US2483409A - Tuning arrangement for thermionic valve circuits - Google Patents

Description

0d. 4, 1949. W, A GOLD A 2,483,409

TUNING ARRANGEMENT FOR THERMIONIC VALVECIRCUITS O SS DS Sapp/.y 60A/TEM. l AMP rg T Inventor 2 rg Ry REMY M//am //exander 60M.

, B ,l 4 /v Oct., 4, 1949. w. A` GOLD 2,483,409

TUNING ARRANGEMENT FOR THERMIONIC VALVE CIRCUITS Filed Nov. 6, 1943 2 Sheets-Sheet 2 Q/ Q2 CA F63 (gf/5.4) (l Ra laf/ Ay e I x I l Ir-zventor Wil/iam //exan der Gold.

Patented Oct. 4, 1949 TUNING ARRANGEMENT FOR THERMIONIC VALVE CIRCUITS `William Alexander Gold, London,=England, as-

signor, by. mesneassignments, to International Standard'Electric Co rporation, NewYork, N LY.,

acorporation of Delaware j Application November 6, 1943,'SerialNo. 509,211 In'Great Britain December 8, 1942 (Cl. Z50-,40)

l v10 Claims.

The present-invention relatesto tuning ar rangements for electrical signal transmissionfsystems,and concerns-particularly the tuning of any number-.of stagesvin such a-system..to..a signal wave of a particular frequency.

Ina radio transmitter, for-example, :it is usually necessary to tune one or more thermionic valve stages to the frequency. of the-masteroscillator; and furthermore,` the aerial circuitvmay` need to be similarly tuned. While 'the invention will' be l' described in its application to a radio transmitter it is. applicable to-any transmission system having stages which need tuningf toa signal' frequency, and the ultimate load isnot necessarily anaerial.

The invention is concernedwith automaticV means for performing the tuning,l and mayv comprise two operations, in the first of which some or all -of the amplifying stages-.are `given a-simul taneous rough tuninl-,` and in the second each stage is separately givenl a .nal accurate tuning. The tuning arrangements are controlled by the variations of a rectied current lin avvalve or rectiiier which passes through a maximum Orminimurn-asthe tuning is varied through theresonance point.

According to the invention therefisprovided in an electrical signal transmission system, lan arrangement for adjusting a tunable circuit to resonate at the frequencyof an applied'signal wave, which comprises an electromagnetic device adapted to vary a reactance forming Vpart of the tunable circuit under theV controll of-a current derived from a thermionic valve or rectier, which current is adapted to vary according to the instantaneous resonance frequency of the tunable circuit and to attain a maximum ora minimum valuewhen the tunable circuit -is in'V resonance with the signal frequency.

The invention will be describedwithreference to the accompanying drawings,V inwhich Fig 1 shows a schematic circuit diagram of one arrangement according to the invention Figs. 2 and 3 show twovother arrangements;

Fig. 2a shows the application of the diode control of Fig. 3 to the'circuitlof Fig. -2.

Fig. 4 shows a control amplifier Which-may be used with any of the circuits shown in Figs. l, 2 or 3;

Fig. 5 shows a diagram of a switching arrangement for facilitating the tuning of a number o amplifying stages; and

Fig. 6 shows a relay circuit which may be employedl for preliminary tuning adjustments.

Fig.'1vshowswtwo stages of a thermionic valve amplier forming .part of. afwave v.transmission system, for example, a power amplier` ina radio transmitter. .It will be assumed that` the valve V2 ati least is operated under class` C conditions, that-is,-r the control grid is negatively biassed well beyond thecut-01T point :and the amplitude of the signal applied to the control grid is such that it iis rdriven positiyeybyl the -positive half waves. The resulting grid current will therefore increase asthe amplitude' of theapplied signal increases, and vice-Versa.

"In: Fig.. 1 the two valves V1 andV2 are provided withk conventional auxiliary arrangements comprising appropriate impedances Zg for connecting the control grids to suitable biassing` sources represented by batteries GB. i The condensers K are coupling condensers of suitable. capacity.

The-anode current ofthe ValvewV 1 is supplied from; the high-tension; source I-IT-ithrough a parallel tuned circuit comprisingv an adjustable condenser C and an inductance L which may also bev adjustable as indicated. The valve V2 may be supplied-,by a similar.l arrangement shown but not designated.

'The condenser Cris mechanically controlled by an electro-magnetic devicey M which is operated through a control amplier CA (to be presently described) the linput vterminals of whichlare connected across a resistance r of suitable value conn nected in. series with the r`control grid of the valve 1V2,.so that the `grid current ows through thev resistance. The 'idevice M is therefore effectively controlledbythe grid current.

- Thisffdevice may be of any suitable type; it may, for example, be similar in principle to one of the well 1. known types of direct-current indicatingr meters, the moving coil or armature being adapted to. rotate a movable vane or vanes or the condenser` C, or otherwise to vary its capacity. The device should in addition be provided with means for locking the movement in any position, preferably.electro-magnetically controlled.

The function of the device M is to tune the circuit L, C accurately to resonance at the frequency of the wavesrapplied to the control gridcf the valve `V1 at the terminal IN. A preliminary rough tuning is vfirst carried out manually or automatically by ad'justingin'ductance L and the condenser C, but leaving the circuit slightly mistuned onone` side-or the other of thesignal frequency according to the manner in which the device M varies the frequency of the circuit L, -C as will be presently explained.

The control-ofthe tuning depends on the varia-Y tion of the grid-currentpfthe valveVz. Having given arconstant signalinput atl the terminalgIfN,

the grid current will increase to a maximum as the resonance frequency of the circuit L, C approaches the signal frequency from either side. Assuming for clearness that the circuit L, C is left mistuned to a frequency slightly lower than that of the signal after the preliminary tuning, as the frequency is raised by the appropriate adjustment of condenser C the grid current of valve V2 will increase until the signal frequency is reached and will thereafter decrease. It will, therefore, be arranged so that when the grid current increases it will operate device M in the direction to decrease the capacity of the condenser C so that the frequency change brought about by the movement of device M will tend to drive it in the direction to increase the change. When the maximum of the grid current is reached the device M will come to rest and will leave the circuit L, C tuned accurately to the frequency of the incoming signals. It is then locked in the rest position. 1

It will be understood that it could be alternatively arranged so that an increase in the grid current of valve V2 causes device M to increase the capacity of the condenser C, in which case the preliminary rough tuning should leave the circuit tuned to a frequency slightly too high. Furthermore, the device M could be designed to control inductance L, or part of inductance L, instead of controlling condenser C.

It will be understood, also, that the preliminary rough adjustment must tune the circuit L, C sufficiently near the signal frequency so that the grid current has begun to flow in the valve V2 in order that device M may be able to control the final :i

adjustment, and moreover, device M must be arranged to control inductance L or condenser C so that the effect of the grid current is to move device M in the direction to improve the tuning.

Fig. 2 shows an alternative arrangement wheref by the device M is controlled by the anode current of the valve V1 instead `of by the grid current of valve V2 (which is not shown in Fig. 2). Assuming that a signal of constant amplitude is applied to the control grid, then the anode current passes through a local minimum value `as the tuning of the parallel resonant circuit L, C is varied through the resonance point.

The anode current passes through the resistance R1 connected in series with the cathode 0f the valve V1 and shunted by the by-pass condenser K. The potential drop across resistance R1 is applied between the control grid and cathode of a reversing valve V3 so that the local minimum may be transformed into a maximum for operating the device M. The resistance r is connected in series with the anode of the valve V3 and operates the device M through the control amplier CA as already desired.

The valve V3 does not need to be a power valve and should preferably be operated under class A conditions, suitable biassing arrangements (not shown) being provided by Well known methods.

Fig. 3 shows an arrangement in similar principles adapted for adjustment of the aerial matching coils in a radio transmitting system, which includes automatic means for carrying out the preliminary as well as the nal adjustment. The aerial A is connected to earth through an inductance coil L1 adapted for the preliminary adjustment, a second inductance coil L2 adapted for the final adjustment, and also through the secondary winding of an output transformer T the primary winding of which is connected to the last y stage of the power amplifier of the radio transmitter, (not shown). The coil L1 is adjusted by an electric motor D which controls a movable contact d on L1 (or which may change the inductance in any other convenient way). The motor D is driven from a power supply connected to the terminals DS, through a starting switch SS.

The aerial A is connected to earth through a diode Vv connected in series with a resistance r3 shunted by a by-pass condenser K1. A blocking condenser K2 may be included if necessary. The control amplifier is connected as before across resistance T3, and operates the device M in series with a relay RY. M controls the adjustment of L2 in the manner previously described, and is short-circuited by the normally closed contacts fyi of the relay RY; and the motor D is connected to the power supply through the normally closed contacts Tg2 of the same relay.

When the adjustments of the coils L1 and L2 are to be made, it is first arranged so that coil L2 is set at one end-or the other of its range, and the starting switch SS is closed to connect the power to the motor D. Since the aerial A is initially out of tune, the signals derived from the transformer T will produce only a small rectified current in the circuit of the diode V1. The motor, however, will be adjusting coil L1 in the direction to improve the tuning, and the rectified current will increase until the maximum is reached when the aerial is tuned to the signal frequency. It is arranged so that just about this time, or perhaps a little earlier, the rectified current reaches a value suilicient to operate the relay RY which opens both the contacts fyi and ryZ. This cuts on the power driving the motor D, which will continue to run for a few revolutions before stopping. It should stop when the inductance L1 has slightly overshot the resonance point. The opening of contacts Tyl when the rectified current reaches the value specified above permits the device M to make the final adjustment of inductance L2 to bring the circuit back to resonance, in the manner already explained, being controlled this time by the maximum of the rectified current in the circuit of diode V1.

It will be evident from what has been said that if inductance L1 is set at the minimum value before the adjustment begins, then the motor adjusts it too high and inductance L2 should therefore be at the maximum setting when device M is short circuited, so that device M may be able to make the necessary reduction in the total inductance. Similarly if coil L1 is initially at the maximum, then coil Lgrmust be initially at the minimum. It will be obvious that the total range of coil L2 must be such as to cover with sufficient margin the amount of overshoot produced before the motor stops.

It will be seen that in general the device M will begin to operate before the motor D has stopped. Owing to the overshoot, there will be no confusion between the two adjustments since the device M tends to correct the overshoot, and will continue to adjust until after the motor has stopped.

It should be mentioned in connection with Fig, 2, that if the valve V1 is operated under class C conditions. and if it is incompletely neutralised, the anode current minimum may not exactly correspond with resonance of the tuned circuit. ln this oase an arrangement .employing a diode circuit like that shown in Fig. 3 may be used instead of the reversing valve V3, so that the device M, which controls the adjustment of con- Lthatetheidevicex'M tcomes to.. rest':whenthe grid ,-.currentiimaximumis reached. I This however may not beztheztcase if 'device Mffhas'fany appreciable Iinertiapanduits movement inayf tend to Aovershoot 74V thee-accurate tuningl point` so that the: grid cur- 'rrent z :of vvalve .-V2 :will zbegin'to decrease again v-denser C1.

messagerie idenser-C;.isitselfzcontrolledebythe yanode voltage :fof z'valve `Viz-.instead fofczby 1- the Ganode current.

and its anode would be connected to the,- anode iifof "valve .Vlafinf'Eig .22;through` a-'blocking contdensergitheavalveV3 .andrits .connections Vbeing 2 omitted.

. Itiwas astatedsabove:.inF connection wwith- Fig. 1

Thisuwill ica-use: fdevcefiMu to' swing. back, andlif it .uis againcarriedrpast the maximum; the resulting lCdecreasez inrgridf.. current A :due :to :the mistuning produced will drive the device still further backswards Vvan-dithe?. circuit may; lbecome mistuned to vf ftheflimit oli-variation Iwhich device M is capable :off producing.i :The samefi tendency is inherent in the'arrangements-shown in Figs. 2 and 3.

` This tendency'mayxbe prevented bykv means of Ithe `control amp1ier1CA,fdetails.of which are' '.ishoWn .in-Fig 4. This famplier' also introduces ;appropriatefamplication for the satisfactory opl. eration '.ofnzthefdevicex'M and relay RY (when .used).

The control amplier comprises two amplify.

ing valves'V4.and`fVs.coupled by a: circuit which includes'a diode VsiResistance R5 is the. anode supply resistance for theinput'valve V4 and the tromagneti'c"tuningl devices Msuch` 'as M1 which corresponds: to 'the Vstage oi ithe 'transmission systernv whichfis'ibeingtuned; the connection being indicated by the dotted linesiinv-:Figfa The .cathodezof u valve yVs is y"appropriately Ebiassed by the voltage-divider f.bleeder.y network comprising resistances'Rean-dm 'connectedto-'thehigh ten- .sion` supply sosthatf there is.` never *at any Ytime f any: appreciablegrid-'fcurrentV AThe "input terminalsfof thecontrol .amplierlare connected 'across V'therresistance.r1corresponding to the stage which 1 isA toi'be'iadj usted. :The same control 'amplier may an appropriateswitch as shown@ inaFigure 5 which qmally 'open,2andscontrolled by theabove-men-v4 .tioned switch, is iconnected iacross the con- The resistanceRz should have' a high nvalue (for #example .megohms) anduthe capacity of the :condenser v'C1f shoul'dmbe large, (for example `1 i znicrcfarad) scthatthe Atime constant'of this `circuit is suiciently large.

For the sakei'of clearness it will be assumed that the controlamplier is connected inthe control gridv circuit of the lvalve V2-in Fig. l, and that atithebeginning'of thefnal tuningoperation, ythe grid-current is small, the-connections being poled so that a small negative voltage-is applied to the controlfgridlof-valve V4. There will bea `rela-tively largeranode current in valve 5511;;vnof eipetentialfe'throughf-thediode-V5. AS v the dbcurrent fofbthe'ivalve Vgl increases during L theiinal tuning: eperationlalready described, Athe oten-tial ofL theanodeofvvalvevi'Will-continually due tofthel increasing negative potential lap lOl-@plied to iitscontrol: grid. `The anode `ofhthe'diode v iV5lWiil1 accordin bepositive'- to its cathode, -a-nd V=:prwid-.edithat the. `Impedance of -the vdiode inthe conducting direction is low-sothat thetimeconflstant'of theicoupling network issuiiicientlysmall, 115`tlie2anodefpote. ial increases-Will -be almost'im imediatelyAappliedthrough the diode 'V5 to the flcondenser CLIM/hose -potentiaL-.Which is also that .fofitheccntrolfgridI-of -valve Vs, will rise accord- I'lcausilng-the anode current,which operates -ridevicev M1;- toincreasein the same-way. If, however, for any reason the grid currenticf-fvalve V2 v-:.(Fig. l) shouldibegin tof decrease, theffpotential Lcrease land valve :V5 -Will immediately 'cease -to '.conduct,l .Y and the pctentiall of l the condenser-C1 will l--not be accordingly reduced '(e'Xcepti'very slightly-through resistance RQ.. Thus, if-thede ing;initial@decreasev inthe grid current of valve "Vm/willnoti-be passed on `and the device 'Mi will beiheld by the-anode Acurrent of valve V6, which ldoes not decrease because thev control grid' voltage remains practically constant.

A reversalloi Vthe#grid currentchangelin Valve V2 may also be produced if the signal Waves are 1 modulated orare-'subject to l interruption or in- -lter-ference. The tuning will be arrested-every f timeithis Loccurs,` but Swillf be resumed directly as lthe gridcurrent increases again, luntil the Y tuning gg-is completed.

V4The'.slowdiscliarge ofthecendenserCi through the resistance R2 will onlyresult in a veryfslight imistuning fwn-ich '-loeing equivalent to affdecrease ofuthegridcurrent of valve V2 will-not'berpassed on through 4the' 'ampliiierso-that it will not initiate fa; complete lmistuning.

A.Assoon asfthe tuningof the stagehas been completedthe@control -amplier Amay be trans- `ferre'drby thefswitch of Figur-e510 Athe resistance 59 r2 anddevice-Mz ofthenext stageto be tuned.

#Atsome point-during the change-over the conlvt-acts S arebmomentarily'closed by `the switch -and blshort 4circuit thecondenser C1, so that the tuning "oi-'theifnew stage =Inaystart at lthe beginning. Any number of vfurtherv stagesmay be tuned in LturnLby.-switc'zhing the-control amplifiers to the corresponding resistance r-and` device M ineach Ityvillfbe evidentthaftlthe -diode device V5 may =Ybereplaced byany other kind of rectier (such as l-af-sel-enum or-cop'peroxide rectifier) arranged so flthatits-forw-ard or conducting direction is from lthea'rodeof valve Vel-tothe control Igrid of'valve VV6.4 1- Alsoitheicontrol grid of valve V6 maybe '65 -biassed infother Wellknown ways, land any-other suitable-conventional arrangements may be used -insteadoffthoseshown. YIt will be obvious, also, "thfat anyy additional amplifying stages necessary may befintroduced in any well known manner.

Similarly the diode .V2 in Fig. 3 may be replaced -by-s`ome other-kind of rectier.

"Itfwil1lb`e understood that several stages'in a transmission systemmay need tobe tuned -to the tfoperating frequency. lFor thisipurpose itf-maybe 75 convenient-'to `couple -tzigether the rough tuning arrangements of al1 such stages so that they can be simultaneously adjusted; then the stages can be arranged to be finally adjusted separately in turn by any ci the methods described, if necessary with the help of the control amplier, Fig. 4. It is to be understood that the separate motor D shown in connection with Figures 1, 2 and 3 is primarily for illustration purposes only and, as explained, a single motor D connected with the tuning means of a plurality of stages, such as one or more amplifier stages I" Figures 1 and 2 and the .aerial connection stage of Figure 3, may be used and is the simpler form.

The preliminary tuning arrangement described with reference to Fig. 3 may be applied with slight modification to the preliminary tuning of all the stages of the system. For clearness the system will be assumed to be a radio transmitter but the arrangement will be equally applicable to other systems, which may have some other final load than an aerial.

Referring to Fig. 3, the preliminary tuning elements of all the stages are coupled mechanically to their respective adjustable tuning means such as coil L1 so that they will be simultaneously adjusted by the motor D. It will, of course, be necessary to arrange so that the separate elements are adjusted at appropriate rates so that they will reach the tuning point at approximately the same time, and further, since the same transmitter may be used with a number of diierent aerials, it may be necessary to provide interchangeable units for coil L1 so that a suitable adjustable coil can be selected ior the aerial to be used.

The rough and nal tuning can be carried out by the methods explained, by the help of a switching arrangement such as that shown in Fig. 5. A pair of coupled rotary switches is shown having iour positions and an oi position. The switch may, of course, have as many positions as there are stages to tune.

The right hand portion of the switch is adapted for connecting the input of the amplifier across the resistance r of the various stages in turn, and L the left hand position is used for connecting the output to the corresponding device M (or to a relay RY1) Position l is used for the rough tuning. The right hand part of the switch connects the input of the amplider CA to the resistance r3 of Fig. 3 and the output to a relay RY1 which controls the contacts T212 of Fig. 3. This relay therefore takes the place ci the relay RY and the device M shown in Fig. 3, the latter being not required for the rough tuning.

The right hand portion of the switch carries a third brush B1 which makes contact with a stud F in position l only. Brush B1 and stud F are connected to the points Q1 and Q2 respectively in Fig. 3 and represent the switch SS. When, therefore, the switch is moved to position l, the motor D is started and the rough adjustment is carried out as already explained, the relay RY1 operating and opening the contacts 'r1/2 as soon as the maximum current through the resistance 1'3 is reached. The left-hand portion of the switch also carries a third `brush Bz which is adapted to make a temporary contact with any of the studs F1, F2, F3 etc. while the switch is passing between the successive positions. These studs, F1, F2, Fs etc., are all connected to ground, and the brush B2 is connected to the point P in the control amplier (Fig. 4). The brush B2 and studs, F1, F2, F3 etc., therefore correspond to the contacts S and ensure that the condenser C1 will be momentarily short circuited immediately before each tuning operation. It will be understood, of course, that when the switch is nally set in any of the positions, the brush B2 is approximately half way between a pair of the studs F.

W hen the rough tuning is completed, the switch may be moved to position 2, by which the control amplifier is connected between the resistance r1 and device M1 of the first stage. The nal tuning then takes place as described. All the other stages may beA tuned in turn by moving the switch one step each time. The position d is allotted to the ne tuning of the aerial A (Fig. 3) and so the studs of the positions i and l on the right-hand side of the switch are multipled together so that the resistance r3 is again used.

The left-hand studs are, however, connected to the measuring device M which controls inductance Lz.

rihe switch shown in Fig. 5 is only diagrammatic and may be constructed in any desired form; some or all of the switching may be performed indirectly in known manner. The switch can be manually or power driven, and may be provided with appropriate timing arrangements so that sufficient time is allowed on each step for the tuning operations to be completed. The switch may also be provided with contacts (not shown) for energising the locking arrangements oi the devices M.

opposition, a resitsance Ra being connected between the anode of diode Vs and the cathode of diode V9. Diode Va is connected with its Ianode to terminal l, and with its cathode connected through the resistance R7 and condenser Cz in parallel to terminal 2. A relay RY2 having a suitable self locking arrangement (not shown) may be connected across resistance Rs and may be adapted to control the contacts r'yZ and ry! (if used) in Fig. 3. A suitable resistance Re may he connected as shown across the terminals l `and 2 to provide a conducting connection at all times.

While the potential 'of terminal l increases positively with respect to terminal 2, the condenser C2 charges up through the diode V8, but since the diode Vs is connected in the backward .direction no current will pass through it. Moreover, after the maximum of the controlling current through the resistance r3 (Fig. 3) has been passed, the potential or" the terminal l will .begin to decrease. Owing to the charge in the condenser Cz, the difference of potential across the diodes is now reversed in sign, and diode Va -becomes non-conducting, and diode V9 discharges the condenser through the resistance Ra producing an impulse which operates and locks the relayA RY2. It will thus be seen that the relay RYz will be operated as soon as the maximum has been passed. Means can be provided (for eX- ample in the switch shown in Fig. 5) for releasing the relay RYz when the tuning has been completed.

What is claimed is:

1. In a class C Vamplifier having a first tube caseros 9 with :a tuned-anode `circuit, -ian--efutput' tube, and at least one grid in said output tube, the ,output tube being-grid f edfrom the tuned anode circuit of the precedingitubepa motion-and tuning means in'said tuned' anode -circuitsthe-method of :automatically tuning said lanode Ycircuit .to secure maXim-um-inputto said grid, `which includes the steps of measuring the current tosaid gridcaucing said current to operate said motor irl-proportion to the current, and :operating said-tuning lmeans fromjsaid motorpto' vary the tuning ofsaid anode circ-uit while said grid current incref-ises,` in a direction tcause still furtherinerease thereof, andi to cease varying'fs'aid tuning -at thecpoint ofmaximum grid current.

2. In a classCl amplifier having a rst tube with a tuned anode'l 'cir'cit,"`an output tube, and at .least oneigridinasaid:output tub e-,f'the output-tube being grid fed from'i a-uned' anode circuit? of the preceding tube, a motor, aid tuning means in said tuned anode circuit, the method of `automatically tuning said anode circuit to secure maximum input to said grid, which includes the steps of measuring the current iiowing in said anode circuit, obtaining therefrom a secondary current exhibiting la maximum when said anode -current reaches a minimum, causing said secondary current to operate said motor in proportion to the current, and operating lsaid tuning means from said motor to vary the tuning of said anode circuit while said secondary current increases, in a direction to cause still further increase thereof, and to cease varying said tuning at the point of maximum secondary current.

3. In an electrical signal transmission :system including a variable reactance and a valve having a tunable anode circuit, said varia-ble reactance comprising part of said tunable anode circuit, means electrically associated with said tunable circuit yfor producing a control current proportionally varying according to the instantaneous resonance frequency oi' said tunable circuit and attaining a critical value at the point of resonance of said tunable circuit with a signal frequency, s-aid control current producing means including a series resistor in said anode circuit and a polarity reversing valve having its grid and cathode connected respectively to the lower and higher potential ends of :said series resistor whereby said control current reaches a maximum when the current through said resistor reaches -a minimum, electromagnetic means responsive to said last mentioned l eans, coupling means between said electromagnetic means and said variable reactance for varying said reactance under control of said electromagnetic means.

In an electrical signal transmission system including a ytunable circuit having a Variable reactance means electrically associated with said tunable circuit for producing a control current proportionally varying according to the instantaneous resonance frequency of said tunable circuit and attaining a critical Value :at the point of resonance of said tunable circuit 'with 4a signal Ifrequency, electromagnetic means responsive to said last mentioned means, coupling means =be tween said electromagnetic means and 4said variable reactance for varying said reactance under control of said electromagnetic means and control amplier means connected in front of said electromagnetic means and adjusted to -operate it in accordance with the variation of the controlling current for causing said electromagnetic device to respond when the value of said controlling current changes in one direction and for preing 'a'C'Cding to the insive to said last mentioned meansfooupling means 'betiv'ceiifsaldelctrngritidmans and said variable reactancedfor varying said reactance under control of sai'd4J l'ectionfagr'etic*freans, and ad- .d-itio%r;al ,-yariable reactance in said tunable =circuit, a driving source varyingsaidfadditional reactance, and a relay connected in series with said electromagnetic means being adjusted to disconnect said driving source when the rectied current reaches a predetermined value not greater than the maximum value.

6. In an electrical signal transmission system according to claim 4, including control amplifier means with its output arranged to be connected in front of the electromagnetic means and its input arranged to be connected across a resistor in series with said control current, va switching arrangement for facilitating the Vpreliminary and final tuning of .a num-ber of similar ampliiier stages, each having control current Iproducing means with -a series resistor and each being provided with an additional variable reactance connected with a driving source common to all stages for preliminary adjustment, and the last or said stages including -a relay and means whereby theY controlling current corresponding to the last tunable circuit of said last stage operates said relay for disconnecting said driving source on reaching a predetermined value not greater than the maximum value, said switching arrangement comprising a pair of coupled rotary switches having one more position than the number of stages to be tuned, said one more position `being for preliminary tuning of all stages simultaneously and said other Ipositions being for the nal tuning of each stage in turn, one switch of said switching arrangement being adapted for connecting in said other positions the input of said control amplifier lacross said resistances of the various stages in turn, the other of said switches being adapted for connecting in said other positions the output of said control amplifier to the corresponding electromagnetic means.

.7.. In an electrical signal transmission system according to claim 6 where said relay circuit includes two rectifying devices connected in opposition :and a relay arranged between the anode of the iirst rectifying device and the cathode of the second rectifying device and shunted by a resistor.

8. In an electrical signal transmission system according to claim 4 where said control amplifier includes two valve amplifying stages and an interstage network including therein a rectifying device, said coupling network comprising :a condenser connected to be charged through said rectifying device when the anode potential of the rst amplifying stage becomes increasingly posi- 1'1 tive, the rectier preventing 1the condenser from discharging when the said anode potential `decreases, whereby change of direction of said controlling current c auses said control amplifier to cease to yield an output. y

9. In an electrical signal transmission system according to claim 8 in which the potential of said condenser is `applied to the control grid circuit of the second amplifying stage, said electromagnetic means being connected in series with the anode circuit of said second stage.

10. In an electrical signal transmission system according to claim 5 Where said relay circuit includes two rectifying devices connected in opposition and a relay shunted by a resistor arranged between the positive terminal of the first rectifying device and the negative terminal of the second rectifying device.

WILLIAM ALEXANDER GOLD.

REFERENCES CITED The foliowing references are of record in the le of this patent:

Number 12 UNITED STATES PATENTS Name' Date Y l Demarest et a1. May 3, 1927 Bernhard Oct. 18, 1927 Hansell May 9, 1933 Guanella Dec. 5, 1939 Guanella etal Feb. 18, 1941 Muller Dec. 16, 1941 Appelton Jan. 27, 1942 Goldstine Sept. 19, 1944 Cunningham May 22, 1945 FOREIGN PATENTS Country Date Great Britain Jan. 20, 1936 OTHER REFERENCES Serial No. 363,862, Publication to Dolle et al. (A. P. C.) published May 25, 1943.

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