Tung-Sol 5881

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♦ i ♦ ♦ ii ♦ 5881—A New Beam Power Tube C. E. Atkins Commercial Engineer Tung-Sol Lamp Works, Inc. As published in Radio & Television News, September 1950.

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OWER OUTPUT TUBES GET ROUGH TREATMENT. case of residual gas, In power tubes especially the PIn the endeavor to obtain maximum output, grid is prone to emit electrons thermionically and, as amplifier designers frequently operate the tubes at, in the case of gas, the resulting grid current changes and sometimes beyond, established ratings. This has the grid bias, often raising the plate current and over- been especially true in the case of the 6L6 and its loading the tube. Of course, in the case of both gas glass equivalents, the 6L6G(A). As a result, failures current and grid emission current, there is a distor- are sometimes too common, particularly in continu- tion of the grid signal which is also undesirable. ous-duty service. Some of these tubes stand up Cathode emission failure is not invariably due to remarkably well, but different production runs of gas ion bombardment. In many applications tubes are the same tube type often exhibit considerable vari- employed where standby operation is a feature of the ability in marginal operating environments where service. In order that electron emission be immedi- some special characteristic is being exploited or ately available, the heaters of this tubes are energized, where it is necessary to rely upon the stability of a while the plate and screen voltages are removed or, in certain parameter under extreme conditions. many cases, a blocking voltage is applied to the con- Power tube failures are usually due to gas. The trol grid sufficient to shut off the plate current. Many presence of gas in the tube results in ion bombard- tubes lose their cathode emission when operated for ment of the cathode so that its emissive capability is protracted periods under these conditions. This phe- ultimately destroyed. Gas difficulties are cumulative nomenon has been called “sleeping sickness.” It is inasmuch as a small gas content may result in grid roughly analogous to the atrophy of body muscles or current, lowering the operating bias and thus organs after long periods of idleness. increasing the plate current. The greater plate cur- For a long time there has been a growing rent produces more gas ionization and hence more demand for a tube with dynamic characteristics like grid current, further decreasing the bias and initiat- the 6L6 but of a design that would cope more vigor- ing a run-away condition. Furthermore, the ously with the problems encountered in a heavy- increased plate current results in greater heating of duty audio output tube. After considerable experi- the tube electrodes which, in turn, may cause the mentation, the Tung-Sol design and development release of additional quantities of gas. engineers have evolved a design which embodies While gas is frequently the final cause of cathode many features which should qualify it as a success- destruction, it may not be the initial culprit. All tubes ful candidate. This is experimental type DT281 (the can be made gassy if heated sufficiently. There are RTMA commercial number is 5881), it has some always at least minute traces of oxygen, nitrogen, intriguing features. carbon dioxide, and other gases in the tube elements The tube is short and stocky to insure mechani- and in the glass or metal envelope. The degree to cal ruggedness. With shorter active electrodes, align- which they are removed during manufacture is a rel- ment is more readily maintained. This is especially ative sort of thing, depending upon the temperature important in a beam power tube where electrode and duration of bake-out and bombardment. If a configuration has the additional function of beam tube approaches this temperature in service, it is formation in order to produce the high density elec- likely to become gassy and, of course, the tempera- tron cloud in the screen-plate space for the suppres- ture of the electrodes and envelope depends upon the sion of secondary emission from the plate. The elec- severity of the application. When a power tube is trodes are carefully secured to arrowhead shaped operated at its maximum rating, various kinds of top and bottom micas, on three edges of which mica spurious behavior may push the dissipation beyond side-snubbers have been pinned, By this means the what is considered safe and normal. Parasitic oscil- walls of the envelope enlist in the support of the lations are by no means uncommon in power ampli- “mount” (tube jargon for the electrode assembly). fiers, and there is reason to believe that tube design is The electrode leads are brought in through a glass a factor in their incidence. Grid emission will, of disc-called a button stem instead of the flat vertical course, initiate the same lethal cycle described in the press stem employed with the 6L6G-GA. This radial

Page 1 arrangement with liberal spacing of the leads in the cathodes of power tubes and this naturally has through the stem is insurance against breakdown an elevating effect upon the grid temperature. due to electrolysis in the glass. Also, it is believed to Because of the temperature the grid can achieve, render the tube less susceptible to certain kinds of plus the likelihood of its surface being contaminated parasitic oscillation. by cathode material, it is easy to see why grid emis- Extra precautions have been taken to deal with sion is a common occurrence. Of course, the emitted gas. Of course, the tube is carefully baked, adroitly current is minute, being on the order of a few bombarded, and thoroughly pumped. The massive microamperes instead of the milliamperes or even plate of carbonized nickel is three times thicker than amperes emitted from the cathode. However, where commonly used. It is painted with zirconium, which there is a lot of resistance in the grid circuit this is all aids in the adsorption of gas during the life of the that is necessary to cause a lot of trouble. tube. Stray gas molecules coming in contact with In the 5881, grid emission has been dealt a the zirconium surface, adhere to the metal and are severe blow by the use of gold plated wire on this prevented from entering the active inter-electrode electrode. Cathode materials do not effectively cont- space to interfere with the thermionic operation of aminate gold-plated grid wires and hence the possi- the device. To clean up exhaust gases and effect the bility of grid emission is greatly reduced when the continual removal of gas from the tube by chemical grid surface is gold. Furthermore, gold itself is not means, a pure barium getter is used. Three getter an efficient electron emitter. Naturally the standard flags are currently employed. power tube practice of copper side-rods, carrying To deal with grid emission, the grid electrodes heat away from the grid to a “black body” radiating are given special treatment. Thermionic emission, member in the ends is used here. so essential in a cathode, can be dangerous and The screen grid, being farther from the cathode, damaging when it emanates from other electrodes is not as vulnerable as the control grid, although it is in a tube. All metals emit electrons thermionically if by no means immune to the same ills. Since it they become hot enough. The free electrons in a absorbs current, it develops heat on its own account, metal, which render it conductive and contribute to unlike the control grid which is heated by other elec- many of its metallic properties, are in a state of agi- trodes in the tube. Small amounts of emission-cur- tated, continual movement. They are prevented rent can often be tolerated, but there is always a from jumping out of the metal at its surface by the limit. In the 5881 the screen grid is painted with a action of electric forces at this boundary, which tend special carbon suspension which is quite porous to keep the electrons inside the metal. When heat is and, of course, very black. Its color, as any physics imparted to the metal the electrons become increas- student knows, increases the radiation of heat away ingly agitated and may develop sufficient momen- from it so that it can run cooler. The porosity of the tum to jump out of the metal in spite of the surface carbon coating is useful when the tube is used under force tending to keep them inside. This surface force circumstances where secondary emission from this varies with the different metals, being low for some electrode may be harmful. It is believed the sec- and high for others. The lower this force the more ondary electrons are trapped in the porous labyrinth. suitable the metal may be for use as a cathode to Also, the porosity is necessary to facilitate de-gassing supply electrons thermionically. When thin layers of this electrode during the manufacturing process.† different metals are built up in a special laminated Cathode failure (or “sleeping sickness”) during fashion, the surface forces are still further reduced. standby periods is combated by the use of a cathode In the oxide coated cathodes now extensively used, sleeve of high purity, grade A, electrolytic nickel. It is some such arrangement is provided which results in generally more difficult to process cathodes with relatively efficient electron emission. this type of sleeve, but exhaustive tests indicate a Now the control grid is necessarily close to the much greater stability than with the nickel alloy cathode. There is never more than a few thou- cathode sleeves commonly used in electron tubes, sandths inch between the cathode surface and the The 5881 carries ratings similar to the 6L6, grid wires. Accordingly, it is very easy for cathode except that the allowable screen dissipation is 3.0 material to condense on the grid wires after evapo- watts instead of 2.5 watts while the maximum plate ration from the cathode. This may occur in the dissipation is 23 watts instead of 19 watts for the process of tube manufacture or later during the life 6L6. The tube has a low loss micanol base. Prelimi- of the tube. The sensitive cathode materials may nary tests give results which augur well for the form films on the grid wires, providing a fairly effi- future of the type. cient source of thermionic electrons. The grid’s † See the comparative study of plate current variation as a function of sec- proximity to the cathode makes it vulnerable in ondary emission in tetrode-connected nickel- and graphite-anode 813 beam another respect. There is considerable heat energy tetrodes conducted by PEARL, Inc., appended to this paper as Pg. 3.

Page 2 0V –10V –20V –30V 500V Graphite plateGraphite 813 Screen & suppressor connected to +300V Curve tracer's step generator Curve tracer's +300V • •

0 ther “mirror-like” in that “inbound” electrons—travelling at approximately 10% of C, the at 10% of C, approximately in that “inbound” electrons—travelling ther “mirror-like” Note that in the graphite case shown directly above, the plate currentNote that is much, in the graphite much higher, case shown directly above, correct,While this is approximately that it is more accurate most “inbounds” stimu- to say indicating substantially lower secondary emission from the more open, “sponge-like” graphite secondaryindicating substantially lower emission from the more open, “sponge-like” considered if somewhat loosely, plate structure the nickel In contrast, can be usefully, surface. ra “bounce off”speed of light—effectively the plate. creating a thereby “secondaries” from the plate, as several late the emission of as many that charge inhibits the ﬂight of “inbounds” from ﬁlament, this causing region of negative the reduction in plate current are nearly equal. seen where the plate and screen voltages

500mA Page 3 Page 0V

–10V –20V –30V 500V Nickle plate 813 Oscillation Screen & suppressor connected to +300V 0 phite respectively) in the plate current regions. curvesphite respectively) in their +100 to 300-plate-volts A comparative study of the secondary study emission within type 813 beam tetrodes having A comparative The beam forming structure in the 813 is connected to a pin on its base so that con- for the beam former connected to the +300V screen supply so as negate was In this case, and to mild dips (nickel Evidence of plate the severe secondary by emission is provided a entional, tetrode operation it can be connected to ground or a negative potential. entional, tetrode operation it can be connected to ground or a negative nickel and graphite anode structuresnickel appears above. v its suppression of secondary particularly emission from the plate, in the operational vicinity equal. are approximately where the plate and screen voltages gr ♦ Verso Filler Page ♦ 5881

TUII-SOL

BEAM PENTODE

I 1.437'MAX. G, G, COATED UNIPOTENTIAL CATHOOE 1------+-,--,- 2JRf HEATER T-II i 6.3 VOLTS 900 MA. , AC OR DC

13A6s'MAX. ANY MOUNTING POSITION

8GTi0M VIEW

��1 B'.,IN·:; OI�GM�M GLASS tJULe oJ (Oft lAC

S"O�T I�TER�EQllT€ S�(Ll 1 PIN OCTAL -1-"1 OUrliNt 11.1 THE THE ELECTRICAL EQUIVALENT TO TYPES 6L6 AND 6L6G EXCEPT THAr 5881 IS THF PLATE AND SCRFEN DISSIPATION RATINGS HAVE BFEN INCREAsrO APPROXI �ArELY 20 PERCENf. IT EMBODIES A COMPLETE MI:CHANICAL REDESIGN WHICH RESULTS IN Gf?EAT[R RESISTANCE TO SHOCK AND VIBRATION. THE USE OF

TREATED GRIOS AND ANODE GREATLY lNCR[ASFS ITS OVERLOAD CAPABILITIES AND rHEREBY PROVIDI:S OESIRABLE IMPROVEMENT IN CONllNUITY OF SU�VrCE. THE ADDITION OF A lOW-LOSS BARRIER TYPE BASE WILL PROVIDE OBVIOUS ADVAN1AGI:S IN CERTAIN APPLICATIONS.

RAT! NGS

MAX IMUM HEATER-CATHODE VOLTAGE VOLTS 2Uu MAXIMUM PLATE VOLTAGE VOL TS 400 MAXIMUM GRln,2 VOLTAGE VOLT5 400 MAXIMUM PLATE VOLTAGE (TRIODE CONNECTION) VOL 400 T5 MAXIMUM PLATE DISSIPATION 23 WATTS MAXIMUM GRID .2 DISSIPATION WATlS 3 MAXIMUM PLATE DISSIPATION (TRIODE CONNECTION) 26 WATTS MAXIMUM GRID RESISTANCE (FIXED BIAS) 0.1 MEGOHM MAXIMUM GRID R I TA C BIAS) 0.5 MEGOHM ES S N E (SELF

TYPICAL OPERATING CONDITIONS ANO CHARACTERISTICS CLASS A AMPLIFIf;.f? SIN{�[ TUBE l -

VOL TAG� 3 VOL "LA TE 250 00 350 TS (;RI VOL VOL 0 " TAGE 250 200 250 TS GRID VaL! AGE VOL TS " --11J -]2.5 -18 �IGNAL VOLTAGE VOLTS P�AK M 14 12. ? 18 i-IMHOS TRANSCONDUCTANCE 6 100 5 300 5 200 PLA TE RESlsrANCE 30 000 35 000 48 000 OHMS GNAL CURRENT ZERO-gi ?LATF 75 '18 53 M' . ZERO-SI GNAI_ CURRENT (;R I 0 " 4.3 ?5 ?5 "' CURRENT M' .. MAXIMUM SIGNAL PLA TE 80 55 65 MAXIMUM SICNAL GRID U RR E 7.6 4.7 M' , " C NT 8.5 OHM LOAD RESISTANCE 2 500 4 500 4 200 S OUTPUT 3 WATIS POWFR 6.7 6.5 11. O AL HARMON 1 C DI.�TORTfON PERCENT T T 10 11 13 5881

nll·IIL

SINGLE TUSE CLASS Al AMPLIFIER - - TRIODE CONNECTION

GN'O .2 co •• FeTED TO �l(TE

PL .... TAGE TE VOL 250 300 VOLTS -18 -20 V L S GRID VOL rAGE O T PEAl( " SIGNAL VOLTAGE 18 20 VOL lS HRD-SIGNAL PLA TE 2 MA. CURRENT 5 78 85 �A)(IMUM SIGNAL PLATE CURRENT 58 MA. A.t..IPLrFICATION F ACTOR 8 T�ANSCONOUCTANCE I) 250 iJ.�HOS LO " 000 O AD RESISTANCE 000 q HMS TOTAL HARMONIC DISTORTION 6 5.5 PE RCE NT POWEll OUTPUT 1.4 1.8 WATTS

CLASS A] PUSH-PULL AMPLIFIER

OLT GE 250 PLATE V A 270 VOL TS GRID " 250 VOL TACE 270 VOL T."> GR I D " VOLTAGE -16 -17.5 VOL TS " 35 PIoA\( GRID TO GRID VOL TAGl 52 VOL TS eTA NeE (F IICH 500 7 0 11MHO$ TR ANSCONOli TIJBE) 5 5 0 RtSISTANCE (E 23 OHMS PLATE ACH rUBlJ 2'1 500 500 ZERO-51 GNAL PLA T( CURRENT 120 13u MA . 112 CURRENT ZERO-SIGNAL GRID 10 11 MA. S GN L PL . MAXIMUM I A A TE CURRENT 140 155 M' MAXIMUM SIGNAL GRIO " CURRENT 16 17 "A. LOAD RESISTANCE 5 000 5000 OHMS POWER OUTPUT 14.5 17.5 WATTS T L 2 TO A HARMONIC DISTORTION 2 PERCE'tH

CLASS AB rUSH-PULL AMPLIFIER 1

VALUES U( FOR TWO TUBfS

360 PLA Tt VOLTAGE 360 VOLTS GRID ., VOLTAGE 270 270 VOLTS GRI VOL AGl -22.5 -22.5 VOL TS D H r " GRID TO GRin 45 45 VOLTS PE AK VOLTAGE ZERO-51 (;NAL PlA If' CURRENT 88 88 MA. " ; IERO-SIGNAl GRID CURRENT 5 "A. MAXIMUM SIGNAL PLATE CURRENT 132 140 "'. G L G D *2 CURRENT 15 11 , MAXIMUM SI NA RI " . L A lS T C 6 600 3 OHMS O D R IS AN E BOO TTS POWER OU TPU T 26.5 18 WA TO AL PERCENT T HARMON Ie DISTORTION 2 2 5881

TUU-SOL

CONTINUED fROM PR(C(OI�G PAGE

PUSH-PULL I CLASS AB1 AMPL 1 F [R - TR lODE CONNECT I ON GRIU '2 CO�NECTED TO PLATE HlHS A�[ FON TWO TUHS

HEAHR VOL lAGE 6.3 VOL IS HEATER CURRENT 0.9 AMP. PLATE VOL TAG£: 400 VOL 15 I Gil 0 VOLTAGE -45 VOLTS PEAK AF GRID TO GRID VOLTAGE 90 VOL 15 ZERO-SIGNAL PLATE CURRENT 65 "'" MAXIMUM N . SIGNAL PLATE CURRE T 130 "' L NC OAD RESISTA E 4 000 OHMS TOTAL HARMONIC DI O T N 4.4 ST R I O I"ERCUH POWER OUTPUT 13·3 WATTS

CLASS AB 2 PUSH-PULL AMPLIFIER VALUES ARE fOR HlO TuBES

T HEATER VOLTAGE 6.3 6.3 VOL S HEATER CURRENT 0.9 0.9 AMI". PL VOL VOLTS A TE TAGE 360 360 GRID .2 VOL VOLTAGE 225 2'10 1S VOL Gil I 0 IIi lAGEVOL -lB -22.5 TS GRID PEAK AF TO CRID VOLTAGE 52 72 VOLlS . ZERO-SIGNAL PLATE CURRENT 78 88 "' ZERO-�IGNAL GRID 112 CURRENT 3.5 5 M' . MAXiMUM SIGNAL PLATE CURRENT 142 205 M'. XIMU SIGNAL MA M GRID 112 CURRENT II 16 ",. LOAO RFSISIANCF 6 000 3 800 OHMS POWER OUTPUT 31 47 WA TlS TOTAL HARMONIC DI STOHllON 2 2 PERCENT

...L-L! ! , , , .- ----.-; - !H' I - , - I I - . - .L-L : . i, ;•- [ ..;.. If : . • · . I I ' , . , · , I , • 'T . - I , +- I i , , - I I , I I , - , I- -T -r q"'r i • I - , - - , -l - , ,,"0 -.!;- -, , , ; I - -, - -" r-i-"- - .

" , I -- 7} • -j - - . r . . I II - ;7.":l -t cP,

PLATE VOLTS �I��Tn I I I I I I ELE C TR O N HOOII " N (W R S (Y •• TUHG- �OL EL E CTR IC IN C ., TU6[ DI V IS I ON , LO J U � . JUNE 1 lq�? P LAT II �� , 5881

, , ; . . . - L _ .- r _l I I- 1 I I ,i , - , , ; - - 0- ._ I -L - - _l., � � � I I 1 + - , I , , I 5881 I PHlfODE CONNECT I - � � I 0' - : ! . -I - HEAT£R: RATED I , 250 - - � �t -jH- �� f- E Volts - i - - el = - -- .J , Ib - - +-, 1j()0 , , - - - IC2 H � ' I � - , I - � I , �,. ' t- I I I i H� ! �- I I -- I - I� � +15 I I f- :: 300 +10 I i -!--- - , .. 8 ;_v +5 +- I - - I - - -t- H ,I ; f- I 1-'- , ,0 ti � 200 EC, I I, , t- -- ..-

-- , � , 5 I TIT L - . - - - . � ; 1-1 10 100 -l -.a- -1 15 - -4 18 -- - .. � 20 .... � 25 � � '0 - 0 0 j 0 100 200 300 �OO 500 600 700 - PLATE YOLTS , l:ffl41�-T-[1I [ [-111 II I I- I

l- 5881 -j PENT 00' CONNECTION - I I I I !� HEATER: > RATED 300 E EC1 �i .. o Va 1t s

250 -

- .. - - ,

- - 250 200 V - .-.- , , - . - - - i - � - - .. - � .... f- 200 � �- I + � 150 , ,. , �- - , � , I � - r-, , f-- , • ,. - 6Cj -t l ! �IOO �- ; rt;- ; � � ,l ,I � - !- + i -+ , . I - . . I , , 1 ; - I ';160 - � , I , I , 50 - - , - . .. + I - .-- i , 5I0 ..r r , I - , , l, I , i -t - .. -t - �H-r- , i _. OV i - 0 200 1j()0 500 600 , 100 300 I " [- , i PLATE VOLTS 1 5881

5881 PENTODE CONNECT10N

HEATER: RATED 250 Vol ts Eb '" - 250 Vol ts Ee2 14 Volts I- 3 ECi" '0.0 � E� RL .. 2500 Ohms t +++-.- I 'r-r-,-++

z o

� 5.0 10 - � "-+++-'')fd \'I3� 5 +� � 7 0 I- 123 � 5 6 POWER OUTPUT (Po) - WATTS I --'-rT iTI-rTri rTl I bin IT-I I I-H 1 1--+-1I

I - 5881 1 PENTODE CONNECT ION - - -1-- I HEATER: RAHD � Vol E 250 ts b -14 Volts ECi '" 250 Vol ts I- Ee2 ,. . - P a 10 Volts RMS 7.5 E$;g o 15 f-- ..... 1 - '..... I- l- I /' z � I- W 0;::: , " "� � ,.,- W I • o. . "-i! I , 5.0 I 1I 10 , - '- - � 0 l I �_ ...... � ,,0't> �( c. '(13 � I o. / ' - - +1- - I- I- I � I- ---I- 0 � r-- I I\.i' vV- I- o. ! 00 I - !; 0 0 i 2.5 V 1/ 5 � -i!� "- w z � 1/ 1/, f- 'ii! o. - - / - {c/ f- � *..... �: - .L-e.- �""., 0 , I 01/ 1000 2000 3000 l1OOO 5000 LOAD RESISTANCE (R L) - OHMS - 1 1 1 1 1 1 - -- 1 1 1 1 1 1 5881

TUII·IOL ULTRA-LINEAR OUTPUT STAGE d.\ • I R,v ;:-- LJ , R, � :� c • vvvvR. Ebb � � �I+ � C, e :� R, � A'VV /'---=. 1\ I R. -+

" �5o)" PEAK AK OIST. = ? R m 0, 1/ �II E eoy. " � f �I i' 112 St; R,.II, 1 0, 112W I). 2 P, ". Cl·C2 "f 600v 100 jJf " '" '�1 " . C, '" " ..

1� THE ULTRl-llnA� Cl�CUll T"E SCRrEN VOLTAGES ARE O(RI�[O fROIt TAPS OR THE PLATE WI�OI"GS Of OVTPH n'�HOR�E�, �E TAPS ARE SO TO APPll TH( a[ T LOCAHO AS �ll OF PUT{ SIG•• l YOLTAG( TO TIl!: SCUU GRID.

T�E PLATE F'�ILT fOR THIS CO"NECTIO� ISS"OWN HIO •• THEH CURVfSIHHonTAll£C Dr STATICALlT VARYING THE PLln VOlIlG( '" INCREII['jjTS .SOUl U[ QlTlESCEMT 'OIU (�OO VOLTS HH£ .10 SUfU SUPPLY) UO SI�UltUEOUS(T CHUGING THf SC�H. VOLTAGE H *310' THE I�C�E�En.l� TH G�Ar" �OT" PlATE UD SC�EU �OLTAG(S �lVE HEN PLOTHO ALONG THE ABSCI�SA.

I , ' ' i , -I - _L , I " I- "I • f-lI ': , I Ti i --t- , I I, , 250 • , ! CONOITIONS • 'co , "ULTRA-lINEARft AMPLIFIER , - - ,'" .. .43 , f., , , 1 - E92 400 + Ep 200 I , , � i w .. � ' " w � I� % i < , - 150 1 � , � , ,'1'" - .. ,. ...l 0 - -

- - " 3100 , . - .. - w I- < � .. � V Jell - 50 ,,,,a " _10 - 1 _60 0 0 100 200 300 'I(l0 500 700 PLATE VOLTAGE 600 271 31� 3 7 �OO ��3 �86 228 SCREEN5 VOLTAGE 529