEYS Nov. 25, 1 924.

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INVENTOR Jaw/ a fi'rl'm I BY Z 3 z nzr ATTORNEYS Patented Nov-25,1924

UNITED STATES PATENT OFFICE.

DAVID GRIMES, OF GRASMERE, NEW YORK, ASSIGNOR TO GBIMES RADIO ENGINEER- ING 00., INCORPORATED, OF GRASMERE, NEW YQRK, A CORPORATION OF NEW YORK.

INVERS -DUPLEX VACUUM-TUBE CIRCUIT.

Application filed December To all whom it may concern:

Be it known that I, DAVID Gnnvrns, a citizen of the United States, residing at Grasmere, in the county of Richmond, State of New York, have invented certain new and useful Improvements in Inverse-Duplex Vacuum-Tube Circuits; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same.

This invention relates to thermionic amplifiers, and is directed, more particularly, to improvements in multi-stage amplifiers of the inverse duplex type.

In my copending application Serial No. 589,108, there is described and claimed a vacuum tube amplification and reception system wherein the amplifier tubes are arranged to effect both radio frequency and audio frequency amplification simultaneous 1y, but in which the order of amplification differs from the previously known reflex method in that the audio frequency amplification is accomplished in the inverse order as compared with the order or sequence in which the tubes successively function to effect radio frequency amplification. Systems of thisv type are now identified by the term inverse duplex.

Prior to the introduction upon the market of vacuum tube amplifiers of especially high efficiency, which have only recently been produced, such, for example, as the vacuum tube amplifier known under the trade symbol UV QOGA, no difiiculties had been experienced with inverse duplex amplifiers arising from audio frequency oscillations being sustained, which condition, as is well known, results in continuous howling. Following the introduction of the more efiicient amplifier tubes, however, it was found that sustained audio frequency oscillations were sometimes produced by the use of these tubes with. inverse duplex systems.

To remedy this undesirable condition is the object of the present invention.

In inverse duplex vacuum tube systems it is the practice to connect the output circuit of one vacuum tube amplifier with the input circuit of the succeeding vacuum tube amplifier through the medium of a radio frequency transformer. After rectification of the amplified radio frequency signaling our- 1, 1923. Serial No. 677,955.

rents, the resultant audio frequency currents are transmitted through the medium of an. audio frequency transformer to the input circuit of the. last radio frequency amplifier tube wherein audio frequency amplification is effected. The amplified audio frequency currents are then transmitted from the output circuit of the last-mentioned tube through the medium of a second audio frequency transformer to the input circuit of the preceding radio frequency amplifier tube, wherein further audio frequency amplification is effected, thus resulting in the production of audio frequency signaling currents in the output circuit of the last mentioned radio frequency amplifier tube. If it is possible for sufficient audio frequency currents to be transmitted from the output circuit of the last-mentioned amplifier tube through the capacity coupling of the radio frequency transformer to effect considerable grid potential variation in the succeeding amplifier tube, sufficient further amplification of the audio frequency currents may be produced to overcome the attenuation of the capacity coupling path through the radio frequency transformer and thus cause sustained audio frequency oscillations by reason of the feedback between the two amplifier tubes.

lVhere it is not nacticable to sufficiently reduce the capacity coupling between the primary and secondary windings of the radio frequency transformer on account of other controlling factors, the ren'iedy, in accordance with this invention, lies in a nunr bcr of alternative circuit arrangements whereby the amplified audio frequency current in the output circuit of a preceding tube is prevented from effecting the potential of the grid of a succeeding tube sufliciently to overcome the attenuation of the capacity coupling path of the transformer. In some instances the audio frequency current is shunted away from the primary winding of the radio frequency transformer, while in other instances it is permitted to pass through the capacity coupling path of the radio frequency transformer, but is prevented from materially affecting the potential of the grid of the succeeding amplifier tube.

In addition to the circuit arrangements provided by this invention for preventing the maintenance" of continuous audio fire quency oscillations resulting from the inherent capacity coupling between the windings of the radio frequency transformer interconnecting adjacent amplifier tubes, a further feature of the invention resides in the provision of circuit connections whereby audio frequency currents which may be transmitted through the inherent capacity coupling of the radio frequency transformer interconnecting the last stage of radio frequency amplification with the detector circuit are rendered ineffective to cause the maintenance of continuous audio frequency oscillations. This feature consists in the provision of a low impedance path from the detector circuit to the cathode of the amplitier, this path being in shunt to the high impedance path to cathode by way of the audio frequency transformer in the detector circuit and the grid of the amplifier tube. By passing these leakage currents directly to cathode instead of to cathode via grid, the potential of the grid is not affected and there is accordingly little or no tendency for the circuit to oscillate.

A number of alternative arrangements which will accomplish the object in view are illustrated in the acconu'ianying drawings and described in detail hereinafter. These several circuit arrangements are shown only by way of example, and are not intended to be exhaustive of all the possible alternative arrangements within the scope of the invention.

Referring to the drawings,

Fig. 1 is a circuit diagram of a two-tube inverse duplex amplifier with a crystal de tector arranged in accordance with the previous practice (that is not embodying this invention) which is provided for the purpose of comparison in order to render a clearer description of the invention;

Fig. 2 is a circuit diagram similar to Fig. 1 but modified in accordance with this invention;

Fig. 3 is a circuit diagram also similar in general to Figs. 1 and 9., but illustrating an alternative modification;

Fig. l illustrates still another alternative arrangement and is likewise based on the circuit of Fig. 1;

Fig. 5 is a further alternative circuit arrangement; and

6 is a circuit diagram of an inverse duplex vacuum tube amplifier and receiving circuit including a single-tube power amplifier.

The circuit shown in Fig. 1 does not, as previously stated, embody the present invention. but is shown more particularly for the purpose of comparison, that is, for the purpose of illustrating the conditions which, it has been found, sometimes result in sustained audio frequency oscillations when vacuum tube amplifiers of high efiiciency are employed. The circuit now under discussion includes two stages of radio frequency amplification, two stages of audio frequency amplification, and a crystal detector. Both types of amplification are ell'ected by the same tubes simultaneously in accordance with the inverse duplex principle. The coil 1 may be a loop antenna, or it may be the secondary of a rariocoiqilcr the primary of which may be connected in an antenna circuit of the usual type. It may be assumed in the presentiust-ance that the coil l represents a loop. Across 1 terminals of the loop l. is connccted a variable condenser 2 forming an oscillatm y circuit with the loop which may be tuned to the frequency of the desired incoming signals. The loop 1 is preferably provided with a plurality of contacts for cooperating with the variable contact 3, which is connected through the conductor t to the grid of the three-electrode vacuum tube an'lplifier 6. The intensity of signal. strength applied to the grid 5 may be \aried by adjusting the moviible contact 3 to one or the other loop contacts. This may be done without appreciably affecting the tuning of the loop circuit. redu g the number of loop turns between the grid and filamentary cathode 7, the effective strength of signals from local stations may be reduced so as to prevent overloading the amplifier tubes. For receiving signals from more distantly located stations, a larger number of turns of the loop would ordinarily be connected into circuit bctween the grid 55 and filament 7, thus in creasing the effect of the incoming signals on the grid. The tube 6 has a radio fre quency output circuit including plate 8, primary winding 9 of the radio frequency transformer ll, condenser 10, which may be of relatively small capacity, and filament 7. The circuit just defined will pass radio frequency oscillations, but, on account of the sinall capacity of condenser 10, will pass little or no audio frequency current. The amplified radio frequency currents are induced from the primary 9 to the secondary 12 of the radio frequei cy transformer ll and thus impressed across the grid 14 and filament 153 of tube 13. A condenser 16 is included in the grid-filament circuit of the latter tube. This condenser will pass the raoio frequency oscillations while blocking audio frequency oscillations. The radio frequency oscillations are further amplified by the tube 13 and passed through the radio fre'qiiency transforn'ier 17 to a circuit including the secondary winding 18 of the last-mentioned transformer, crystal detector l9, primary winding 20 of the audio frequency transformer 21 and condenser in parallel with the latter wind ing.

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harness flhc amplified radio frequency oscillations are rectified in the usual manner by the crystal detector 19 and converted into the form of audio frequency signaling currents. The audio frequency currents are induced from the primary 20 into the secondary 23 of transformer 21, the terminals of which are connected respectively to the grid l-l and filament 13 of amplifier tube 13. 'lhe latter tube thus functions to effect the first stage of audio frequency amplification. The audio frequency output circuit of the tube 13 includes plate 2%, primary winding oi the radio frequency transformer 17, primary winding 26 of the second audio frequency transformer 2'7, battery 28 and filament 15. The various connections may be readily traced. The amplified audio frequency signaling currents are induced from the primary 26 into the seconoary 529 of transformer 27. The terminals of the secondary winding 29 are connected respectively to the grid 5 and filament 7 of the vacuum tube (3 through a circuit which includes a portion of the turns of the loop l. Further audio frequency amplification is effected by the tube 6, the audio frecuiency output circuit of which includes plate 8 primary winding 9, telephone receivers 230, battery 28 and filament 7.

Between the primary and secondary windings of each of the radio frequency transformers 11 and 17 there exists an in herent capacity coupling which is illustrated in dotted lines and referred to by reference numerals 31 and 32.

Taking note of the fact that the audio frequency output circuit of the tube 13 is associated through the medium of the audio frequency transformer 27 with the input circuit of the amplifier tube 6, it follows that if the capacity coupling 3., and amplification factor of tube 6 are relatively high, a sufiicient amount of audio frequency energy will pass through the capacity coupling 31 to appreciably affect the poten tial of grid 14 and thus maintain con tinuous audio frequency oscillations. One remedy for the condition described is to employ vacuum tubes having a relatively low amplification facton that is, of low efficiency; another remedy may be to separate the primary and secondary windings of the radio frequency transformer to such an extent as to greatly reduce the capacity coupling therebetween. Each of the foregoing remedies, howeven results in lowered elliciency.

According to the present inventiom a number of different circuit arrangements have been found whereby this objectionable condition can be overcome without sacrificing the increased efficiency insulting; from the employment of close coupling and high amplification tubes. i i hile the several alteri'iative circuit arrangements illustrated appear to differ radically in form, there is an underlying principle common to all of them.

Still referring to Fig. 1, let it be assumed that the grid 5 is for an instant rendered negative by the amplified audio frequency potential impressed thereon. This will result in a stoppage 0 reduction of flow of space current within the tube The in-- ductive discharge from the telephones 30 in following the path of least resistance will pass through the capacity coupling 31.. thence tl'irough grid '14. to filament 15 and battery 28, thereby coi'npleting; a discharge circuitfor the phones 30. lhe action just enumerated results, of course, in a change of potential on the grid la, the further re sult of which is an. amplified reproduction of the audio frequency current variation in the output circuit of tube 13. This audio frequency current variation is then repeated back to the input circuit of tube 6 through the medium of transformer 27. The condition is therefore a regenerative one, resulting in the maintenance of oscillations the frequency of which is in accordance with the constants of the circuit.

A similar condition exists between the amplifier 13 and the detector circuit, arising from the inherent capacity coupling 32 of transformer 17. Any substantial audio frequency leakage through the capacity path will tend to flow to cathode by way of grid 14 and the capacity path between the windings of transformer 21.. lhis condition is overcome in each of the several alternative circuit arrangements in the manner hereinafter described.

In Fig. 2 the phones 30 are connected between the plate 8 and the primary winding 9 and are shunted by a sma l condenser 33. In this circuit arrangement there is little or no tendency for current discharges from the telephone receivers 30 to seek a path to filament through prid 14 because of the fact that there is a very much easier path to filament through the battery 28. Of course. when the resistance of the space path through tube 6 of Fig. 2 is increased by reason of increased negative potential on grid 5, there is some tendency for the current from battery 28 to follow the. capacity coupling path 31 to grid i l and thence to filament, but the potential of battery 28 is small as compared with the instantaneous induced potential arisingfrom the sudden collapse of the n'iagnetic field of the tcle iihone receivers 30 so that there is a vast difference in the resultant effect on the potential of grid 14;. This ar ane'enient does actually eliminate the condition described which resulted in the maintenance of continuous audio frequency oscillations and permits the use of high efficiency vacuum tubes and. radio frequency transformers haying :cnsiderable capacity coupling.

in the present circuit, as well as .111 each of those to be described hereinafter, the detector circuit is tied to the cathode 15 by conductor it) which provides a shunt path for audio frequency leakage through the capae ty coupling 32. By careful examination of the circuit it will be seen that the conductor 4.6 parallels a somewhat tortuous circuit which may be traced as follows: from the output circuit of tube 13 tl'irough ti. 0 ca orudty coupling 32, one or both sides of the detector circuit to primary winding 20, which for the present consideration functions as a condenser plate in cooperation with the secondary winding 23 functioning as the other plate, thence through grid 14; and the space path to cathode 15. Any sul'istantial current flowing through the path justtraced would, of course, affect the potential of grid 14. But since the current which would otherwise tend to flow in that path is shunted directly to cathode tl'irough the conductor 16, its effect is obviated.

1n circuits like that of Fig. 2, radio frequency transformers having inherent capacity coupling between the windings of .001 microfarad have been employed with good results. So far as is known, any good commercial radio frequency or audio frequency transformers may be employed with this circuit as well as with the circuits to be hereinafter described. The voltage of the plate battery 28 may range from to 130 volts with goo-d results. it is thought that still higher voltages may be employed without any detrimental effect when using vacuum tubes designed for higher plate. voltages.

In the circuits of Fig. 3 the audio frequency output in the plate circuit of vacuum tube 6 is substantially prevented from reaching the primary winding 9 of the transformer 11 by means of a small blocking condenser 3 1. The capacity of this condenser may be of the order of .001 microfarad. The audio frequency output circuit of tube 6, which includes telephone receivers 30, is connected in parallel with the primary winding 9. ln order to prevent the transmission of radio frequency currents through the audio frequency path which might otherwise occur due to the capacity of the telephone cord, a radio frequency choke coil 35 is inserted in the audio frequency output circuit as shown. The inductance of coil 35 may range from 25 to 200 millihenrics. These values are given only by way of example, and are not to be regarded as limiting. It is thought to be obvious without further discussion that very little, if any, audio frearness quency current can reach the primary winding 0 through condenser 34;, and therefore little or none can pass through the inherent capacity coupling of the transformer 11 to affect the grid 14: of the succeeding amplifier tubes. Any audio frequency current which may pass the condenser 34 has a low impcdance path to filament through conductor 4;? and therefore has very little tendency to pass the coupling capacity 31. The circuit of Fig. 3 may be easily understood by comparison with that of Fig. 1, from which it differs only as already noted.

The circuit of Fig. f is soinewl'iat analogous in its operation to that of In this case, however, no provision is made against the transference of audio frequency energy through the inherent capacity cou pling of the radio frequency transformer 11. ltn order to prevent such energy being impressed on the grid 14:, a small condenser 36 of capacity .0005 microfarad approximately, is inserted in the grid-filament circuit of tube 13 as shown. The audio frequency input to grid 14 is connected between the grid and the condenser 36, a radio frequency choke 37 being preferably included in this audio frequency input circuit. In some instances it may be possible to omit the choke coil 37. Whether or not this coil may be omitted depends very largely upon the pres ence or absence of radio frequency shunts across the terminals of the secondary winding This fact applies equally to the circuit of Fig. 3. In all instances where it is found that the audio frequency input circuit to the tube 13 provides a substantial path for the radio frequency currents, it is desirable, but not necessarily essential, to insert the coil 37. The inductance of coil 37 mayrange from 25 to 200 millihenries. These values, however, are merely given by way of exam ple, and are not to be regarded as limiting.

1n the circuit of Fig. 5 no provision is made to guard against the transference of audio frequency energy through the inherent capacity coupling of the radio frequency transformer 11. In this circuit the secondary winding 23 of transformer 21 is connected in series with the secondary winding 12 of the radio frequency transformer 11, and is interposed between the grid 14 and winding 12, there being a small condenser 38 shunted across the terminals of the winding 23. lVith this arrangement the winding 12 is connected directly to the filament circuit. The path to filament for any audio frequency energy which may be transferred through the capacity coupling of transformer 11 by way of the grid 14: is of such enormously greater impedance, on account of the inclusion of Winding 23, than the direct path to filament from the winding 12 via conductors 39, 40 and 4-1, that the potential of grid 14 is not likely to be materially afiected by such energy. The capacity of condenser 08 may be of the order of .0005 microfarad with good results.

The circuit of Fig. 6 is similar, in general, to that of Fig. 3, but includes an additional power amplifier tube 42. The primary winding 43 of transformer 4:4: is inserted in the audio frequency output circuit of tube 6. The secondary winding a5 is connected in the grid-filament circuit of tube 42. The telephone receivers 30 are connected in the plate circuit of tube 42. In so far as the present invention is concerned, the disclosure of Fig. 6 is identical with that of Fig. This circuit is shown, however, in order to make it clear that a power amplifier can be combined with the inverse duplex circuit described in the other figures.

In all of the several circuits shown, the telephone receivers 30 may be replaced by a loud speaking instrument.

In each of the circuits shown the crystal detector circuit can be replaced by a vacuum tube detector circuit without detriment.

I claim:

1. In an amplifier including at least two electron discharge devices each comprising anode, cathode and control electrode, wherein said devices both operate simultaneously for effecting both radio frequency amplifica-- tion and audio frequency amplification, the method of preventing the maintenance of audio frequency continuous oscillations which consists in reducing or preventing the flow of audio frequency current from a current source in the output circuit of one of said devices to the cathode of the last-mentioned device through the medium of the control electrode and cathode of the other of said devices, thereby reducing or preventing audio frequency potential difierence variations between the last-mentioned control electrode and cathode arising as a result of audio frequency current variations in said output circuit.

2. In an inverse duplex amplification system, two electron discharge amplifiers each comprising an anode, a cathode and a control electrode, said amplifiers being operatively associated so as to consecutively effect radio frequency amplification in a prede termined order and audio frequency amplification in the inverse order, a radio frequency output circuit for one of said amplifiers and a radio frequency input circuit for the other of said amplifiers, said radio frequency circuits being associated to effect radio frequency transmission from the first radio frequency amplifier to the second radio frequency amplifier, an audio frequency input circuit for said first radio frequency am plifier and an audio frequency input circuit for said second radio frequency amplifier, means for impressing audio frequency currents on the audio frequency input circuit of said second radio frequency amplifier, the audio frequency currents being amplified by the second radio frequency amplifier, an audio frequency output circuit for said second radio frequency amplifier, said lastmentioned audio frequency output circuit being operatively associated with said audio frequency input circuit of said first-mentioned radio frequency amplifier for effecting the transmission of amplified audio frequency currents from the output of said second radio frequency amplifier to the input of said first radio frequency amplifier, and means for substantially reducing audio frequency potential difference variations be tween the cathode and control electrode of said second radio frequency amplifier arising from audio frequency current variations in the output of said first radio frequency amplifier.

3. An inverse duplex amplification system including two electron discharge amplifiers each having an anode, a cathode and a grid, a radio frequency transformer comprising a primary and a secondary winding for inductively interconnecting th output side of one of said amplifiers to the input side of the other of said amplifiers for the transmission of radio frequency signaling current therebetween, said primary and secondary windings having inherent capacity coupling capable of providing a path for audio frequency signaling currents of sufficient energy to effectively vary the potential of the grid of the last-mentioned amplifier if permitted to flow to said grid, as a result of which condition undesirable audio frequency oscillations might be sustained, and circuit means adapted to prevent suflicient audio frequency currents flowing through the path including the said capacity coupling and said lastmentioned grid to the cathode of said first amplifier to effect sufiicient audio frequency amplification to sustain audio frequency continuous oscillations.

4:. An inverse duplex amplification system comprising at least two electron discharge amplifiers connected in cascad and adapted to effect radio frequency and audio frequency multi-stage amplification simultaneously, circuit connections for transmitting radio frequency signaling current through successive amplifiers in a certain order, circuit connections for transmitting audio frequency signaling current through said amplifier successively in the inverse order with respect to the order of radio frequency amplification, and circuit connections arranged to prevent the effective transmission of andio frequency signaling currents from the output side of one amplifier to the input side of a successive amplifier in the order predetermined for radio frequency amplification.

5. An inverse duplex receiving system comprising a plurality of three-electrode electron discharge amplifiers, each of said amplifiers comprising an anode, a cathode and a grid, a detector clrcuit operable to convert modulated radio frequency currents into audio frequency currents, circuit connections for operatively associating said amplifiers and said detector circuit in such manner that said amplifiers are operable to effect multi-stage radio frequency amplification in a predetermined order and further operable toeffect multi-stage audio frequency amplification in the inverse order as compared with the aforementioned predetermined order, the arrangement being such that the fully amplified radio frequency current is converted by said detector circuit into audio frequency current, the output side of at least one of said amplifiers being associated with the input side of the succeeding amplifier through the medium of a radio frequency transformer the primary winding of which is interposed in the output circuit of the first-mentioned amplifier and the secondary Winding of which is interposed in the input circuit of the second-mentioned amplifier, a signal indicating device interposed in the output circuit of said firstanentioned amplifier between the anode thereof and the primary Winding of said radio frequency transformer, and a radio frequency path in shunt to said indicating device.

6. In a. signal receiving system, a threeelectrode electron discharge amplifier comprising anode, cathode and grid, a radio fre quency input circuit for said amplifier including said grid and said. cathode, a radio frequency output circuit for said amplifier including said anode and said cathode, a radio frequency transformer comprising a primary Winding and a secondary Winding, there being inherent capacity coupling be tween said windings, said primary Winding being connected in the radio frequency output circuit of said amplifier, detector circuit including the secondary winding of said radio frequency transformer, an audio fre-- quency transformer comprising a primary winding and a secondary winding, the primary winding of said audio frequency transformer being connected in said detector circuit, an audio frequency input circuit for said amplifier including the secondary winding of said audio frequency transformer together with said grid and said cathode, the arrangement being" such that modulated radio frequency currents are amplified by said amplifier and transmitted through said radio frequency transformer to said detector circuit, said detector circuit being operable to convert said radio frequency currents into audio frequency currents which are trans mitted through said audio frequency transformer to the audio frequency input circuit of said amplifier, and a conducting path of low impedance from said detector circuit to said cathode, said path being operable to form a short circuit for audio frequency currents which may pass through the inherent capacity coupling of said radio frequency transformer, thereby preventing the maintenance of continuous audio frequency oscillations in said system.

.ln testimony whereof I affix my signature.

DAVID GR IMFJS.

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