US2077550A - Radio circuit - Google Patents

Description

April 20, 1937. DALPAYRAT 2,077,550

RADIO CIRCUIT Original Filed March 9, 1929 2 Sheets-Sheet l INVEHTOR Henr'z Francois Dalpaqral' BY 41 4 W HTTORHEY H. F. DALPAYRAT A ril 20, 1937.

RADIO CIRCUIT 1929 2 Sheets-Sheet 2 Original Filed March 9,

mvsmon: Henri FfanCplS D h qraf HTTORNEYI Patented Apr. 20, 1937 UNITED STATES RADIO CIRCUIT Henri Francois Dalpayrat, New York, N. Y., assignor, by mesne assignments, to Radio Corporation of America, a corporation of Delaware Original application March 9, 1929, Serial No. 345,654. Divided and this application July 23, 1929, Serial No. 380,273

21 Claims.

10 creasing the efiiciency of the circuit.

Another object of my invention consists in providing a radio circuit for utilizing four-electrode tubes for improving the operation of the tube, such as the removal of objectionable space charge 1 3 and at the same time, increase the efiiciency of the circuit associated with the tube by utilizing the fourth electrode as a means of reaction on the operating circuits.

Another object of the invention consists in using 2O four-electrode tubes in connection with amplifying circuits for securing an easily and finely adjustable regenerative action.

These and further objects of my invention will become more apparent by the following detailed 25 description taken in connection with the accompanying drawings. I wish it to be understood that the drawings are illustrative only of some forms of embodiment of the invention and that they are not intended in any way to have the 30 effect of narrowing the interpretation of the invention short or" its full and comprehensive scope as pointed out in the appended claims.

Figure 1 shows a simple circuit in which a fourth electrode in the form of a space charge electrode is arranged to react on the output circuit so as to produce an increased efficiency.

Figure 2 is a similar circuit to Figure 1 showing an indirectly heated cathode for the vacuum tube.

Figure 3 is a complete radio receiving circuit 40 embodying my invention, using tuned coupling stages for the amplifying tube and Figure 4 is a similar ci cuit as Figure 3, using resistance coupled tubes.

Referring to Figure 1 of the drawings, a vacuum tube of the conventional type is shown schematically having a heated cathode 2, a control or grid-electrode 5 and an anode l. M is an auxiliary electrode for removing the space charge which, in accordance with the above mentioned parent application, is arranged underneath the cathode wire in close proximity to it and within the sphere of the action of the space charge cloud established around the filament. This arrangement of a space charge grid has the great advantage that the main path of electrons between the filament and the anode is free from any obstruction and the maximum efficiency of the emission is thus secured.

The plate element 7 is connected through the primary 23 of the transformer to the positive terminal of the anode battery 24. This anode battery may be shunted by a potentiometer 25. The negative end of this battery is connected to one of the terminals of the filament at 26. The filament current is supplied by a filament heating battery 2? through a controlling rheostat 28. The auxiliary electrode I4 is connected through a separaw coil 29 to a tap 30 on the potentiometer 25. This coil 20 is shown to be connected to the voltage supply from the battery 24 in a winding relation which is opposite to the connection of the plate coil 23. A secondary coil-3| of the transformer is connected to output terminals 32. By providing a potentiometer tap to supply the potential to the auxiliary electrode, I can apply a potential which is sufficient to neutralize the space charge. At the same. time, a current will flow through this tap and through the coil 29 and since the total emission of the filament is limited and consists of the emission to the plate l and to the auxiliary electrode M, then. when the plate current through the coil 23 is being changed by variations of potential on the grid 5, there will be corresponding variations in the current through the auxiliary electrode i l, but in the opposite phase relation, that is when the plate'current is increasing, the current through [4 will be decreasing and vice versa. By virtue of the connections as illustrated and the above phase relation of currents, the effect of the two windings will be added together and the resulting pulsations in the secondary winding 3| will be the sum of the two. It will be clear from the above that each of the coils 23 and 29 carries output currents.

Figure 2 shows a similar circuit to that of Figure 1 with'the exception that a tube is used with an indirectly heated cathode. Cathode l'l preferably having a cylindrical shape and filled with refractory material, contains a filament l8 within its inside which is directly heated by an alternating current source. 19 represents the auxiliary electrode which in this case, may be arranged between filament and the outside surface of cathode IT. I9 is a normal space charge grid electrode which surrounds the outside of the cathode ll up to a short height and which is intended to assist the action of the electrode l9 arranged inside of the cathode. The electrode 19 however is not absolutely necessary and in many cases, may be omitted. In particular, the input terminals 22 are now connected to the grid 5 and cathode ll while filament l8 has leads to a filament supply transformer 33 which may be fed from an alternating current supply 34. There is a potentiometer 35 across the filament supply leads and a biasing battery 36 between the central point of this potentiometer and the cathode connection 31. The auxiliary electrode i9 is con nected to the coil 29 and potentiometer 25 in exactly the same manner as in Fig. l. The biasing battery 36 serves the purpose of placing a potential between the filament and cathode by means of which additional heating of the cathode is secured by insulation leakage currents. This is the usual type of connections for this type of tube and is for the purpose of illustration only, but does not interfere with the operation of the auxiliary electrode |9.

Having described a certain connection to the auxiliary electrode, I wish it to be understood that I am not limited to this type of connection but may change it to suit the requirements of different uses of this tube, having the following relations in view:'

If it-is desired to decrease'the electronic new from filament to plate, the 4th element (auxiliary electrode) may be connected to a source of high positive potential of such magnitude that this 4th element will attract more electrons than the plate. On the other hand, if it is desired to increase the electronic flow from filament to plate, a potential may be applied to the 4th element, just sufficient to remove the accumulation of the negative electrons around the filament forming the space charge. On removal of this space charge, as already explained above, the amplification of the tube is increased enormously. The 4th element may also be connected to a tuning circuit in such a way that its potential is modulated or varied by the plate circuit. These voltage variations may then be applied to another circuit to be further amplified. The 4th element may also be used for neutralizing purposes. This is done by providing a coupling relation between the auxiliary electrode and the plate and arranging this coupling relation to eX- ercise a suitable action for stabilizing oscillations. For this purpose, the coil 29 may be also used with reversed connection to the one shown. Under such condition, the complete oscillation control circuit is closed through the internal capacity existing between the 4th element and the filament and also the capacity between the filament and the grid and the plate, and these capacities are then adjusted in order to obtain the desired result. This adjustment may be carried out during the manufacture of the tube. For example, the capacity between the 4th element and the grid could be made equal to the capacity between grid and plate or any other desired relation. This capacity can be very easily increased by turning over theedges of the metallic plate constituting the 4th electrode M as in Fig. 1 or by increasing the size of the external control parts IQ of Fig. 2.

This tube may be further used to reduce or eliminate what is known as audio resonance when a vacuum tube is applied to audio frequency amplifiers. Such audio resonance is generally caused by inductance of the coupling transformer, its distributed capacityand-the relation of this capacity to the tube capacity. Such effect may sometimes be sufficient to cause the circuit to sing or else to distort the higher notes of the speech or music transmitted, causing a sort of buzzing noise. The usual manner to overcome this, is by introducing a high resistance across the secondary winding of the coupling transformers, that is by introducing losses and reducing the volume of sound. My system however, does not introduce any resistance. It is only necessary to apply a counter electromotive force through a separate winding to the transformer to be connected to the auxiliary electrode and in this way, oppose resonance conditions. Such a connection,

is, for instance, illustrated in Fig. 2 where the transformer, consisting of coils 23 and 3| is shown to have an iron core 3! and be suitable for audio frequency amplification. I am not restricted however, to this particular connection as the result may be attained differently, as by coupling the 4th electrode to either the plate or the grid circuit.

Referring now to Figure 3, I have shown a complete radio receiving circuit, utilizing the principle of my invention, wherein the cathode is heated directly or indirectly by alternating current supply. In this figure, item 4| is an antenna with a ground connection 42 and a coupling coil 43. The secondary 44 coupled to 43 applied its energy through the intermediary of a tuned circuit consisting of variable condenser 45 and a coupling coil 45 to a second tuned circuit consisting of coil 41 adjustably coupled to coil 46 and variable condenser 48a. This latter circuit applies the potential to the grid of the first tube 48, the plate of which has a coupling transformer 49 to the next stage of amplification. The auxiliary electrode of this tube 50 is connected to the intermediary circuit 44, 45 and 46 and through it, to an adjustable tap 5| on the potentiometer 52. across the source of direct current supply 54 which may be a battery or a suitable filtered rectifier output. I have provided condensers 5,5 and 56 across the potentiometer tap in order to facilitate the passage of alternating currents. The negative end of the supply is connected to the center point 57 of the filament potentiometer 58, the latter being in parallel with the filament supply from the transformer 59 fed by alternating currents.

The transformer 43 applies its energy to the secondary 66 in parallel with a tuning condenser 6i and thence to the grid of the second tube 52. The connection of this second tube is different from that of the first. The plate circuit of this tube is connected to the primary 63 of another transformer and thence to the positive end of the direct current supply 54. Coupled to the coil 63, is coil 64 with a. tuning condenser 65. This latter circuit connects the auxiliary electrode 55 of the second tube through a coupling coil 61 to the tap 5|.

The secondary 68 of the latter coupling coil 61 is then directly connected to the grid of the third amplifying tube 69. It will be noted that the grid circuits of the latter three tubes are connected to the biasing battery NJ and hence to the central point 51 of the filament potentiometer 58. By-pass condensers 7| and 12 may be provided across the two halves of the latter potentiometer to allow a free path for the radio frequency currents.

The plate circuit of tube 69 has a different kind of coupling to the next or detector tube 13. This coupling consists of a primary 14, a secondary 15 with a tuning condenser 76 and a coupling coil H. The coils 15 and "I1 with condenser 16 are connected between the auxiliary electrode 18 of the tube 69 and to the tap point 5|. Coil 19 coupled to coil TI is supplying energy to the grid of the detector tube 13 through a grid condenser 88 with a grid leak 8|.

With reference to these three stages of'amplification I have shown in the first stage input to tube 48, an intermediate tuning circuit 44, 45 and 46 coupled to the grid of this tube and acting in the auxiliary electrode 59. I have found that such regenerative coupling can be made and yet avoid oscillations. This circuit has the great advantage of enabling a very fine adjustment reaction. In the usual reactive circuits, in which reactive energy is taken from the output or anode circuit of the vacuum tubes and fed back into the input or grid circuit, it is difficult as known in the art, to adjust the amount of reactive energy which is very small as compared to the output energy up to the most critical point, at which the receiver would function as an oscillator. A number of circuits have been devised to secure a very fine regulation of the feed-back energy, but most of these circuits are difficult to adjust and to operate easily by unskilled persons. In the circuit according to this invention where the feedback energy is taken not from the output circuit but from the circuit including the auxiliary electrode such as space charge grid, it is obvious that a more accurate control of the feed-back energy is possible. This constitutes one of the major advantages. of the novel radio circuit arrangement in accordance with my invention.

The coupling between tube 48 and 62 is of the usual tuned grid circuit type. The coupling between tube 62 and 69 is a novel arrangement utilizing the auxiliary electrode 66 in such a way that its potential may be modulated or varied by theplate circuit. These voltage variations are then applied to another coupling transformer 61 and 68 for further amplification. Thus, the original radio frequency impulses from the antenna as amplified by the first tube are made to operate the second tube 62 near its oscillation point. But to avoid oscillations and feed-back action, the potentials in this circuit are prevented from being excessive by connections to an auxiliary electrode only and. by grounding by- pass condenser 55 and 56. I have thus secured a suitable stage of amplification with a regenerative action by coupling between the auxiliary electrode and the plate circuit of the tube.

Between the third stage of amplification in the tube 69 and the detector 13, I have provided a different type of coupling in which the intermediate circuit 15, 16, I! connected to the auxiliary electrode 18, includes in series with the tuning elements, the coupling coil 11 so that the circulating current in this tuned circuit exercises its full effect on the secondary 19 connected to the detector grid. Such an arrangement results in less energy loss in the intermediate circuit as the full energy is coupled to the secondary, but in general, it would require a special transformer construction. On the other hand, the arrangement as illustrated in connection with the plate circuit of tube 62, possess the advantage of using standard transformers and yet securing a high amplification.

For my detector tube I3, I have shown a tube of the alternating current type described in Fig. 2, wherein there is an indirectly heated cathode 140.. Between this latter and the heater filament a, I have supplied a potentiometer 16a, the central point of which is connected through a biasing battery 1'! to the cathode. This arrangement is, in general, the same as is used in the indirectly heated cathode tubes. The plate circuit of this detector is connected'to the primary 18a of an audio frequency transformer, the secondary 19 of which includes the auxiliary electrode 89a of the tube 13 and a second coupling coil No of another audio frequency transformer. The object of the intermediate circuit comprising the auxiliary electrode 80a with the coils 19a and 81a is to avoid direct electromagnetic coupling between the detector plate circuit and the grid of the first audio frequency amplifier. Such an arrangement avoids interstage resonance and consequent distortion. By properly choosing the relative direction of the windings of the coils 18a and 19a all tendency to feed-back at audio frequency is prevented.

The first audio frequency tube 82 has a grid circuit supplied by the secondary 83 coupled to coil 8Ia. and properly biased by the battery 84. The positive end of such biasing battery may be connected to the midpoint of another potentiometer 85 across the filament supply. It will be realized that the filaments of one or several of these tubes may be of different voltage, in which case they would be connected to separate special transformer secondaries. I have shown the plate circuit of tube 82 coupled by means of coils 86 and 81 to the auxiliary'electrode of the tube 82 and by means of coil 89 to the grid of the last amplifying tube 89. The object of the coupling coil 81 is to introduce a counter E. M. F. through a separate winding to the transformer output circuit of this tube, and in this way, to safeguard this stage of amplification against resonance of certain audio frequency notes. Such a phenomenon of resonance is known to occur in audio amplifiers when the inductance of the coupling transformer is in resonance with the tube capacity. It is then manifested in the increase of certain high frequency audible notes which is sometimes sufficient to cause the circuit to sing or at least to distort the higher audible frequencies near that note or else to cause a steady highpitch buzz. In the known means of avoiding such buzzing or distortion, resistors have been placed across the secondary winding of the coupling transformer or else resistance had been incorporated in the design of the transformer. It is evident that such resistance would decrease the volume or muflie the tone of reproduction. Such muffling would be especially noticeable in speech.

Thus, by providing a back coupling coil 81 to my auxiliary electrode, I avoid all of these drawbacks. If there is a tendency for audio resonance at some frequency, then by providing the necessary capacity relation within the tube to the auxiliary electrode and to the grid and by introducing an opposing counter E. M. F. through the coil 81, this tendency can be prevented.

The plate circuit of tube 89 is shown to be connected to a loudspeaker 99 while in this case, the auxiliary electrode 9| may be directly connected to its biasing potential. I have shown for the biasing potential of the auxiliary electrode of tubes 82 and 89, a separate potentiometer tap 92 which may be on the potentiometer 52 or as shown on a separate potentiometer 93 with separate by- pass condensers 94 and 96 designed to be suitable to audio frequencies.

Referring to Fig. 4, I have shown a resistance coupled circuit comprising 4 stages of amplification and a detector and have omitted the audio frequency amplifiers which may be connected in a manner similar to Fig. 3 or in any other well known manner.

The elements corresponding to Fig. 3 are indicated by similar numbers. It will be seen that the first amplifying tube 48 is coupled to the antenna 4| in the same manner as in Fig. 3. However, the output of this tube is connected to a resistance coupling element 96 which may have a small by-pass condenser 91. This resistance couples by means of condenser 98 to the grid of the next tube 82. A grid leak 99 is provided. The next two stages of amplification. may be identical or else the by-pass condenser 9.7 may be omitted. I have shown, in each case, the auxiliary electrode of the tubes 62 and following, connected directly to the tap SI of the potentiometer 52 across the source of supply 54. The auxiliary electrode 50 of tube 48 is connected in the same manner as in Fig. 3. In this case, the plate circuits of each of the amplifying tubes are shown connected to a 180-volt tap on the supply source 54, except the plate circuit of the last amplifier tube 63, which is connected through coupling transformer I00 to a 90-volt tap. The out-put circuit of this transformer may be tuned by condenser !0! and connected directly to the detector tube '13. In this diagram, I have not shown any alternating current supply to the filament but have simply connected the grids to the one side of the filament. To one skilled in the art, it will be evident that for the alternating current supply to filaments, the grid return circuits wil be modified to be the same as in Fig. 3.

By supplying an auxiliary electrode in my tube as applied to resistance-coupled amplifiers, I have made it possible to secure a very high amplification. In other words, I secure the full advantage of the high amplification of this new type of tube without the disadvantage usually present in tuned radio circuits such as instability or complicated connections. By this means, I can utilize a tube without any special provision for screening the plate or stabilization and yet secure amplification factors at radio frequency of the order of 110 to 200, which is far in excess of that possible with the old type of tubes.

I claim:

1. In a radio circuit comprising an antenna circuit, an amplifying tube having cathode, anode and grid electrodes and including an auxiliary electrode located near the cathode of said tube, a grid input circuit connected to said tube and an intermediate circuit between said antenna and said grid circuit tuned to the incoming frequency and connected to said auxiliary electrode.

2. In a radio circuit comprising an amplifying tube having cathode, anode and grid electrodes and including an auxiliary electrode located near the cathode to avoid the space charge, an input circuit, a grid circuit to said tube, a plate output circuit from said tube and an intermediary tuned circuit coupled between said input circuit and said grid circuit and connected to said auxiliary electrode.

3. In a radio circuit, comprising radio frequency amplifying stages, a four-electrode amplifying tube, an auxiliary electrode located near the cathode to counteract the formation of space charge, an input grid circuit to said tube, an output plate circuit from said tube and an intermediate tuned circuit coupled to said plate circuit, a connection from said intermediary circuit to said auxiliary electrode, a fourth circuit to be connected to a subsequent stage of amplification and a coil connected with said intermediary circuit, but external to it, said coil being coupled to said fourth circuit and further connected to said cathode through a biasing potential source.

4. In a radio circuit, comprising radio frequency amplifying stages, an amplifying tube having cathode, anode and grid electrodes and including an auxiliary electrode located near the cathode to counteract the formation of space charge, an input grid circuit to said tube, an output plate circuit from said tube and an intermediate tunable circuit coupled to said plate circuit and connected to said auxiliary electrode, a fourth circuit to be connected to a subsequent stage of amplification and a coil in series with said intermediary circuit but external to it and coupled to said fourth circuit.

5. In an audio frequency amplifying circuit; a four electrode tube including a. cathode, a control electrode, an anode, and an auxiliary electrode to counteract the formation of space charge; a plate output circuit for said tube possessing inherent resonance characteristics; a back coupling circuit associated with said output circuit and balanced by virtue of associated capacities to counteract said resonance characteristics and connected to said auxiliary electrode; an input circuit coupled to said control electrode; and an output circuit coupled to said plate circuit.

6. In an amplifying circuit, a first tube, a second tube, said first tube including an auxiliary electrode to counteract the formation of space charge, a plate output circuit for said first tube possessing inherent resonant characteristics, a back coupling circuit associated with said output circuit and connected to said auxiliary electrode of said first tube and an input circuit connected to the grid of said second tube and coupled to said plate circuit, said back coupling to be arranged in magnitude and direction with relation to the internal capacities between the electrodes of said tubes and to the inductances of said circuits, to balance said resonant characteristics of said plate output circuit.

7. In a radio circuit comprising a thermionic discharge tube having anode, cathode, and control electrodes, an input circuit associated with said control electrode, a space charge electrode for said tube disposed in close relation to said cathode at a position opposite from said anode, and tunable circuit means to produce reaction in said input circuit from said auxiliary electrode.

8. In a radio circuit comprising a thermionic vacuum tube having incandescent cathode, an anode in operative relation with said cathode, a control electrode to influence the fiow of electrons from said cathode to said anode, input and output circuits associated with said control electrode and said anode, respectively, a fourth space charge electrode having a potential applied to it with respect to said cathode and disposed relative to said cathode at a position opposite from said anode, and tuned circuit means to produce reaction in said input circuit from said fourth electrode.

9. In a radio circuit comprising an antenna circuit, an amplifying tube having cathode, anode charge electrode disposed in close relation to the cathode at a position opposite from the anode of said tube, an input circuit, a grid circuit to said tube, an anode circuit from said tube and an intermediary tuned circuit coupled between said input circuit and said grid circuit and connected to said auxiliary electrode.

11. In a radio circuit comprising an amplifying tube having cathode, anode and grid electrodes and including a fourth electrode disposed in operative relation to the cathode of said tube and located at the side of said cathode opposite from the anode of said tube, an input grid circuit to said tube, an output plate circuit from said tube, and an intermediary tuned circuit coupled to said plate circuit, a fourth circuit to be connected toa subsequent stage of amplification, and a coil in series with said intermediary circuit but external to it coupled to said fourth circuit.

12. In a radio circuit comprising radio frequency amplifying stages, an amplifying tube having cathode, anode and grid electrodes and including a fourth electrode disposed in operative relation to the cathode of said tube and at the side of the cathode opposite from. the anode of said tube, an input grid circuit to said tube, an output plate circuit from said tube, and an intermediary tunable circuit coupled to said plate circuit and connected to said fourth electrode, a fourth circuit to be connected to a subsequent stage of amplification and a coil included in said intermediary circuit and coupled to said fourth circuit.

13. In a radio circuit comprising a thermionic discharge tube having anode, cathode, and control electrodes, operating circuits connected with said control electrode and said anode, a fourth space charge electrode of said tube disposed in close relation to said cathode and located at the side of said cathode opposite said anode, and tunable circuit means to cause reaction of said fourth electrode upon one of said operating circuits.

14. In a radio circuit comprising a four electrode thermionic tube, operating circuits associated with said tube, the fourth electrode of said tube being disposed in close relation to the cathode of said tube to counteract the formation of space charge and located at the side of the cathode opposite from the anode of said tube, and tuned circuit means to cause reaction of the fourth electrode upon one of said operating circuits.

15. In an audio frequency amplifying circuit, an amplifying tube provided with cathode, anode and control electrodes, an auxiliary electrode positioned near the cathode and arranged so as to counteract the formation of space charge in the vicinity of the cathode, input and output operating circuits connected to said control electrode and said anode, means for energizing the cathode, a source of space current included in the output operating circuit, means for connecting the auxiliary electrode to said source of space current, means including a back coupling circuit to produce reaction between the output circuit and the auxiliary electrode, said lastnamed means being arranged so as to oppose the eifects of any inherent resonant characteristics of said operating circuits to thereby balance the effect thereof in said operating circuits.

16. The combination with an electron discharge device having an electron emitting cathode, an anode, a control grid, and an auxiliary electrode, said auxiliary electrode being arranged relative to said cathode at a location opposite from said anode; operating input and output circuits connected to said grid and anode, respectively; and regenerative circuit connections between one of said operating circuits and said auxiliary electrodes.

17. The combination with an electron discharge device having an electron emitting cathode, an anode, a control grid and an auxiliary electrode, said auxiliary electrode being arranged relative to said cathode in a manner as to substantially not obstruct the electron stream to said anode through said control grid; operating input and output circuits connected to said grid and anode, respectively; and regenerative circuit connections between one of said operating circuits and said auxiliary electrode.

18. The combination with an electron discharge device having an electron emitting cathode; an anode, a control grid and an auxiliary electrode, said auxiliary electrode being located relative to said cathode in a manner as to substantially not obstruct the electron stream to said anode through said control grid; operating input and output circuits associated with said grid and anode, respectively; and regenerative circuit connections between said input circuit and said auxiliary electrode.

19. In combination with a radio circuit comprising an electronic tube having cathode, anode and control electrodes, a further auxiliary electrode of said tube, grid and anode circuits associated with said tube, input and output operating circuits operatively associated with said grid and anode circuits, respectively, a tuned intermediate circuit connected between an operating circuit and the associated tube circuit, said tuned intermediate circuit being conductively connected to said auxiliary electrode, and, means for applying a biasing potential to said auxiliary electrode through said intermediate circuit.

20. In combination with a radio circuit comprising an electronic tube having anode, cathode and control electrodes, a further auxiliary electrode of said tube, grid and anode circuits associated with said tube, input and output operating circuits operatively associated with said grid and anode circuits, respectively, a tuned intermediate circuit connected between an operating circuit and the associated tube circuit, said tuned intermediate circuit being conductively connected to the auxiliary electrode, said intermediate circuit having a coil with one end connected external to it, said coil being coupled to the associated operating circuit, and a biasing potential source connected between the remaining end of said coil and the cathode of said tube.

21. In a radio circuit comprising a plurality of cascaded stages, an amplifying tube provided with a cathode, a control grid, an anode and an auxiliary electrode, a control grid circuit, a source of signal energy, a source of space current connected between the anode and the cathode of the amplifying tube and arranged so as to maintain the anode of the tube at a positive potential with respect to the cathode thereof, an intermediate tuned circuit for coupling the source of signal energy to the input control grid circuit, and

. means for applying a positive potential to the auxiliary electrode with respect to the cathode of the tube comprising a connection between the auxiliary electrode and the source of space current, said connection including said intermediate tuned circuit.

HENRI FRANCOIS DALPAYRAT.

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