USRE22576E

USRE22576E - High-frequency signal-translating

Info

Publication number: USRE22576E
Authority: US
Priority date: 1938-10-22
Publication date: 1944-12-12
Also published as: US2188504A; GB535969A

Description

Dec. 12, 1944.- R. FREEMAN HIGH-FREQUENCY SIGNAL-TRANSLATING STAGE Original Filed Oct. 22, 1938 2 Sheets-Sheet l ancneouo FIG.2.

FIG.\.

INVENTOR Grid Vohuql FlG.4.

Grid Voliuqu AVG ROBERT L. FREEMAN FIG.5.

ATTORNEY Dec. 12, 1944. R. L. FREEMAN ;;Re. 22,5

HIGH-FREQUENCY S IGNAL-TRANSLATING STAGE Original Filed 001;. 22, 1938 2 Sheets-Sheet 2 3G ,ooooooooooooo 0O 0 O O 0000 Q 0 o o 0000 62- h-DOOO 0 O 0 film 0000 o o o 0000 36 OOOOODOO0 +5 INVENTOR AVG R'yfiBERT L. FREEMAN ATTORNEY FIG. IO.

Res-ea Dec-12,1 9 Ref-22,516

' UNITED *STATES PATENT Y OFFlCE,

men-memoir srcuurrmsmrmd s'rsca Robert L. Freeman, Flushing, N. Y., assignor to Haseltine Corporation, a corporation of Dela ware Original No.\2,188,504, dated time so, 1940,

Serial No. as

,540, October 22, 1988.

Amalia-- tion for reissue Apr-i129, 1941, Serial No. seam c 25 Claims. .(01. 179-111) This invention relates generally to high-fraquency signal-translating stages and particularly to stages for operation at such highfrequencies that the conductance of the input circuit of the stage is an appreciable factor in the response.

in the response of the stage. Thereason for this condition is that the maximum impedance which can-be developed across the input circuit of the stage at these frequencies is limited by'the input conductance of the tube.

It is an object of the present invention to provide an improved high-frequency signal-translat ing stage which is not subject to the abovement'ioned disadvantages.

It is a further object of the invention to provide a high-frequency signal-translating stage, the response of which is not substantially limited by the input conductance of the vacuum tubes utilized in the stage.

- In accordance with the invention, a high-frequency signal-translating stage comprises a vacuum-tube electrode structure effectively including a plurality of space discharge paths in parallel'. Certain of the discharge paths comprise the elements of a conventional vacuum tube including input electrodes having an appreciable positive conductance therebetween. Others of the discharge paths comprise input electrodes including a control electrode and a cathode, togetherwith means for forming a virtual cathode adiacerit the control electrode and between the control electrode and the cathode, whereby a negative conductance exists between the input electrodes. The spacing of the electrodes and their normal operating potentials are so proportioned that the above positive and negative conductances are complementary over the operating range of the stage. \4

In one embodiment of the invention, the tw discharge paths may be comprised in separate vacuum tubes. In accordance with the preferred embodiment of the invention, the means for forming the above-mentioned virtual cathode comprises two electrodes between which the neg- -atively biased control electrode associated with tentials are applied to the two,above-mentionei electrodes to form the virtual cathode, thereb: providing a negative conductance between the control electrode and cathode, that is, between input electrodes of the tube.

In another preferred embodiment of the invention, the two above-mentioned space discharge paths are comprised in the same vacuum tube, the tube having conventional electrodes disposed in one of the space paths and means for formin the above-mentioned virtual cathode disposed in the other of said space paths.

The novel features which are believed to be characteristic of the invention are set forthwith particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with other and further advantages thereof, will best be understood by reference to the following speciflcation taken in connection with the accompanying drawings, in which Fig. 1 is avsimplifled circuit diagram utilized to derive the input conductance characteristic of a conventional vacuum tube circuit; Fig. 2 is a fragmentary View of the basic electrode structure of a tube providing a negative input conductance; Figs. 3 and 4 are input conductance graphs of a tube having the electrode structure of Fig. 2 and of a conventional vacuum tube, respectively; Fig. 5 is a circuitdiagram of a high-frequency signaltranslating stage in accordance with the invention utilizing :two vacuum tubes; and Figs. '7, 9 and 11 illustrate high-frequency signal-translating stages in accordance withthe invention utilizing the unitary tube structures of.Figs 6, 8,

and 10, respectively.

' The input conductance of conventional vacuum tubes is causedby three factors: (1) the dielectric losses of the tube insulators and the tube base; (2) feedback between the cathode and grid circuits through the grid-cathode capacitance of the tube and the self-inductance of the cathode lead, the latter being'common to the output and input circuits; and (3) the transit time of electrons through the tube. The input conductance caused by dielectric losses is usually small compared to that caused by feedback through the grid-cathode capacitance and the self-inductance of the cathode lead and also is small as compared to that component of input conductance dependent on the transit time of electrons through the tube.

put and output circuits of a high-frequency sigrial-translating stage utilizing a conventional vacuum tube, reference is made to Fig. 1. Fig. 1 comprises a high-frequency signal-translating stage including a vacuum tube i0, inputtermlnals ll, 12 and output terminals ll, ll. The grid-cathode capacitance of tube III is represented by condenser I, while the inductance of the cathode lead is ,represented by inductance IS. The impedance of the output circuit is represented schematically by impedance II connected across output terminals ll, II.

If V is the voltage applied to the input terminal ll, l2 and i the input current which flows because of this voltage, the following equations are; obtained neglecting the components of input conductance due to dielectric losses and to transit time of the tube:'

where is is the cathode current and is the angular frequency.

where VI is grid-cathode voltage and 9k is the change in cathode current for a change in grid voltage. Therefore As the cathode-lead inductance of conventional tubes is usually of the order of 0.04 microhenry, while the grid-cathode capacitance of such tubes is of the order oi 7 micromicrofarads, the resonant frequency of these two reactances is about 300 megacycles. Therefore, for frequencies up to 100 megacycles Jim is small compared to l/mClS and the former may be neglected. The input admittance Y of the tube may then be expressed as:

Or if rationalized and the termwhLm' in the denominator is neglected From Equation 4 the expression for the component of input conductance Gr due to feedback through the grid-cathode capacitance and the inductance of the cathode lead is found to be:

The component of input conductance oi. a conventional vacuum tube dependent on the transit of electrons through the tube, has been obtained mathematically by D. 0. North (see Proceedings of the Institute of Radio Engineers, volume 24, No. 1, January 1936, page 108 et seq.) A mechanical explanation of the effect of the transit time of electrons on the input conductance of a vacuum tube has been developed by W. R. Ferris (see Proceedings of the Institute of Radio Engineers, volume 24, No. 1, January 1936, pa e 82 et seq.). These authors have shown that the component of input conductance Gt of conventional vacuum tubes dependent on the transit time of electrons through the tube is:

conductance caused by feedback between the in- Certain vacuum tube structures under certain operating potentials exhibit a negative input conitive potential. This basic tube structure for'a tube having co-axial electrodes, is represented by the fragmentary view of Fig. 2, wherein a control electrode 20, associated with a cathode 2|, is shown interposed between an accelerating or space-charge grid electrode 22 and an anode 23. Experiments with a type of tube equivalent to that of Fig. 2, wherein the first or accelerating grid 22 was connected to a source of potential of +50 volts, and the anode operated at a potential of +200 voltage, showed that a negative conductance of several micromhos obtained between the grid 20 and the cathode at 6 megacycles with the grid 20 biased to 3 volts. Further, this negative conductance decreased with increasing negative bias-on the grid 20. The grid conductance bias characteristic of; such a tube is shown in Fig. 3 while the corresponding characteristic of a conventional vacuum tube conventionally connected in a signal-translating stage is shown in Fig. 4.

It has also been determined experimentally that the negative input conductance of a vacuum tube, as represented by the characteristic of Fig. 3, varies somewhere between the 1.5 and 2.0 power of the operating frequency. This, of course, is nearly the same law of variation as that of the positive input conductance of conventional tubes, as indicated by Equations 5 and 6 above. It is seen that the characteristic curves of Figs. 3 and 4 are generally similar in form but opposite in polarity. It is, therefore, proposed, in accordance with the present invention, to utilize space discharge paths in parallel having difierent types of characteristics and so to adjust the spacing of the electrodes and their operating potentials that the conductance characteristics of the several space paths are complementary over the operating range of the stage. The component input over the frequency range as well as the grid-bias will be noted that Equations 5 and 6 are of simrange of the stage, that is, for all operating con-' ditions of the stage.

An arrangement of the type just described is shown in Fig. 5 which comprises a conventional electron discharge device and. an electron discharge device having an electrode structure including the basic form illustrated in Fig. 2 to provide a virtual cathode between its control electrode and its cathode. The high-frequency signal-translating stage of Fig. 5 comprises input terminals 21, 28 to which are coupled the primary winding of selector transformer 29, the secondary winding of which is tuned by condenser 30. The input electrodes of each of tubes 25 and 26 are connected across the selector circuit 29, 30 in a conventional manner. A tuned circuit 3|, 32 is included in the common anode circuit of tubes 25, 26, while output terminals 33, 34 are coupled across tuned circuit 3|, 32. Suitable operating potentials are applied to the electrodes 0! tubes 25, 26 from sources indicated at +80 and +B while a suitable amplification control potential is applied to the signal input grids from a suitable source indicated at A. V. C.

In considering the operation of the stage of amplification Just described, it will be seen that tube 25 has the conventional type of grid voltage-input conductance characteristic shown in Fig. 4 while tube 20 has the type of characteristic shown in Fig. 3. It is seen that these characteristics are approximately complementary over a" wide range of grid-bias voltage which may be adjusted either manually or by a conventional automatic amplification control system as indicated by A. V. C. Therefore, the resultant input conductance of the high-frequency signal-translating stage is materially reduced and may, in fact, with the proper choice of tubes, be reduced substantailly to zero by the operation of tubes II and 26 in parallel.

The two parallel space-discharge paths provided by tubes II and II of Fig. are replaced by a single composite tube in the embodiment of the invention illustrated in Fig; 6. A signaltranslating stage including the electrode structure of Fig. 6 is shown in Fig. 7. The electrode structure of Fig. 6 comprises a cathode II, a helical control or signal input operating range and coextensive with cathode 3i and a cylindrical anode 31 coaxial with cathode 35 and electrode 38. A suppressor grid 38 connected directly to the cathode 35 by conductor 39 and a screen grid 40 is.

electrode structure of Fig. 6 includes also the electrode structure of Fig. 2 so that, for this portion of the tube, a virtual cathode exists adjacent the control or signal electrode 38 and between the control or signal electrode 36 and the cathode 85. The portion of the tube in which this virtual cathode is eflective tends to provide a negative input conductance. In order to provide the virtual cathode an auxiliary or accelerating electrode, commonly known as a space-charge grid, is disposed between the control or signal electrode 36 and cathode is and extends along .only a portion of the cathode surface so that it is effective over only a portion of the cross-sectional area or the electron stream emitted by the cathode. the auxiliary electrode 4| being adapted to be operated at a potential positive with respect to that of the cathode 35. As thus arranged, the control or signal electrode I i effective over a substantially larger portion of the cross-sectional area of the electron stream than is the auxiliary or accelerating electrode ll.

A second auxiliary electrode 42, axially coextensive with auxiliary or accelerating electrode H and connected to cathode 35," is disposed between electrode 4| and cathode 35 in order to reduce the amount of current which would otherwise be drawn by the accelerating electrode II. It will be understood that in order to provide a suitable transconductance the control or signal electrode 30 must be located reasonably close to cathode II and it may be found expedient to locate only that portion of electrode ll which is coextensive with electrodes 4i and II at a sufll- I space-charge grid Ii is adjacent the cathode over the remainder of the cathode surface.

A circuit utilizing the'electrode' structure of Fig. 6 to provide a stage of high-frequency ampliflcation having very low or substantially ssro input conductance is shown in Fig. 7.. The circuit is generally similar to that of Fig. 5 while the electrode structure isthat of Fig. 8, similar circuit elements being given reference numerals identical to those of Figs. 5 and 8. It is believed that the operation of the circuit of Fig. '1 will be apparent from the description which has been given above with,.reference to Fig. 5 and with reference to the electrode structure of Fig. 8, the space path including electrodes 4| and 42 corresponding to the tube 20 of Fig. 5 and the space path excluding such electrodes corresponding to the tube 25 of Fig. 5.

The tube structure of Fig. 8 and the circuit of Fig. 9 are generally similar to those of Figs.

' a 7 respe y. and similar circuit elements have been given identical reference numerals. The primary diflerence between the tube structure of Fig. 6 and that of Fig.'8 is that auxiliary electrodes 4| and 42, the effective surfaces of which are restricted to a, particular portion of cathode 35, have been re'placed by electrodes ill and 52, respectively. Electrodes SI and 52 are each in the form of a helix, the length 01' which is approximately equivalent to that of cathode 35, but the pitch of which is such that there are appreciable portions of cathode 35 upon which electrodes 5i and 52 are not effective, these portions of the tube providing space discharge paths which are eflective to provide a positive input conductance in the normal manner.

The tube structure of Fig. 10 and the circuit of Fig. 11 are identical with those of Figs. 8 and 9, respectively, except for the fact that electrodes II and 52 having a uniform spacing between the turns thereof have been replaced by electrodes 6! and 62, respectively, each of which forms a helix having a, variable pitch.

While there have been described what are at present considered to be the-preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A high-frequency signal-translating st'age charge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having appreciable positive conductance therebetween, others of said discharge paths comprising input electrodes including a control electrode, a. cathode, and a positive electrode between said control electrode and said cathode, .whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are substantially equal in magnitude for all operating, conditions of said stage.

2. A high-frequency signal-translating stage comprising vacuum-tube electrode structure effectively includinga plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventionalelectrodes includconductance therebetween. othersoi' said disa tsve ing input electrodes having appreciable positive' chargespaths comprising two electrodes and input electrodes including a control electrode disposed between said two electrodes. and means 'ior applying a positive potential to each of said two electrodes, whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are substantially equal in magnitude for all operating conditions of said stage.

3. A high-frequency signal-translating stage for operating over a wide range of frequencies comprising vacuum-tube electrode structure efiectively including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having appreciable positive conductance therebetween, others oi said discharge paths comprising input electrodes including a control electrode and a cathode, means for forming a virtual cathode .between said control electrode and said cathode, whereby a negative transconductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are approximately equal in magnitude over the igequency range of said stage.

4. A high-frequency signal-translating stage comprising vacuum-tube electrode structure's!- iectively including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having appreciablepositive conductance therebetween, others of said discharge paths comprising input electrodes including a control electrode, a cathode, and a positive electrode between said control electrode and said cathode, whereby a negative conductance exists put and output electrodes and having. appreciable positive conductance between said input electrodes; others of said discharge paths comprising two electrodes, input electrodes including a control electrode disposed between said two electrodes, and output electrodes; means for applying a positive potential-to each of said. two electrodes, whereby a negative transconductance exists between said last-mentioned input ,electrodes; said two sets of input electrodes and said two sets of output electrodes being respectively coupled in parallel in said stage; and the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are substa'ntiallycqual in magnitude for all operating conditions of said stage.

6. A high-frequency signal-translating stage comprising vacuum-tube electrode structure ef-, fectively including a plurality of electron discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including a-eathode and a control electrode an appreciable positive conductance therebetween,

others oi said dischargepaths including two elec-' trodes, a cathode, and a control electrode disposedbetween said two electrodes, means for applying a positive potential to each of said two electrodes, means for biasing each of said control electrodes negatively with respect to its associated cathode, whereby a negative conductance exists between said'last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are substantially equal in magnitude of said stage.

'1. A high-frequency signal-translating stage comprising vacuum-tube electrode structure eiiectively including a plurality of.clectrondischarge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having an appreciable positive conductance therehetween, others of said discharge paths comprising two electrodes and input electrdoes including a control electrode disposed between said two electrodes, means for applying a positive potential to each of said two electrodes, whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are approximately equal in magnitude whereby said conductances vary as substantially the same function. of frequency over the operating range of said stage and the efl'ective for all operating conditions zero over said operating range;

8. A high-frequency signal-translating stage comprising vacuum-tube electrode structure ctfectively including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having appreciable positive conductance therebetween, others of said discharge paths comprising two electrodes and input electrodes including a. control electrode disposed between'said two electrodes, means for applying a positive potential to each oi said two electrodes whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so proportioned that said conductances are approximately equal in magnitude, whereby said conductances vary between the limits of 1.5 and 2.0 power of the frequencyv over the operating range of said stage and the eilective input conductance to said stage is substantially zero over said operating range.

9. A high-frequency signal-translating stage comprising vacuum-tube electrode structure e1- iectively including a plurality of electron-discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having an appreciable positive conductance therebetween, others of said discharge paths comprising two electrodes and input electrodes including a control electrode disposed between said two'electrodes,

means for applying a positive potential-to each of said two electrodes, whereby a. negative conductance exists between said last-mentioned input electrodes, said input electrode being operable over a wide range of bias voltages, the spacing oi said electrodes and their normal operating potentials being so proportioned that said conductances vary substantially as the same function discharge paths in parallel, certain of said discharge paths comprising conventional electrodes including input electrodes having an appreciable positive conductance therebetween, others of said discharge paths comprising a cathode and a control electrode, means for forming a virtual cathode between said cathode and said control electrode, whereby a negative conductance exists.

tween said cathode and said control electrode,

e spacing of said electrodes and their normal.

operating potentials being so proportioned that said conductances are substantiall ual in magnitude for all operating conditions of said stage.

11. A high-frequency signal-translating stage comprising a single vacuum-tube electrode structure eflectively including a plurality of electrondlscharge paths in parallel, certain of said paths comprising conventional electrodes including input electrodes having an appreciable positive conductance therebetween, others of said discharge paths comprising two electrodes and input electrodes including a control" electrode disposed between said two electrodes, and means for applying a positive potential'to each of said two-electrodes, whereby -a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal ture' effectively including a plurality of electronover another portion of the cross-sectional area of said electron stream.

14. An electron discharge device having a cathode for providing a stream of electrons and an anode for receiving said electrons, an accelerating electrode near said cathode and between said cafiiode and anode eifective over only a portion of the cross-sectional area of said electron stream, a signal electrode positioned between said accelerating electrode and said anode over said portion a of the cross-sectional area oi said electron stream operating potentials being so proportioned that i others of said discharge paths comprising two electrodes and input electrodes including a cathode and said control electrode, said control electrode being spaced between said two electrodes and the one of said two electrodes between said control electrode and said cathode having an effective area which is appreciably smaller than said control electrode, thereby being effective only upon said others of said discharge paths, means for applying a positivepotential to each of said two electrodes, whereby a negative conductance exists between said last-mentioned input electrodes, the spacing of said electrodes and their normal operating potentials being so propcrtioned that said conductances are substantially equal in magnitude for all operating conditions of said stage.

13. An electron discharge device having a cathodefor providing a stream of electrons and an anode for receiving said electrons, an accelerating electrode near said cathode and between said cathode and anode effective over only a portion of the cross-sectional area of said electron stream, and a signal electrode positioned between said accelerating electrode and said anode over said portion of the cross-sectional area of said electron stream and positioned next to said cathode and positioned next to said cathode over another portion or the cross-sectional area of said electron stream, and a screen grid positionedbetween said signal electrode and the anode.

15. An electron discharge device having a cath ode and a coaxial anode surrounding said cathode. a space-charge grid adjacent said cathode but extending along only a portion of the cathode surface, and a control electrode surrounding said cathode and space-charge grid and coextensive with the cathode surface and positioned between said space-charge grid and said anode over said portion of said cathode surface and positioned next to said cathode over the remaining portion of said cathode surface.

,16. An electron discharge device having a cathode and an anode, a space-charge electrode adjacent to said cathode effective only over a portion of the surface of the cathode, and a signal electrode coextensive with said cathode and positioned between said space-charge grid and said anode over said portion of said cathode surface and positioned next to said cathode over the remaining portion of said cathode surface.

17. An electron discharge device having a cathode and a coaxial anode surrounding said cathode and a space-charge grid positioned adJacent to said cathode but extending over only a portion of the cathode surface, and a control grid surrounding said cathode and space-charge grid and positioned between said space-charge grid and said anode over said portion of said cathode surface and positionednext to said cathode over another portion of said cathode surface, and a screen grid positioned between said. control grid and said anode.

18. An electron discharge device having a cathode and a coaxial anode surrounding said cathode and a space-charge grid positioned adjacent to said cathode but extending over only a portion of the cathode surface, a control grid surrounding said cathode and space-charge grid and positioned between said space-charge grid and said anode over said portion of said cathode surface, and a screen grid positioned between the control grid and the anode, said control grid formed to have a portion lying next to that portion of the cathode not covered by said space-charge srid.

19. An electron discharge device having a cathode, a space-charge grid, control grid and anode, said space-charge grid and control grid being so formed that the control grid is next to the cathode over only a portion of its surface and the spacecharge grid is adjacent the cathode over the re: mainder of the cathode surface.

' 20. An electron discharge device having a cathode and an anode and control means positioned between the cathode and the anode, an input circuit and connections between the input circuit and said control means for applying potentials from said input circuit on said control means, said control means undesirably absorbing power from said input circuit, and means within said.

device for causing part 01 said control means additionally to deliver power to said input circuit.

' 21. a circuit including a first electron discharge device having a cathode, control grid and anode, and a second electron discharge device having a cathode, space-charge grid, control grid and anode, an input circuit connected between the control grid and cathode oi the first electron discharge device,-said last-mentioned control grid undesirably absorbing power from said input circuit, an output circuit connected to the anode and cathode of vthe second electron discharge device, a pair of direct electrical connections one as; as electron discharge device having a cathodeandananodeandtwocontrolelectrodepor-- tions positioned between said cathode and said anode, an input circuit and connections between said input circuit and said control electrode-portions for applying potentials from said input circuit simultaneously on said control electrode nor-- tions, one oi said electrode portions undesirably absorbing power irom said input circuit, and mans within said device for causing the other of said control'electrode portions to deliver power to said input circuit. each of said control electrode portions operating on electrons originating between the two control grids and one between the two cathodes of said electron discharge de- Vices, and means for applying a positive unidirectional potential to said space-charge grid to cause the control grid of said second discharge device to deliver power to said input circuit. i

22. A circuit including a first electron discharge device having a cathode, control grid,- screen grid and anode, and a second electron discharge device having a cathode, space-charge grid, control grid and 'anode, an input circuit connected be-- from diflerent parts of the suriace of said cathode.

24. An electron discharge device having a cathode, an anode, a space-charge grid adjacent to said cathode but extending along only a portion of the cathode surface, and a control electrode coextensive with the cathode suri'ace and 'positioned between said space-charge grid and said anode over said portion of said cathode surface and positioned next'to said cathode over another portion of said cathode surface.

25. An electron discharge device having a cathode, an anode, a space-charge grid positioned adjacent to said cathode and extending over only a portion of the cathode surface, a control grid positioned between said space-charge grid and said anode over said portion of said cathode surface and positioned next to said cathode over another portion of said cathode surface, and a control grid of said second discharge device to deliver power to said input circuit.-

, screen grid positioned between said control grid and said anode.

' ROBERT L. FREEMAN.