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♦ PDF Cover Page ♦ ♦ Verso Filler Page ♦ Triode Operation of KT88s ROBERT M. VOSS AND ROBERT ELLIS';'
In response to many requests, and after a great depl of effort, the authors present a higher-powered version of their ten-whtt all-triode amplifier.
HE BRITISH KT88 power pentotle. hig brother of the much respected KT6t3, Tquickly became popular after its ill troduetion ill this country not too long ago. Audiofaus no longer had to couple four tubes in push-pull parallel in order to get enough powel' to t:lrin' a low effi ciency spl'ake�·. In addition to its tapped-screen IIdapt ability, the KT88 11 Eb 485 v 10 170 ma I"wz Big 180 11111 560 oh illS (pel' tube) Ein (g_g) 70 v R,(A-A.) 4000 ohm POlit 27 '" D 1-3% First to clttch our eye were the zero lind maximum sigll:!1 currents; the total • 697 West End Au., New YorK 25, N.Y, Fi9. 2. B::ttom view of the amplifier. Note 'twisted filament leads and ground buss. \'arilltion is less than 6 pel' cent, mean illg that Olll' power supply requirements w('I'e grelltly simplified. So fal' so good. The load impedance of 4000 ohms is, howeyer, a problem. \Ve know of 110 pre viously dcsigned tubes mellnt to work into sHch a load except in ultralincar operlltion, and transforlllers with screen taps lire expensi\'e. vVe settled for a Triad S-42A which is wound for 4500 Oh:IIS, :Iud, by jnggling the operating pal'llilleters a bit, ,vere still able to ob Fig. 1. Top view of tllin 25 watts.1 The S-42A is rated lit 50 completed ampli \\'atts, which mellns that power at fre fier. quency extrcmes will not drop off be cause of poor output transformer powel' hllndling capability. The amplifier deli\' ers the full 25 watts lit 18 cps, (the lower limit of our generator) and only slightly less at 20,000 cps. The Power Supply As we mentioned beforc, the low cur rent fluctuation of the output stage sinl- 1 Au ud"autage of triodes is that using too high a load impedance will not increase distortion although it will decrease power. 30 AUDIO • MARCH, 1961 plifies the power supply. A single GZ34 l'ectifier feeds what is essentially a ca 0.1 pacito r input filter. A fter an LC net ��"��"--����++�----������P-"�� +-��4---� work to the output stage supply, B+ goes I '.0 - through resistance-capacitance filters to - r ----1--+-l-+-H-+-H----+-++-++ -++t+--'·08••�a="·.J'>I- ''.>� ""�I'J',�t+f.++- ---4 the earlier stages. The preamp power 110.. ,-H4--�i ' take-ofI sockt't will supply sufficient ����I��--+--+-+-r�+++---r-r-r;��Hr-���- - -- -H--ilft+rr---I-++H-tt+t--+--'V-:>- · f--- J' power to most un powered control units. ! I. The 10-ohm resistor is a device recom GAIN, .8 f INPUT FOR 25 WATTS OUTPUT . -HUM & NQISE,(BELOW 25 WATTS OUTPUT)I--+-f-H+tt-ft---t mended hy t.he Dyna Co. to avoid _r- II INPUT OPEN-73 db ground loops. If you 're 110t powering a I-+++f-t-T-H--...L.-t-l-+-+m INPUT SHORTED-90 db preamp, make up a shorting plug with t--f- ju mpers fl'om pins 6 to / (to turn 011 the amplifier), and pins 4 to 8 (to place bias on the heaters). -'- I-+-+-+-ht-t++ The Audio Circuit '0 100 I • � 1000 I. "0000 IfXJOO!> To drive the output stage, we used the FREQUENCY IN CYCLES PER SECOND cathode-coupled phase splitter, with Fig. 4. '!Frequency response. which we and other d('sig lers have had l value capacitors, using them to e�tend men(ls s('pn rate cathode resistors for the excellent results. Driving the inverter, the low-frequency response below' that output tuhes, we used Il com1110n res is and determining its operating point, is of the output transformer. We cho e the tor, mainly hecause we are using a n conventional voltage amplifier. The J second alternative, because the firs� pre- matched pllir of tubes. TI�e control grid phase splitter, which is capable of de � arc th sents p ase-shift problems which nke nn 2 Better yet; if you have the equipment, connect the feedback loop and adjust the balance control for lowest Dr at about 2 Fig. 3. Schematic diagram of 25-watt amplifier. watts. 32 AUDIO • MARCH, 1961 thc ft'edlJal'k l'('sistor and capacitor in position, shake the hits of wire and sol der out of thc chassis and you're through. Performance 'fhl' SIJeI'itications look nice pnoug'h, but it is thc sound of this amplifier that you'll like the best. The extreme stabil ity (we used 20 db of feedback, leaving a stahility margin of more than 15 db) of the ulllplifiel' is demonstrated by the fact that it recovers immediately from d.c. Jlulses of considerable magnitude applil'd at the input, with no ringing or hangover. We t rit'd several records whil'h we had considered to be substandard as far as over-all clarity is concerned. Thl' n('w II.llIplifier reversed our thillking thl' dl't'1' bass had been there all the time, and the l"I'I'ol'd �cl'lltl'h wns not IIrn!'ly as prominent as II I('ss smooth amplifier had led us to h!'li!'YI'. This U1uplifil'!' ('o:;t close to a $100 to build," whi('h yon lIIn�' ('onsi(II'I' ('xtnl":1- gant ill vil'w of the fact that one can buy 50-wnlt aJllplifit'l's for that priet'. HOWI'WI', the sound frolll it is so e\ean at norlllal allll \\'l'll ahove 1101'IIIal listt'lI ing levels, that we feel it is well worth the eost. JE • The stereo version would cost about $150. It could use as a power transfornler a Triad R·!!5A, and a C-20A for the choke. For tho input stage, eaeh channel could use half of a 12AX7 to eliminate the filament drain of the 6A V6. PAIn'S LIST R, 470,000 ohms, % watt R., Rm R", RZJ 10,000 ohms, % watt H. 1000 ohms, % watt, 5% R, 30,000 ohms, % watt, 5tfr. R, 180,000 ohms, 1 watt, 5'70 R, 1 megohm, % watt R" R,. 18,000 ohms, 2 watts, 5'70 H., R, ::!:?OOO ohms, 1 watt, 5% H,. 25,000 ohm, potentiometer Hili RI1 220,000 ohms, % watt H,,, H" 2iO ohms, % watt HlI 3900 ohms, 2 watts H,. 150,000 ollms, % watt H,. 10 ohms, % watt H2O 280 ohms, 20 watts (selected 250 or 300 ohm) H" 200 ohms, 20 watts C, 47 !l!lf, 5% C, .:?5 !tt, 600 \' C"C, 1 !If, 600 v C, 300 !If, 150 v, electrolytic C, 16 !Jf, 'j00 v, electrolytic C, 20 !If, 600 v, electrolytic, COl C, 40/40 !If, 500 v, electrolytic (can) C,o 12 !tf, 500 \', electrolytic T, Triad R-71A T, Triml S-42A Cll, Triad C-15A V, 6AV6 V, 6SN7GTB V" TT, KT88 V, GZ34 FJ 3AG, 3 amp ♦ Verso Filler Page ♦ United States Patent Office 3,153,766 Patented Oct. 20, 1964 1 2 privilege of the amplifier designer. The positions 3,153,766 chosen will determine the two different sets of MODE OF OPERATION OF THE PENTODE OR. characteristics between which the power output TETRODE THE OUTPUT STAGE OF TUBE IN tube will alternately operate. AUDIO POWER AMPLIFIER AN Renato E. Tancinco, Meycauayan, Bulacan, Philippines 5 Filed Sept. 8, 1961, Ser. No. 136,817 The New Circuit Claims priority, application Philippines Sept. 12, A good amplifier must not only be good in perform 1%0 Claims. (Cl. 330-1'22) ance but it must meet the require ents of the manu 5 m facturer, the amplifier designer, and above all satisfy the This invention relates to a new concept in the mode 10 ultimate user, i.e., the owner. of operation of a pentode or tetrode power tube in the It is a fact that in the reproduction of music, the output stage of an audio frequency amplifier. Up to high power peaks are relatively few and of very short the present stage of the art, the output tubes in an audio durations, that, most of the the amplifier may be time frequency amplifier operate only of char under one set considered as idling. Low consumption of power dur- acteristics which is determined by the mode of opera ing idling is therefore of paramount importance. An 15 tion assigned to them. The following are the modes of amplifier is usually rated at the maximum power that it operations under w ich a pentode or tetrode power tube h can deliver during these peaks, with a minimum of dis may be made to operate : tortion and good stability. The maximum power that (1) Triode connected pentode or tetrode output stage. a pair of power tubes can deliver is usually limited by i of (2) Distributed load pentode or tetrode output stage or 20 the maximum plate dissipation rat ng the said power the so called "Uutra Linear Amplifiers." tube. To get therefore the most out of a pair of power (3) Straight pentode or tetrode output stage. tubes, the plate circuit efficiency of output stage should (4) Unity coupled output stage. be high. It is also very desirable to obtain high power (5) " xtended Class Amplifier" using a pair of triode outputs without the necessity of using very high signal E A connected push pull pentode or tetrode paral el grid drive voltages. In other words, the power sensi in l 25 with another pair of the same type of tubes con tivity of the output stage must be high. It is therefore nected as straight pentode or tetrode. clear that Class B operated pentode or tetrode power tubes appear to ideal for the output stage. The re The sixth mode of operation is offered by the under be quiremen of low distortion in ou put stages using the signed and is the subject of this patent application. t t In conventional Class pentode or tetrode power tubes is, this new modc of operation, the output pentode or 30 B however, not easily met. The cause is the presence of tetrode tube alternately operates between the two sets a form of distortion, called the notch distortion, which of characteristics assigned to them. These two sets of occurs as a notch or fuzziness in the output signal wave- characteristics are determined by the position of he two t form. In reality, this fuzziness in the signal taps on the plate winding of the output transformer to 35 output waveform is a form of shock excited self oscillation at which the anodes of the back to back connected diodes, some ultra-sonic frequency in plate winding of the feeding the screen grid of the said power tube, are con the output transformer. The leakage inductance between nected. The particular set of characteristics under which the plate windings of the output transformer forms an the output tube operates is determined by the polarity oscillatory circuit in combination winding of the grid signal voltage relative to the quiescent poten 40 with the capacitance and other stray capacitances. This oscil tial of the control grid, otherwise known as grid bias. the latory circuit is shock excited into self-oscillation every Some of the objectives of this invention are as follows: time the plate current of the power tube is driven to (1) To obtain high quality reproduction of sound with- cut off. Unfortunately the frequency of this fuzziness or notch distortion is so high that is beyond the effec out the necessity of using expensive special output 45 it transformers or resorting o o pl itr . control of the inverse feedback loop that may be t c m ex circu y tive (2) To be able to obtain the most power output avail applied in the amplifier. Furthermore, the plate cur able for a given power output tube and a given B rent of the power tube which is the most logical part supply voltage, commensurate with high plate circuit �f the amplifier that can prevent the said distortion, has efficiency, low distortion, and low power consump been driven to cut off. The said power tube, being then 50 tion during the absence of signal or during idling. inactive, is therefore not in a position to prevent the (3) To att the above objectives without the neces occurrence of the said notch distortion. So far there ain sity of using more than one pair of power output tubes are two successful solutions to the problem of prevent or more than one power supply or B-SuppJy. ing the occurrence of the notch distortion. They are exemplified in the Unity Coupled mplifiers and the (4) To reduce the output impedance of the power out 55 A put stage before the application of any inverse feed Extended Class amplifiers. A third solution is offered A back. by this invention, which is believed to be an entirely (5) To be able to attain the above objectives without new concept in the mode of operation of pentode or the need for very high signal grid drive voltage to the tetrode power tubes in the output stage of an audio power power output tubes. amplifier. 60 (6) To give the amplifier designer a free hand in the The foregoing objectives will become more apparent design of the whole amplifier. The said freedom is from the following detailed description of various em achieved by virute of the following: bodiments of the invention and from the accompanying (a) Absence of inherent inverse feedback in the drawings wherein: output stage. The manner of application of the FIG. 1 is an electrical schematic diagram illustrating 65 inverse feedback is left entirely to the amplifier a push -pull tetrode power amplifier incorporating the designer. switching diodes in circuit between the screen grids and (b) The designer is free to choose any type of output transformer. pentode or tetrode power tube for the power FIGS. 2-4 are views similar to FIG. 1 but illustrating amplifier stage. various modifications for connecting in the switching 70 (c) The choice of the position of the taps on the diodes; and plate winding of the output transformer is FIG. is a detail of one of the switching diodes in- the 5 3,153,766 3 4 dicating the convention observed in designating its cath former T-2 can be any standard output transformer ode and anode components. ordinarily available. Even though tube V-a is biased Referring to 1 of the annexed drawing which to the projected plate current cut off value like in con FIG. p of is ventional Class B amplifie s, its plate current can never forms a art th patent application, there is shown a r push pull tetrode power amplifier, which operated as reach cut off. The action of the control grid in forcing is 5 a conventional Class output stage except for the man a plate current cut off during the negative halves of B e in which the reen r o he p w e o e the grid signal effectively counteracted by the n r sc g ids f t o er t tr d s voltage, is V -a and V-b are fed. In a conventional output stage rise in the potential of the screen grid which is then using Class B pentodes or tetrodes, the screen grids of tied to the plate due to the conduction of diode V-1. the tubes are fed by the B-Supply for the plates or, if The low plate impedance of the then triode connected 10 a different voltage is equired for the screen grids, by a V-a effectively damps the pl te winding connected to r a separate screen grid power supply, It will be noticed it, thus preventing the winding from ever being shock excited into self-oscillation. possibility therefore, that the screen grids of the power tetrode tubes V-a The and V -b are connected to the common cathodes of two of the occurrence of notch distortion, can entirely be pairs back to back connected diode s, V-I, V-2, V-3 precluded. The fact, that the tube co ected of 15 V-a is nn tetrode during the positive halves of the grid and V-4. The anodes of the diodes V-1 and V-2 are as a connected respectively to taps No. and No. on the signal voltage, makes it possible to obtain a relatively 1 2 corresponding plate winding of the output transformer large power output at high plate circuit efficiency and T-1 and similarly the diodes V-3 and V-4. When the at very low power consumption during idling, like in tetrode V-a grid is driven to the positiVe direction by 20 the conventional Class B tetrode power amplifiers. We therefore have now an amplifier performs very the signal, the plate swings to the negative direction that - . much like a conventional Class B tet ode amplifier with relative to the B-Plus terminal of the B Supply Such r a si uation makes tap No. 2 on the plate winding more all its advantages but without its disadvantages, i.e., the t positive than the tap No. causing diode V-1 to cut notch distortion. It should be noted however, that the 1, off while diode V-2 remains conducting. Diode V-2 25 grid drive voltage requirements are low and are the same therefore, effectively ties the screen grid to tap No. as when the output tubes are connected as in a conven 2 on the plate winding. Tube V-a will therefore behave tional Class B tetrode power amplifier. In the prototype amplifier that was tried by th under in the same manner as when its screen grid is per e manently connected to tap No. of the plate winding of signed a pair 807's were used for V-a and V-b and 2 of the output transformer. On the negative half of the 30 one pair of 6 x 4's were used in place of the diodes signal wave, the grid of the power tetrode V-a is driven V-I, V-2, V-3 and V-4. The common cathodes of the to the negative direction. The plate of the tube V-a 6 x 4's feed the screen grids of the 807's. The output then swings to the positive direction relative to the B transformer used was a "Merit" 55 watt P.A. output Plus terminal of the B-Supply. Tap No. transformer with a plate to plate rated load of 1 therefore 3,300 of the plate winding of the output transformer then 35 ohms. The circuit used is shown on FIG. 2 of the draw becomes more positive than tap No. 2, thus causing ing. Although the output stage operates as a Class B diode V-2 to cut off and diode V-1 to conduct instead. tetrode power amplifier, the output impedance, without The conduction of diode effectively ties the screen any inverse feed back is and the same as that of a V-I low grid tube V-a totap No. in plate winding of the out triode amplifier. This low utput impedance is e of 1 o the r put transformer T-1. Tube V-a then, will t erefore be- 40 sult of the alternate Tetrode-Triode operation of the h have as its screen grid is permanently connected o output tubes. if t tap No. o the plate winding of the output transformer. of the drawing shows a mode of operation in 1 f FIG. 3 In the absence of a signal, the two diodes -1 and V-2 which the power output tubes operates as a distributed V load tetrod s or "Ultra Linear" amplifier during the are both conducting and are sharing the screen grid cur e rent drain of the tube V-a. It can be seen therefore halves of the grid signal voltage and as triodes 45 positive that the characteristics of dur g during the negative halves of the grid signal As the power tetrode V-a in voltage. the p ositive half of the grid signal voltage is entirely in FIG. 2, the possibility of the occurrence of notch dis different from those, during the negative half of the grid tortion in this case is non-existent. The circuit in FIG. is the same as in FIG. 2 except signal voltage. The tube V-a therefore automatically 4 the anode of the diode V-1 is c nnec operates alternately between two different sets of char- 50 that o ted to the tap on the plate winding of the output transformer acteristics, with the aid of diodes V-I and V-2. The No. 1 In this case the positive voltage applied to the polarity of the grid signal voltage determines under T-4. screen grid of tube V-a thru diode V-1 during which set of characteristics the power tetrode V-a will the negative half of the grid signal voltage i just enoug perate . It must be noted however that the positions of the s h o to prevent the plate current of he from taps No. and No. 2 on the plate winding of V-a are 5 t tetrode V-a 1 5 reaching the point during the maximum power identical respectively to those of the taps No. and No. cut off 4 output of the amplifier. on the plate winding of V-b. The proper positions 3 the foregoing discussions, the principles involved of the taps on the plate windings are left to the discretion In in the operation of this new amplifier can be fully un of the amplifier designer. They can be anywhere from 0 derstood. In reg rds to the diodes V-1, V-2, V-3 and the B-Plus terminal to the plate terminal of the plate 6 a semi-conductor diodes may be used in lieu of windings. The limiting positions of the taps is shown V-4, thermionic diodes, provided however, that the peak in F . this drawing. Here, tap No. is at the in IG 2 of 1 verse voltage of the diodes are not exceeded during the plate terminal ''Of the plate winding of the output trans delivery of maximum power the load or when the former while the tap No. 2 is at the B- lus terminal of to P amplifier is unloaded while in operation. B-Supply. In FIG. 2 of the drawing, power tetrode 65 the operates as a straight tetrode on the posi ive halves Advantages of Invention V-a t This he grid signal voltage, while during the negative of t The advantages that may be derived from the appli halves, operates as a triode. On the positive half of . it cation of this invention in the manufacture of high qual the grid signal voltage, the screen grid of tube V-a is 70 ity audio frequency power amplifiers may be summed up tied to the B-Plus terminal of the B-Supply while on the as follows: negative half of grid signal voltage, the screen grid is (1) The application f this invention may be con o connected to its plate. Automatic switching is ac sidered the most practical approach to the design and complished by the back to back connected diodes V-I manufacture of high quality reliable audi power am . o and V-2. this. mode of operation, the output trans- plifiers that are capable of relatively high power output In 75 3,153,766 5 6 with low distortion, high plate circuit efficiency, low power the power output tubes, less power consumption and saV consumption during idling, and at a reasonably low cost. ings in money for both the manufacturer and the end user. (2) The amplifier does not require specially designed (11) The choice of the mode of operation of the power output transformers. fact, standard output trans output tubes is entirely left to the discretion of the ampli In formers commonly available can be successfully used. fier. The 5 desired two different sets of characteristics The e is no need for sectionalized and intermeshed plate under which the power output tubes will alternately op- r windings that are required for the successful operation erate may be obtained by the amplifier designer with the of the conventional distributed load or "Ultra Linear" proper choice of the points in the plate windings of the amplifiers, nor is it necessary to use a special, bi-filar output transformer to which the anodes of the back to wound output transformer as in the case of the unity back connected diodes are to be connected. The ampli 1 coupled Mclntosh and Gow amplifier. The' presence of 0 fier designer is therefore completely free to choose the the full B-Supply voltage between the turns of this bi two different sets of characteristics under which the power filar windings greatly affects the reliability of the whole output tubes will alternately operate to obtain his desired amplifier. results. (3) The amplifier requires only one power supply (12) The circuitry of this amplifier is very simple. 15 and this power supply is operated at signal ground po The simplicity of the circuitry makes it easy for the man tential. The "Extended Class" A-2 amplifier requires an ufacturer to duplicate the performance of the original additional well regulated screen grid power supply for model during the production run. Simplicity of the cir its successful operation. The "Circlotron Amplifier" of cuit contributes to the stability and reliability of the whole Electro Voice, Inc. requires two separate identical amplifier. and 20 power supplies, one for each power output tube, and In class B power amplifiers, the push pull output tubes furthermore, these power supplies are operated above alternate in the delivery of power to the load. The signal ground potential. The additional cost of the ex- power output tubes deliver power to the load, only dur ing the positive half the grid signal voltage. During tra power supply is worth considering, and moreover, of operating the power supplies above signal ground poten- negative half of grid signal voltage, the plate current 25 the tial can affect the frequency characteristics and the of the power tube is cut off. The power output tube is sta inactive and therefore serving no useful purpose. bility of the whole amplifier. then (4) Only one pair of power output tubes are required The essence of this invention is to make the said inactive for the successful operation of this amplifier. To equal tube do some useful work, i.e" act as an effective damp the power output of a conventional amplifier using one ing element to the then inactive plate winding of 30 the of power output tubes, the "Extended Class A Am output transformer. During the negative halves of the pair plifier" requires two pairs of power output tubes. Con grid signal voltage, the output tube is operated as a tri sidering the present cost of good audio power output ode or near triode to prevent its plate current from ever tubes, the additional cost of the extra pair of power reaching the cutoff point and to act as a low impedance shunt across the inactive plate winding of the output tubes is certainly worth savings. 35 (5) The output impedance of this amplifier before transformer. The action of this, otherwise inactive tube the application of any inverse feed back is low and the during the negative halves of the grid signal voltage, effec same as that of a triode power amplifier. The alternate tively prevents the occurrence of the notch distortion. claim: operation of the power output tubes as a triode or near I triode during the negative halves the grid signal volt- 1. As an output stage for a push-pull audio amplifier of 40 age accounts for the low output impedance of this am the combination comprising first and second tubes con plifier. nected for push-pull anode loaded operation, each said (6) Elimination of the notch distortion is accomplished tube including a cathode, anode, signal grid and a screen in this amplifier without resorting to Unity Coupling or grid, a push-pull output transformer, said output trans- Extended Class A operation with their attendant disad- former including first and second primary portions respec 45 vantages. tively connected to and loading the anodes of said first and (7) Compared to Unity Coupled amplifiers, the sig- second tubes, first and second pairs of common-cathode nal grid drive voltage requirement of this amplifier is back-to-back arranged diodes connected in shunt respec tive y with said first and second primary portions said very low. It is not necessary to design a special driver l of stage to deliver the high signal voltages at low distortion output transformer, and circuit means respectiVely con 50 as is called for in the Unity Coupled amplifiers. It is necting the common cathodes of said first and second very possible for the driver stage to contribute most of pairs of diodes to the respective screen grids of said first the distortion the output of otherwise distortion and second tubes for effecting automatic switching of each in an free Unity Coupled amplifier. Specially designed driver said screen grid alternately between the terminals of the stages can increase the cost of amp er and add to corresponding primary portions of said output an lifi 55 trans the complexity of the circuit. former. A push-pUll audio amplifier output stage as defined (8) This amplifier is not very particular as to the type 2. in claim wherein said first and second portions of the of the power output tube. The manufacturer or the am 1 plifier designer is free choose any power pentode or primary of said output transformer are provided respec- to tetrode for theoutput stage of this amplifier. tively with first and second pairs of taps, and said first 60 (9) The absence of inherent inverse feedback in the and second pairs of diodes are connected respectively to output stage this amplifier is a distinct advantage. said pairs of taps for shunting the same to thereby effect of The amplifier designer has therefore a free hand in the choice said automatic switching of each said screen grid alter- the manner i which the inverse feedback may be nately between said taps. of n Inherent feedback in the output stage A push-pull audio amplifier output stage as dermed applied. inverse 65 3. claim wherein said first and second pairs of diodes is inevitable in Unity Coupled amplifiers. This inherent in 1 feedback accounts for the very high grid drive signal are connected respectively in shunt between the apper voltage requirements of Unity Coupled amplifiers. taining anode terminals and the positive terminal of the (10) The fact that the notch distortion is non-existent power supply connected between the cathodes and a point the amplifier, makes it advantageous to set the grid on said transformer primary intermediate the first and in 70 bias of the power output tubes to the projected cut off second portions thereof to thereby effect said automatic value of the plate current. The amplifier is therefore switching of each said screen grid alternately between capable of relatively high power output at high plate cir the apertaining anode terminal and said power supply cuit efficiency, attendant to low power consumption dur- terminal. idling. This will mean a very much longer life for A push-pull audio amplifier output stage as defined ing 75 4. 3,153,766 7 8 in cla wherein said first and econd portions of the connected respectively in s unt between the appertaining im 1 s h primary of said output transformer are each provi ed tap and the positi e terminal of the power supply con d v with a tap and said first and second pairs of nected between cathodes and on said trans diodes are the a pOint connected respectively in shunt between the appertaining former primary intermediate the first and second portions tap and the an de terminal the appertaining tube to thereof to th reb effect said automatic switching of each o of 5 e y thereby effect said automatic switc ing f each said screen aid screen grid between th corresponding h o s alternately e grid alternately between the corresponding tap and the tap power supply terminal. . and said anode terminal. in the file of this patent amplifier output stage as defined References Cited 5. A push-pull audio in claim 1 wherein said first and second portions of the 10 UNITED STATES PATENTS primary of said output transformer are each provided 2,710,312 Hafter et al. ______June 7, 1955 with a tap, and said first and second pairs of diodes are 2,868,896 Gabor et ______Jan. 13, al. 1959 OCT. 20, 1964 R. E. TANCINCO 3,153,766 MODE OF OPERA nON OF THE PENTODE OR TETRODE II'i THE OUTPUT STAGE OF AN AU O POWER AMPLIFIER DI FILED SEPT. 8, 1961 T-1 T-2 o < o ... FIG. 1 FIG. 2 T-3 T-4 FIG. 3 FIG. 4 ANODE [>ICATHODE ...... - ELECTRON FLOW INVHNTOR �c��.T�'O FIG. 5 RENATO E. TANClNCO ♦ Verso Filler Page ♦ Three-quarter view of high-fidelity amplifier chassis. W ALTER T. SELSTEO* and ROSS H. SNYDER** High-Power T riode AmpliFier The authors describe a high-quality audio unit for-exacting applications. .\ ,\t'!lI,' ,\)II'L"'JJ::R of higb gain. exceed the requirements of measure "'ith tht'�l' rClluirt'JIlClltl! ill "il'w, high II\,Wt'r. high stllbility, wide ment, but good "oltllge regulntion of t h(' I(,ng !,!lIooth cur\'(':< of t h(' Inrg<'r A frr"'luenc�' r.mge, 100v distortion the output would hn\"e to n('(,oU1pnn�' triode 11\1\\'.:-r tube;: !tMlkt>d IIttrlwti\'\�. "",Il,,\\" lwi;:e (·ontent was required at the respon!'t'. Becllu� thE' nmplifier All Oppt1rtl:nit;\, h1 e\'ulul1h' tll<' r(!ilult,. ,,1lI;.et 11 ... of 11 ;:eries of measurements would be U,.ed at Iow 08 well as high tt, lIE' hlltl :'r .. m the high COllt,. u,;unll�' .. t" 1,11111:111 I)t':lring, loudspeaker per output le\'el&, internnl noise would h:l\'C' n"sucillte,1 with triocl(· .ll·lIiJot 11,. tllll" . . . . rlll:IIH'e, :lIId ret'Ording fi(lelity. Be to be kept below ouilibilit�· eOlllpnr('(1 "f(�nH-d it-elf nl�.:,. ":III�l' thi!! unit wai! to be a link common to the zer(l 'Vu 1('\'('1. It WitS olltici · Low Cost Components , ,, :Ill tlU' lllen�lIre11lents, its character· pated that the lllt>ailUft>1n('ntll {or wlli, ,1 . FUlld" too\' t h(' l·xIM·rimcllt.. w�rc Ui. i�J i.'" hnel to exceed !;uhstantially the thil' equipm('ut \\,11" t' 0 11 '" t rue t (. d mort> )l1t'lItiiul thllll u!lulIl. 1'0 ttlC )(1,;. I.·", "Xl'e,\·t:ltion� of the electro-mechnn would take ;;ome w('Ck", Of montllll tliloilit�· . . , u"iuJr 1,\w-.·. . "t wnr-,.urplus i"nl '�llIiplIlent im'oh'ecl. to complete. nnil !'int'e the ,lata nc tul)('" \\";1,. ,··xplorl·.l TIll' l'ntl. ..r formi:I, C ':d"l1Jnticl)) inili" nted thnt with loud cumulated ,,'ould be eorrelnt('(l, cxtra nhl,' :!11 L,· for r. lOll;: whi le, \x"{'n in -I"·:lk,·r� (If :I"t'rage ('fficienc;\' pe.lk be It efJ llir ordinary stnbi1it� ",oul,l r .. ' '·"I'inu,. "�:l'I,b' ut I'fi('I'" C'tllllllllrnhll' I",m·r· .. I' 11'; much ns 2:, ,,,,,tts might he llIt'nt in the :lmpliner. Finolb'. (!\'('r�· tll t hn ...· .. :' rl.'l·C'iw·r t�'I"'" P"w"r IlIlJlJlb' ""1·.',·.1 to ntwin th(' desire.l 80111JtI ren80nabl(' ]lre<.'aution hnd to he tokell r''tlllire·lJIt.!:tS>. 1l1w:I�'" "",·,'re- ill tl'iMP 1,·\. I. .\ frt>OlueJl(,�' rllJJgc of 30 to 8goinllt intrt)()ucin� cii;;tortion prolith'e lit-teller. nlllfl1lg rll,I: .. III1lIlTt'uril t'.\r IN,wI'r «'fllliJl- Fig. I. Schematic of audio amplifier. .. wall, carLon. I: I :11(1.1)1"1 hlll-, 1 � 11. 117 I·' ·•. 1,)I.I,OOO,,,hm "irt',wounol 1'01t'nlionwlt'r. ".. tOOK Tut' I:::. I; J 1-1000 ...11111;, 10 lI'all�. wirp·wolln.1. T'e I::,. J:��. 1:1�, IlB--:;n,OOO ohl1l", 10 wall., wir·�.\H.untl. 1:1, - I �er Jpxl I, 11. t�OK $ I;;, 1:')-1;;0,01)(1 "hl11 •. I � W:l11. ,·arloon. I: 111··-:;0.000 olum. 1 � wall. carhnn, J: 1·1. 1:1;; --3(10,0·:1) ohm�, ! � wall. rarl,on, I: 1(,- 11100 olm!>, :;0 wall�. FIi£ler ad.iu;oJ,'d 10 670 ohm�. 1:J7-:iO,ohm +,,·all poJPnliomelf.'r. wir� wuull.L J: 11\-,�5,OOO ohlll'. 1l) wall�. wire,wound, Cl C7 1:19-100.000 ohm�. 200 walls. wilh slidpr. wire,"·ound. tO� to'l 1.1- lOO ""f. mira. Tt (2. r3�.5 "f. 600 \"olt�, oil·filled, C�. C5-0.1 "f. (,()() \'olt�, oil·filled, (.6. Ci -20,0 "f. 600 \'oh�. oil · filled, tUOY f.s. C9-17,5"f. 1500 \·oh�, oil·fill<,,j, TI LT.I.. rnl·3 T2 l.T.C. �3 n LT.C. S�5� n LV:. !'.(,::, 11 I'.V·. �,31 AUDIO ENGINEERING • FEBRUARY. 1949 9 dilllillatt....t . b�' it,. ncce",.it�· for CI.ou. "tllllt rendjuetllwlH with tulte ol.l'it,,:. Tht' stability al,,1 gain of the OOn, '·(,lItiOIl,,1 . ftooting'rarnphllH' in\'ert� were Ilttractive. but itl! UL'\"l'r-quit". IIt'rfl1C,t hal"nc!o WI\3 a di$4d\"autnll". rill he ('ud n mOlliticRtion llf tho Illttl"r t.\·'It� I'ro\'l"I "nti"fnctor�': thl� lon,l on till' input tll� "'"s "arial hy llhuntinl[ Iti with HR. CIf \'ur.\'iIlJe "nl1l1'" ollU\'" II mf'/-tohm, IIntil :I drtulIlI�' Il('r('<" hnlnnl'e Ilf n.f. to the ,Irin'r ;:rill" \\'11., "htu iut.... !. The tlo"ting-purl'l' ha:'C lIetil.lIi 1I111intnilll'e.l the h"lamt'e o,'er 1""!lIIt'rill,l, Fig. 2. Frequency response �e of audio amplifier at 30 watts output� IIf lI"l', Note th.t it Is flat within leSl than one .db from 30 to 20,000 cycles. The titl"I retinement ill tht' "ircuir ment in tbi� nlllgt' ha. lW\Ilted in hil.l'h-potl'ntinl end of the IIJ5 l'hlt(· rl" wo. the addition llf Il hurt .. time ron· quantity r�l\k'tron ()f the requircci siators the.oe tubea, oper"ting lit IWllrl.\· shmt tilter ill the illput. l·lI1,·ul"t..... I tu tranaformer. At tolerable COil. Hf,h- full rnte.:1 clillllillntion in Jlu,.h-pull. �.. unter:ld the hillh·f�'lllt·'\I'�' 10,.,..', \"oltaare ron,\elll!er1l were 11l"llilnble "'ouM tlri\"(· the :Bb hl8t "t·�·"IIC\ tIll! within the omplifier duo t., .1i:5tributeol throUl'i, wllr ,urpl"". .\n,1 11 hridge I't'oluirement". "h 11 lit �·lIpIlCitllnCC,.. l[jllh·fr('flUenl!�· n.... ·titier "'",. fell"ibll� with tht' 10"'- The tri",).·" .Ii.·, nat'tli"m'lIlu clrh'('r In ..",.,. ,ltl(, tn 11 m('r I·ff,'!·t, '.,ftt'n ex ...·· Jlrif't'd ;;R4ffY f('(·titil·r t\l�. with it.- tllt('<1 tlat' UM' of ullotht'r IItU/-tI' lOt' ,· .. It· ri'·lIl·l'C.\ in high,ill1l'l! 10 AUDIO ENGINEERING • FEBRUARY. 1949 The intermodulation I!ur,'e of Fig. ;; I}(r.;es, the rinal j llol�,.'llIl·lI( would /a'I\',. was taken with the Pidcering nnalyzer. to be th:lt of prolongecl li.tcnill:': ,,�. using input frequencie:i of 100 an.! critical �ll'le, The natllr(' ,',i the tlis· ';'000 oycles at 10 decibels difference in tortions which offend a "!roMen ear" lel"el. Tot.al harmonic dL4tortion analy· :ttiU defi� :'naly�i�. hilt whatc\""er their sis gln'e reading of i/a to 11 � the numer' causes t.h�e <,ffI'IlSeS hllll to he el:dutll'tl lcal ",ruues of the DID figures, at C.'l· from the pre5ent amplitier if it were responding powers. to be useful. The operation W:IS ap The frequency-response curve oi parently :1 SUCC�8, for the equipment Fig. 2 is a power line, based on 30 watt. .. 11'8' later u,;eU with IabMator�··prt)ducctI and is therefore of significance at much tl'llnsducer,; L';lJluble of a,:cllrate re more than usual operating lel"e1a� Thr production (of sounds ill rh" :!O,lJf�1 10\l"-frequency drop is attributed to out· cyde aren without pain to tht' l't�rhaJl� put·transformer core-saturation. h,.ypercritil'al listening of the �1)4>uhr Xoise level was measured with 11 eTe4tors. Prolonlfed use has ('Ollnrlllc.\ !lhielded 100,000 ohm resistor across thl' the original ')pinion that t.he amplitil'r input terminals, with the gain control is contribmiilg no unpleasantlle�s, It� wide open. Further amplification of tht' low intern.,l impedance is autlibly :111 output was required to distingUish it; etJectil"e d.unp on oscillll tOQ' ,·xcursion character audibly. It was the usual ran· of bass s:p(';,kers emlllo,\'('t\ with it. dom hiu superimposed upon a tower For all l't the eX('{'lIl'nt eh;lractc'r level of hum, The absolute level oi iatics obt;,illt',1 here, no fill'! i� meullt noise at the output under these condi· to be addt't\ to the fire uf acadellli,' tions was -52 dbm. or nearly 96 deci· argument ,wt>r tTiotles and be.llll tullt's. bels below the nominal 30-watt output The high-frt-quency equiPllleut men level. tioned abol"e has been used witll beam Effective internal impedance . at the amplifiers employing ample feedbnek 15-ohm tap is 8 ohms. with no decrease in listening 5.ltisfa.·- Thirty watt. of output was reached _.HoD. It is belie"ed that the requirt·· with about 0.1 Tolt drive, giving • ments met by the present amplifier nominal 85 db maximum gain. could hal"e been reached t.>qua1\�· \\"('11. Although all the testa indicated that and at lower cost, with beam tetrodf" the amplifier was �tabte fo� �ta pur- using gen('rous feedback. ♦ Verso Filler Page ♦ PENTODES & TETRODES OPERATING AS TRIODES by C. C. McCallum screen current 15 one-fifth of the INeE commercial radio receiver H.T. + S designers have nece!'s.lI·ily to anode current. Thi!'< relation�hip can and public address equipment be assumed to remain suhstantially provide a maximum audio power constant throughout the ranges of output for a mllumum D.C. power potc'ntial in which we ale intere!>tt'd input (to reduce cost of H.T. supply and (�nable� the "triode" anode equipment) and .1 minimum audio currt'nt to be derived by addition of input swing (to reduce cost of early INPUT screcn and anode current. 'Vhere stages), the pentode and its successor the manufacturer gi\·cs a family of ,he beam tetrode has been almost anode currt'nt-control grid vult age universally chosen for the output OOTPUT lOAD curves for identical anode and �crt'en stage. voltages, th� procedure of conversion .T.- to anode current-anode voltage curves The result of this concentration on H is simplified. When' the anode power efficiency has very naturally currcnt-control grid voltage cuncs· cau&ed the valve designers to neglect are given for varying anode voltages triodes and to devote their skill to and fixed screen voltages, interpola the still further improvement of the HI+ tion methods will have to be adopted. pentodes and tetrodes. This can be In the ca�e of a true pentode, as dis· very readily seen by looking through tinct from a beam tetrode, the screen, current catalogues and finding that supprcssor grid and anode must the old original PX4, PP3/:ZSo, TETROOE all linked together in making an PPS/400, etc., are still the be PX:zS, artificial triode. " ultimate" in triodes much as they were over ten yea'rs ago. INPUT The adjoining chart ha;; been pre pared to indicate' the power OlltPUt to Now among a large number of be expected from a representati\'e quality-conscious enthusiasts triodes range of British and American are still used extensively because, OUTPUT lo\O . triodes, pentodes and beam tetrode�, \\ • R8Cl1o Dhision. The i::4i1Oll Swan Electric Co. From this it mav be seen that the sensitivity. r-- 100 90 80 70 DO.26. 60 50 �o _ 40 .45 .2,043 30 6r60 .,J.042 .00.24 I I I I I' I Ipps/«JO - .AC.Pl1!Jl20 PP3/250 -- •• p�� ?A� 15 £�2e 4 �15 Kf6�• . !'''' 6i .KT66 � 606. AC� " .AC.P4 L.: 6K6. .AC.P .. g 10 • 6 I N. Cl) - rC .... 8 � • � • AC6,P€4 7 .g P. 'N.45 IEL6 . +-' • AC,4,liN 6 _CLJ � .1'.41 I 5 .El • AC2P£N .. 3 • TRIOD£S • PENTODES & T£TROO[s I ,- 'C;: 2 �,S 1-5 Audio Outp.ut watts - ' 15 ·2 '3 ... ·5 '6 '7 -I -8 I' 1'5 2 2-5 .. 5 6 7 8 S f o I I I I I I I 1 1 I I I "{ I I I I I I J Chart of typical British and American trlodes, tetrodes, and pentodes d.Alsifled accord In, to power output and Input volta,. swinl_