US2048232A - Vacuum tube - Google Patents

Description

July 21,1936. H. A. SNOW 2,048,232

VACUUM TUBE Original Filed Dec. l5, 1930 2 Sheets-Sheet l @2q .P279 Z jig, f

July 21, 1936. Hv A, SNOW 2,048,232

I `VACUUM TUBE Original Filed Deo. 13, 1950 2 Sheets-Sheet 2 VOL 546i I I Ill llllI INVENTOR HAROLD A. SNOW y BY wwe/'J v ATroRNEY YPatented July Z1, 1936 PATENT OFFICE VACUUM TUBE Harold A. Snow, Mountain Lakes, N. J., assig'nor to Radio Corporation of America, a corpora- Y tion of Delaware Original application December.,i13, 1930, Serial No. 437,225. Divided and this application July 2, 1931, Serial No. 548,298. Renewed June 12,

The present application relates to space discharge tubes of the type including a cathode, a control grid and an anode, and is a division of my. application, Serial No. 437,225, filed Decem- .ber 13, 1930.

The problem of preventing modulation distortion in an amplifier stage, or in cascaded stages, has imposedsevere limitations upon the range of signal voltages which may be applied to the am- 1(3p1ifler. In V:radio receivers, for example, high sensitivity is desirable for the reception of weak signals, and some form of manual, or automatic, control must be provided to reduce the amplifier mtransmission, or gain, when stronger signals are lf'received. When a receiver of high sensitivity is operated in the vicinity of a broadcasting station, it is not unusual to final that the signal voltage applied to 'the zst carrier wave amplifier is greater than the voltage required on the detector ``for normal output Vat the loud speaker. Vlth the present'types of electron discharge tubes, it is usual to adjust one of the operating potentials applied to the tube electrodes to decrease the amplification as the received signal strength increases.

Within the range of relatively low signal strengths, this reduction of ampliiication is not accompanied by modulation distortion, but with increasing signal strengths distortion is intro- 30? duced when the amplification rate is adjusted to maintain an approximately constant output. Furthermore, within the range of higher'signal strengths, it is frequently difficult to adjust the amplification to maintain constant output since the transconductance of the tube changes very rapidly for small changes in the transmission controlV voltage. This restricts the amplification control to a small range of applied control voltages, and unfortunately, the rate of change of amplification is more gradual in the range of high amplication where a rapid change of amplification for small changes in control voltage would be permissible.

.3 Cross-talk effects in radio frequency amplifiers depend upon the high-order curvature parameters of the tube, and are to that extent related tothe problem of distortion discussed heretofore. It can be pointed out that the term cross-talk 50 is employed to designate that species of interference which originates in the radio frequency amplifier tubes by modulation between two, or more, signals. The improvements discussed herein in connection with an electron discharge tube am- 55 z-plier to-reduce distortion will also reduce a large s claims. (C1. 25o-27.5)

part of the cross-talk. Reference is made to the Proceedings of the Institute of Radio Engineers for December, 1930, wherein in an article entitled Reduction of Distortion and Cross-talk in Radio Receivers by Means of Variable-Mu Tetrodes there is demonstrated the intimate relationship between the problems of distortion and cross-talk in radio frequency amplifiers, and their elimination by means of variable mu tubes.

It is, therefore, one of the main objects of the present invention to provide a space discharge tube having such operating characteristics that no distortion is introduced when, for increasing signal strengths, the operating potentials are so adjusted that the amplification rate is reduced to a small fraction of the maximum amplification.

A further object of the invention is to provide an electron discharge tube having such characteristics that, when the potentials are adjusted to give a relatively -low amplification of strong signals, the change in transconductance for a given change in the gain control voltage is much lower than is the case with the known types of' tubes.

Another, and important object of the invention is to provide a high frequency amplier tube capable of adjustment to give an undistorted output of approximately constant magnitude over a wide range of applied carrier voltages.

A more specific object of the invention is to provide an electronic amplifier in which different portions of the electron stream are influenced at different rates by the voltages applied to the control grld.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organziation and method of operation will best be understood by reference to the following description taken in connection with -the drawings in which I have indicated diagrammatically several arrangements whereby my invention may be carried into effect.

In the drawings,

Fig. 1 is a perspective view, partly in section, of a screen grid tube embodying the invention,

Figs. 2a and 2b are diagrammatic views illustrating two embodiments of the invention,

Fig. 3 is a curve sheet showing the variations of plate current with grid bias for a tube such as shown in Fig. 1, l

Fig. 4 is a curve sheet showing the relation between control grid voltage and transconductance for tubes embodying the invention,

Fig. 5 is a curve sheet showing the relation between permissible maximum input voltages and control grid bias voltages, and

cathode. As is well known,this particular type of tube comprises an evacuated envelope enclos'y ing a cathode C, heated by a resistance (noti shown) within the cathode, an inner grid CG, an outer grid SG, a plate, or anode P, andan outer screen S which is electrically connected to the outer grid. Except for the novel' construe-- tion of the control grid CG and its novel functional relationships to the remaining elements lof the tube, the several elements of the tube, andV their relative physical arrangement, may be substantiallythe same as -that employed in the present commercial tubes. 1

In tubes of this general type, the control gridY comprises a helical winding supported by one or more wires YI. In this particular embodiment of the present invention, the helical winding is not continuous, asin the known constructions, butv comprises two sections 2, 2 that are separated by a distance of the order of twice. the pitch of the winding; The windings of each section are of the same pitch,7which may be the same asthat nowjV employed in tubes of this type. This particular physical embodimentV of the invention, therefore physically diiers from the known construction, of ythe same generalfphysicaldesign, by the absence of two complete circumferential turns of the control grid winding. r y l 1v This particular constructionresults in a tube inV which the control exercised upon the electron stream is not uniform over the entire extent thereof. Y Y

The operation of the tube shown in Fig. 1- may berexplained as follows: e f

The controll grid has been shownto be divided into two sections which are mounted with a gap between them. -At low negative biases'the entire cathode is operative, andthe tube has about the same characteristics it would have Vif the gap were in place.v As' the grid kbias increasesA negatively the electron current through the upper'andlower parts ofthe control grid are cut off leaving a.- low mu control through the gap. At these bias volt-v ages the tube acts las if the.. upper fandlower sections ofthe control.- grid were formed of 'solid' metal, andl .controlled the current fthrough the gap inthe ordinarymanner;V 1 Y., n An eccentric arrangementof one,- or more,elements may be employed, Fig. F2a'.V showing Vthe axisof the cathode C inclined tothe axes of theother elements, and Fig.2b showing the cathode C parallelyto but co-axial with, theotherf'elef ments. g

. The.l mode of operation of lthe modifications disclosed'in'Fig's. 2a" and 2b can be'- explained' in* the same manner as has'been explained in vconnection'` with the' Y embodiment shownV in Fig, `l. Essentially, and basicallycons'idered, all rofthe tube Vstructures disclosed herein provide aradio"V frequency arnpliiierA tube having a Inu-,factor which` decreases continuously. with increasing` negative grid'bias. e y Y It is well known the amplification of a vacuum tube may be regulated by adjusting the bias voltage upon the control grid, vthe amplication decreasing as the bias voltage becomes Y more negative. lCurves showing the relation be'- tween plate current and grid bias, i. e., transfer characteristics, afford an indication of the ,ampli-V cation at different bias voltages, since the slope of the curve at any point is a measure of theY amplication when the tube is biased for opera.-

non at that point. the latter having a separate heater for the vIn Fig. 3, the solid line curve A is the transfer characteristic for a tube such as shown in Fig. 1,

'and the dotted line curve B is asimilar curve for a commercial screen grid tube of the same -general type but having a continuous control grid. v winding of `uniform pitch. An examination of 1curve A shows that, with tubes embodying the invention, a control of amplification extends over a range of control grid bias of from zero to more Y than -30V volts.V With the known tubes, the

curvatureV of the transfer` characteristic ap-VV proaches zero at a controlr grid .biasV of about l5-volts.: i, n. y

AIn other words, an increase of the grid bias above approximately l5 volts negativewill not be accompanied yby a decrease in amplicationfwhen Vthe known type oi"V tube construction is employed,

but with tubes Yembodying the invention,k the amplification may be varied with changes of control grid bias throughoutY a range of from zero to upwardly of -30 volts.- The tubes will still pass signals, lby leakage transmission when the control grid biases exceed these respective values,

but control of yampliiication isno longer possible in regions where the Vtransconductance curvesV become substantially horizontal.

The curves of Fig. 4 show the relation between*Y control grid bias andtransconductance for two tubes embodyingvthe invention, and for a similar tube which has the usual grid construction..

Curve A is the transconductance-vcontrol grid biascurve for a screen grid tube ofthe Vtype shown in Fig. .1, having two turns omittedV fromV the center of the control grid. The data for this curve and for curve A' of Fig. 3 relates to the same tube.V Curve A is a similar curvev for a screen grid tube in which only one turnwas removed frorn the center 'of the controlgrid, andv curve vB shows lthe characteristic properties of the conventional type of tube having a continuous control grid winding.

` An'examination of these curves shows-that the useful range of transmission control is considerablyextended by the present invention. With theV known constructions, a decrease of thet'ransconductance from about 500 micromhos to the value, about 0.8 Vmicromho, at which leakage transmission prevents,furtheramplification control, correspondsto a change incontro] grid` bias of aboutten volts.,y The correspondingranges of control grid bias for the tubes of curves A' and.

A are, respectively, about 30 andA 60 volts.4-

As stated above, Vmodulation distortionv may Y occur when, `for a givensignal strength, kthe amplificationV is so adjustedT as toV bringthe output down to a desired, or standard, level. v Since such .distortion 'is due-toVv the curvature of the transfer characteristic it VWill be apparent that a tube having a curve of lower curvature can transmit, vwithout distortion, `higher 'voltageV signals than ,an tube having acharacteristic .which` exh ibits a region of higherv curvature.-Y -An Vexamination of the, curves of Fig. 3 will showthat the 'laA y Modlatn v distortion introduced l, byv` at tubesoalieftnbegandmeasuring modxilationrot l outpntisignait. Fonsi-nail?iinputtsignalsfetheatubes introducesfpracticallyi:noschangeeimsmodulatiorm. Wiiemthe 'zinputiusignal-lincreasesubeyondal cer- 102i tait': vaidectheemodulatiomot the 'i outputsignal increases rapidly due to the curvaturefinfthettubes transfricharacteris'tica. Thisfirrcreaseriimnodla'f tiom(modulati`on distortion). limitswtheimaidmunri y input'signalithatimayfbetransniittdeby theftube 15?. withoutifdistorti'ontl. Y'

Th'efcurvesoi 4Figiishiowffth'e frelatiombetweenn.

control gridfbiasfvoltagesandf the :maximum-input signatfvoltagesfwvhich:produce aatwentyfperz centi.

aos.

35i nuttedsvnnnobmorethanzzo syrzdismmonnsabout:

vefvoltss.

When-etherbiasrjsladiusted tocmaintainna'rcone stant output signal, theemaximurmiinput Signat. voltage fthaticam be rappll'edxofthetuberwithbless 4tlthanc20%\di'stortiomis ab'outzvolcorresporrde ing'stooalcontrotf. gridibi'as; oi approximately,yv 12 5': voltsnegativef. Fonfgreaterssignal strengths, thee increases :oflzontolgridbiasidofrrot faltertheiams. pliiibatiomz. but do aicti theemaximunr signal .45l'fstre1rgth1whiclr may beftransmittediwithlessithann 20%1 distortionv.- For? ai single: tube;- 'signatz strengths-,failings outside the range-t on? vohslnieffv controlcannotnbe shandliedalby:r thbfamplirrif 'ag constant output is essential, but in cascadedan'mf fti'pliersihaving; twotopxnore controlled stages, the maximum voltages, as shown by the dotted line portion of curve B, may be transmitted by the first stage when amplification control without additional distortion is provided in a subsequent stage.

A similar analysis of curve A will show that the maximum signal strength which may, without undue distortion, be transmitted to give the desired constant output voltage is about 1.1 volts, corresponding to a control grid bias of about -38 volts. For the tube with two turns removed from the control grid, curve A shows a permissible input voltage of 3 Volts, with a bias of 95 volts. Furthermore, by so restricting the control grid bias voltages that the maximum can never exceed about 28 volts negative in the case of the tube of curve A', and about 65 volts in the case of the tube of curve A, the maximum carrier voltages which may be transmitted in approximately 7 and 22 volts, respectively.

The -observations have been verified by tests made with a commercial radio receiver having three radio frequency amplifier stages employing commercial screen grid vacuum tubes of the 224 type. Measured carrier wave voltages, 850 kilocyolennodulated thirty iper icent fat fiofcyclespwere impress ect' upon ftherstampliiier.andrthefampl fication wasladjustedrto-'maintaina constant-car; rier 'voltage foni-th'ev-'dtector which,l for =undistorteda carrierramplication; correspondedato e260 fcycle? 5..

audio: frequencyi'output of,` 4? voltsr. acrossfthe; speakeriterminalss Mdulationr:distortionrlinf `thierampliiierwisxeviL dencedf by'f'ani increase/in: laudio equencyeoutput voltage when1 tithe* carrierl lcomponentL of *fthe dei 10 tectoniinputTremainsconstant; By-maintainingfaid constant carrier voltage at `the fvdetector :the idemodulated audio frequency output will remain f constantup' torthe :point fat lwhiclimodulationx dis'- tottionlbeglns... Beyond that point; the audioffreeA l5 'i quencyloutput .iwilllriseveverr thoughi'theicarrier'r voltagei'acrosstheIdemodulatorismaintai-nedfconi; l stant'as fthesigxial-` strengthincreases:

Inzsligsf curveD shows the?relationshipnb'eL tweenmudio.outputiandxcarriermvavei input fwherr'20 i commercia1=224 atbes averelisedzin the amplifier?- Toc eliminateedlstortioneirn thes'fthirdxradio fre-wv quencystageeonly the rst two :stages fWereiad-i` justed` .toic'zontrol'ftheamplication;l Theffdis'tor; tionzwasso the lsame: values iwliether.' theibias225 onzthe' control: grid of thefrstftubeiwas variedjorr the f bi'ascontrol' .was'extended to include'bothftheev irst andf secondtubes;` Fomfzcurvecl) .it will: vbe 1 noted-rfthat the modulationiriseabeginsiat a can'ier inputsofnabout :15 volt, an'dln'eaches .-20% '.at` .023 f. 30 signatvoltagefonthe -cfirst tube;

Curveielil'was :plotted ffronrzdata@obtained-With# ther-same .radio1 receiver' *when'tubes .f embbdyingf the invention were substitutedlinxtheradio"freequencygstagesn. 'Ihe=tubeswereof'theitype shown 35 "3 infFigg Iii. e.';.of standardv 2242?A typeconstruction"Y except 1 thatil two f, turns were omitted,` from: the c: center othe cont-rol grid.:r Withfz-increasingfsigw nal-.'strengthy: .the idetector input :was maintainedconstant :byadusti'ngthe :control-grid-bias simule. -40if be noted that the carriertinputacross.theffrst` tube increased to 10 volts before :ai-modulation rise-was :apparentL .andathat itfreachednl'l -volts before ftheernodulation-A Vrisef:reached 20% Thef45fi variation-fofcontrolugridebiaswith inputfsignal i strengthris shown-.bycurveFx 'I-hesez'resultsswerecchecked :qualitatively-.- bya listeningntest-twith -.ordinazsy broadcast (music) modulation-c1 It -was found .that the'distortionfon A50= high modulation peaks became apparent when the input on the rst amplifier was from to 20 volts, becoming worse as the input was increased beyond volts.

These observations of actual performance in a receiver check closely with the results plotted in Figs. 4 and 5 for single stages. By employing tubes constructed in accordance with the invention, the permissible input voltage was raised from 0.3 volt to 17 volts, i. e. Volume control with 60 good quality reproduction can be had with input voltages about 57 times as great as those which may be applied when the known commercial form of tube is employed.

It is to be understood that the invention is not limited to any particular type of tube, but is, in general, applicable to all tubes employed for amplication control. The physical construction of the control grid, or the geometric and structural relationships of the tube elements are subject to Wide variation so long as the control grid exercises different rates of control at different portions of the electron stream. Considered broadly, the invention provides in a single tube, the electrical equivalent of two, or more, amplier tubes operating in parallel, one Atube having a relatively high a Avoltage (10W mu).

less amplication control and substantial reduc- V ratio of plate voltageto control grid voltage (high mu), and one, or more. of theremaining tubesv having lower ratiosv of plate voltage to control grid In fact, the same Adistortionand convenient than the parallel tube arrangement. Y i

Although the above discussion has been lim'- ited to aconsideration of modulation distortion in radio receivers, it will beapparent that the curvature of the transfer characteristic gives riseto'.

other forms of distortion which limit the range of continuous wave and audio frequency voltages -within which a tubeactsas a substantially linear The invention provides afmeans for` amplifier. extending the range of signalV voltages which may be transmitted Without distortion, the signals 'be-.V ing either of.audio or radio frequency,.and if of radio frequency, either continuous wave or modulated. By transconductanoe is meant the ratio of theV change'in the current in the circuit of an electrode to the change in the voltage on anotherl electrode, under the condition thatV all other voltages remainl unchanged. Byr mu-factor is Y meant the ratio of the change in one electrode voltage-to a change in the other electrode voltage, under the conditionthat a specified current remainsunchanged. v

' 5 While I have indicated Vand described several systems for carrying my invention into eect, it

Will berapparent to one .skilled in the' art that:

my invention isY by no means limited to the particular organizations shown and described, but that many modifications maybe made withoutdeparting from thel scope of my inventionvas set f 'i ment having its longitudinal axis inclinedto the forthin the appended claims.

What I claim is:

1*. An electron dischargetube comprising an emission electrode,v a positive cold electrode surrounding said emission'electrode, and an -auxiliary cold electrode disposed between said emission electrode and said positive electrode, the axis 2,048,232J vi 2. Anelectron discharge tube comprising'an r equi-potential cathode,` a cylindrical anodesurf" rounding saidcathode, a cylindrical screen grid electrode surrounding said cathodel andi disposed l 1 between said anode andcathode, and acylindrical controlgridelectrode surrounding saidcathode Y and disposedfbetween said screenr grid and 'saidi' y cathode; said anode,y screen grid and control elec-1A Y trode having aV common axis, and Ysaid cathode;

being disposedwithits axis out of alignment with.v 10

straight unipotential. electron emitting electrode...

a tubular control electrode surrounding'said emitting electrode, and a tubular-cold electrode 15y surrounding said control electrode, the longitudinal axis of one of. saidelectrodesdiverging from., .1 V'

the longitudinal'axis of another ofV said electrodes, 4. An v.electron discharge tube Vcomprising a.;

tubular plate and a tubular1controlgridrcoaxially 20 mounted along common longitudinal axis. an .Y

elongated, thermionic cathode mounted insideV said control grid with itsk longitudinal axis parallel to and non-coincident with said common axis ofsaid control 'grid and said plate. Y 5 5. An electron discharge device comprising-an elongated thermionic-cathode, a cylindrical main cold electrode, Vand a cylindrical auxiliary cold p electrode concentric with said main electrode and disposed :between said cathodeand said main 0 electrode, theV longitudinalaxis Vof said cathode being eccentric to said -coldelectrodes.V s

6. An electron discharge tube comprising Va cy.- lindrical equipotential cathode electrode andtubular Agrid and plate electrodes surroundingsaid 3 cathode, the longitudinal axis of Ione of said electrodes being -inclined to the longitudinal axis 'of another ofvsaid electrodes. Y

7. An electron discharge tube comprising cy-z lindrical cathode, grid, and plate elements, twoV Ai0 of saidelernents being coaxialfand the third elev common axis orf-said other two elements. 8. A11-electron discharge tube comprisingacylindrical equipotentialcathode and coaxial tubu` 45 Y Y HAROLD a. SNOW. 50

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