Vacuum Tube Heaters

8/2/2019 Vacuum Tube Heaters

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With the advent of new, exotic devices,the vacuum tube has been de-emphasized by many engineers ..However, lube aesigners are continually striving to design better tubes.One area of their interest has been the heater.

,A significant number of improvements hove been madein the heater over the last few years.

The Materials and Shapes 01

By W. A. HASSElSenior EngineerWestern Elecfric Co.Laurelclale, P a.

For proper cathode temperature, the heater nor-mally operates at about 1100 to 1200 C. It maysometimes reach 1600 C in tube processing. Underthese rigorous conditions, only the most carefullyselected and controlled materials can be used. There-.. fore, the choice of heater materials is limited tothose elements which are characterized by highmelting point, low vapor pressure, chemical inert-ness and low cost. Of the available materials, tung-sten, which is used as the heating element andalumina, which is used as the electrical insulatingmaterial, meet these requirements.

N vacuum receiving tubes, heat can be supplied tothe cathode by either of two conventional methods:"Directly," by means of a current passed through afilament of base metal to which the emissive mate-rial has been applied; or "Indirectly," by means ofa separate, insulated heating element which ismounted within the cathode structure. Because thedirectly heated cathode is rather simple in construc-tion, we will discuss only the heating element in anindirectly heated cathode.

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-4 Heater MaterialsIn its natural form tungsten is usually obtainedfrom the minerals wolframite (Fe, Mn) W04 andschoelite (Ca W04). Because 70% of our originaltungsten resources have been depleted, methods havebeen found for purifying relatively poor-grade ores.The quality of tungsten heater wire depends upon

many factors, and the materials and manufacturingprocess are carefully controlled. The powder usedto produce the ductile metal is initially of high pur-ity. For the purpose of inhibiting grain growth, how-ever, very small quantities of partly volatile alkalisilicates and non-volatile oxides such as silica, alu-mina, thoria, or calcia are added to the tungstenpowder.After being mixed, the tungsten powder is pressedinto bar ingots. Ingots are then sintered at a tem-perature of approximately 30000 C. The time-temperature relat.ionship at which the ingots are

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-r2- [ 3 Fig. 1 : Graph showsth e effect of temper-ature on the elec-tr ical conductivity O falumina.

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sintered is carefully controlled to assure a densebar which, in turn, determines many of the proper-ties of the finished heater wire. At a temperatureof about 1300 C, the sintered bar is worked into aro d by mechanical hammering or "swaging." Duringthis process the cross-sectional area is reduced by15% each time the rod is run through a successivelysmaller die. After the swaging process, the tungstenrods are drawn hot through a tungsten carbide die.The final, smaller wire sizes are drawn through highlypolished diamond dies. As the wire is drawn and re ..duced in area, its tensile strength increases to as muchas 500,000 lbs/sq. in.The electrical, as well as the chemical and physi-cal properties of tungsten have been intensivelyinvestigated. Although the resistivity of tungstenis not as high as that of some other materials, itshigh melting point of 3400 C makes it a desirableheater material.At room temperature, small variations in resistiv ...

ity are found among tungsten wires, depending upontheir previous treatment. Despite these small varia-tions, however, tungsten wires display similar elec-trical resistivities at high temperatures. This char-acteristic is important because it enables the massproduction of reproducible heaters having a uniformcurrent at operating voltages. No other material hassuch an outstanding combination of desirable prop-erties at elevated temperatures in vacuum.

A '~ R E P R I N Tof this article can be obtained b y writ ing o n c om p an y le tt er he ad t o

Th e EditorE L E C T R O N r c I N D U S T R I E S t Chestnut fJ 56th Sts., P h i l a . 3 9 , Pa.

alumina is a modification containing sodium in itscrystal structure; and gamma alumina is encoun-tered in the low-temperature calcination of alumi-num compounds. The alumina used for heater coat-ing is a very high-purity alpha form.Large fragments of the fused alumina are reduced

to very fine particles by grinding in an iron-ball ...mill.After particle-size reduction, the material is cleanedin acid, washed, and heat-treated to remove anycontaminants. This preparation results in the pure,carefully controlled alumina particles which are im-portant in the deposition of the alumina on theheater.An important property of alumina, which depends

upon the crystal form and purity, is its extremelylow electrical conductivity. Fig. 1shows the effectof temperature on the electrical conductivity of alu-mina. Thus curve represents an average based uponthe work of several investigators. The thickness ofthe alumina coating required on a heater is a func-tion of its dielectric strength, and usually dependsupon the bias to be applied between the heater andthe cathode. It is generally agreed that one mil ofthe coating is required for each 75 volts.Because heat energy is primarily transferred fromthe heate-r to the cathode by radiation, the thermal

conductivity of alumina, although good, is not tooimportant a factor. The chemical stability, highmelting point, and electrical resistivity are the im-portant properties that make alumina a dependableinsulating coat for vacuum tube heaters.

AluminaAlumina, alone or associated with silica, is a major

constituent of the earth's crust. The principal alu-mina ore is bauxite (AI20a2H20).The three principal crystalline forms of aluminaare designated alpha, beta, and gamma. Alpha alu-mina is formed at high temperatures. It is foundin the natural mineral corundum and in fused alu-mina formed from the solidification of a melt; beta

Fig. 2: A drag-coating operation fo r apply-ing an alum ina coating to tungsten wire .

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Tube Heaters (Continued)Methods of Alumina Deposition

One technique used for the application of the alu ..mina insulating layer to the tungsten wire is the"drag" ..coat method. As the name implies, the baretungsten wire is passed or dragged through a spe-cially prepared alumina suspension. This suspensionis composed of a very pure, fused and milled aluminain a solution of methanol, aluminum nitrate salt, anddistilled water. The alumina particle size usuallyranges from 5 microns to 40 microns. The methanolacts as a suspending agent for the fine al umina par ..ticles and evaporates quickly as the wire passesfrom the suspension into an air furnace. The alumi ..num nitrate salt acts as a low-temperature binder.It cements the alumina particles together as eachlayer is built up during the "drag" operation.

of each heater strand. After the proper number ofstrands are wound, the heater is automatically cutfrom the continuous length of spooled wire. Simul ..taneously, a small section of the coating is removedfrom the heater legs to expose the wire at the endsfor welding to the tube stem leads.

Cataphoretic CoatingCataphoresis or electrophoresis is defined as "themigration of coloidal particles under the influence

of an electrical potentia1." Cataphoresis, as appliedto heater coatings, is the process by which positivelycharged alumina particles are deposited on a nega ..tively charged tungsten heater wire. The aluminaused in the suspension consists of very fine particles,usually in the one-to-five-micron size range. An in-creased number of ionized groups on the aluminasurface results when the particles are surface ..charged by the addition of small amounts of selectedsoluble inorganic salts, such as aluminum nitrate.The charge and stability of the alumina particlein the suspension is due to the preferential absorp-

tion of a particular ion. By the application of apotential, the positively charged alumina particlesare deposited on the negatively charged tungstenheater, and a layer of alumina is built up to formthe insulating coating. Fig. 3 illustrates the deposi ...tion of alumina on a heater. Generally, the amountof alumina deposited on the wire depends upon themobility of the particles, the concentration of theparticles in the suspension, and the potential be..tween the anode and the cathode. In production, aclip holds a number of heaters, which are submergedin a suitable alumina suspension, while a fixed volt-age is applied. The coating thickness depends uponthe value and the duration of this voltage. Afterthe heater is coated, it is sintered at 1600 C for ashort time in a hydrogen-atmosphere furnace.

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ANODE (+)(+ )CHARGEOA l203 PARTICLES CATHODE (-)

.....................................--.....TUNGSTENHEATER

SUSPENSION: ALUMINUM OXIDEF ig . 3: A l um i na c oa tin g is applied to tu ng ste n f ila m en t wireby using an electrical po tentia l to ob tain cha rged pa rticles.

Fig. 2 is a sketch of the "drag"-coating operation.The grooved ceramic roller rotates partially sub-merged in the alumina suspension, and applies athin layer of coating to the tungsten wire passingover it. The specific gravity of the suspension andthe speed of the machine are adjusted so. that after8 or 10 passes of the wire over the ceramic roller,the coating is built up to the desired diameter. Asthe wire leaves the ceramic roller, it enters an airoven. Oven temperature is between 6000 and 8000 Cto dry and bake the coating. The coated wire is thenpassed through a hydrogen-atmosphere furnace. Itoperates at approximately 1200 C to chemicallyreduce any tungstic oxide which may have formedon the wire.The coated diameter is controlled automatically

by means of a photoelectric cell. The cell operates asolenoid valve to release a measured quantity ofaluminum nitrate solution into the suspension, thusadjusting its specific gravity. The coated wire iscarefully controlled for diameter, smoothness, con-centricity, flexural strength, and weight.Heaters are fabricated from the coated wire byspade ..winding. In this operation, a length of wireis folded over razor-sharp edges set a predetermineddistance apart, depending upon the linear dimension

Spray CoatingHeater coatings are also applied by the spraytechnique. As in the drag and cataphoretic coat sus-

pensions, the spray suspension is specially com-pounded for optimum results. High-quality sprayingis obtained by control of the viscosity and the dryingrate of the suspension. Organic solvents are addedto aid dispersion and to prevent the settling of thefine-grained alumina. A nitrocellulose binder is usedto produce a tough coating that can be handledeasily. The desired coating texture is obtained byadjustment of the air pressure and of the area ofthe orifice of the spray gun. A smooth, dense coatingis desired because it produces a strong coatingwhich facilitates insertion of the heater into thecathode during tube mounting.

R E F E R E N C E PACESThe pages in th is section a re perfo ra ted fo r ea sy re-m o val and retention as valuable r ef er en ce m a te ria l.SOMETHINC NEW HAS BEEN ADDEDA n extra -w ide m argin is now p rovided to perm itthem to be punched with a sta ndifrd th re e-h ole-punch without o blite ra tin l a ny o f th e text. They ca nb e filed in standard th re e- ho le a ete bc ek s or folders .

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A few of the heater designs now in use are brieflydescribed in the following section. The basic designsfall into the following three groups: the spade-wound folded heater, the single helical heater, andthe double helical heater.

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Fole /e el He ate rThe folded heater, made from drag-coated wire,is simple and easily manufactured. There are manydesign modifications in this type of heater. However,

three principal forms are the staggered apex, thestraight apex, and the sloped apex. The staggered-apex type is designed so that each succeeding foldis shorter than the other; the straight-apex typehas each apex opposite another; and the sloped-apextype has the top and bottom apices sloped parallelto each other.The staggered-apex heater, shown in Fig. 5a, ismostly used in round cathodes requiring a closely-packed heater. In such an arrangement, the shorterapices nest between the strands of the longer apices,thus preventing their direct contact. The straight-

apex heater is best suited for a flat cathode whosecross-sectional area permits a certain amount ofalignment of the heater strands, and permits theapices to spread. Because the folded heater is versa-tile, it is used in either round or flat cathodes. Thechoice of strands is determined primarily by the fitof the heater within the cathode. In practice, thefolded heater is commonly used in octal tubes of therectifier type and the power amplifiers. The heatersused in these tubes are of extremely rugged con-struction, and they typify the design of most of thespade-wound heaters.

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F ig~ 4 : H e ater-cath ode construct ionsthat were used in early tube designs.

The heaters are mounted into a clip. The clip isplaced in a rotary spraying machine having sprayguns positioned at selected points. At each revolu-tion, the sprayed alumina is deposited in thin layerswhich are dried by infrared lamps. The desiredcoating weight and thickness are obtained after sev-eral revolutions of the coating machine. For sinter-ing of alumina, the heater is fired at 1600 C in ahydrogen atmosphere furnace.

Heater ConligurationsHeater designs have varied considerably since1927 when the first indirectly heated cathodes were

introduced. At that time, a hairpin tungsten heater'was supported by an extruded ceramic insulator,surrounded by a nickel sleeve. Fig. 4a shows aheater common to the early detector- and amplifier-type tubes. This heater operated from a 2.5 voltsupply. It had a warm-up time of 20 to 30 seconds.Fig. 4b shows a 2 mil wire spirally wound on analumina insulating tube. The return lead passedthrough the center of the insulating rod. Usuallythe wire was covered with an outside aluminacoating.Fig. 4c illustrates a 70 mil diameter tungsten wirewound around an alumina insulator. A molybdenumro d passing through the center of the tube acts asa support rod. Such heaters were designed to oper-ate at 5 volts and 60 amperes. Fig. 4d shows a 25m il diameter spiral heater wire supported inside anextruded insulating tube.All of these heaters were made in various sizesto meet different heater-power requirements. Theceramic insulating sleeves were usually made ofalumina, magnesia, thoria, beryllia, or electricalporcelain. Many factors, such as high cost, con-taminants in the ceramics, and slow warm-up time,resulted in the decline Of these heaters.

C A ) (6) (e )Fig. 5: Th ree type,s o f heaters that ar e used in today's tubes.

S in gle H e lica l HealersThe three single-helical shapes commonly em-ployed in vacuum tubes are the inverted "V," or

hairpin, the inverted "U," and the "M" shape. The"V," or hairpin shape, is used to accommodate singlecathodes, whereas the inverted "U" or "M" shapesare used to accommodate 2 cathodes, depending uponwhether the heater bridge between the cathodes isat the top or bottom of the tube cage construction.These heaters are made by winding tungsten wirearound a metal mandrel to form a helix. Helix iscut to the acquired length and bent into the desiredShape. Because the heater current depends upon

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Tube Heaters (Concluclecl)the total wire length rather than the helix length,the turns of wire are precisely spaced so that eachheater is accurately reproduced. After the heateris formed, it is cataphoretically coated with aluminaand sintered at a high temperature in a hydrogenatmosphere. The core, usually molybdenum, is re-moved by an acid-dissolving process.Because the extremely high number of turns perinch obtainable with this heater permits more wireper unit length, it is possible to use a single helicalheater in thirty-mil-or-less flat or round cathodeswhich normally would require tightly packed foldedheaters. Fig. 5b illustrates a single helical hairpinheater, the most popular shape of the helical heat-ers. It is used extensively for minature tubes inwhich power requirements dictate heater designsinvolving long wire lengths.

D a ub le ..H elica l H ea tersUse of the double-helical heater is usually re-stricted to round cathodes having diameters of 30mils or larger because the mechanical forming tech-niques make it difficult to make smaller sizes. Theheater wire is cut to the desired length, fed into acoil-winding machine, and wound around a mandrel,

After the coil is removed from the mandrel, thealumina insulating layer is applied by spray or cata-phoretic coating techniques. To increase the amountor wire in a double-helical heater, a single helix wireis frequently shaped into a double-helical heater bywinding on a mandrel. This modified design not onlypermits a greater length of wire to be placed in thecathode, but also takes advantage of low hum char-acteristics of the double-helical heater. Fig. 5cshows a double-helical heater used in octal or minia-ture tubes requiring low hum characteristics.Of the many complicating factors that enter intothe design of a heater, such as the relative emissivi-ties of the heater and inner surfaces of the cathode.the thickness of the heater coating, and the heaterfit within the cathode, the dimension of the cathodesleeve is of prime importance. This dimension de-termines the heat that the heater must furnish tomaintain the proper cathode temperature. Theheater design temperature is calculated from theappropriate tungsten resistivity formulas or deter-mined from nornographs specially constructed forthe purpose.

References1~Shaw t G. R. and ShardlowJ L. R., "Heaters and Heater-

Cathode Insulation," Vacuum Tube Design Handbook, RCA,Harrison, N. J. pp 24-33.2. Troelstra, S. A., "Applying Coatings by Electrophoresis,"Philips Tech. Review, Vol. 12. 1950-51, pp 293-303.3. Navias, L., '4Extrusion of Refractory Oxide Insulators forVacuum Tubes," Jour. Am. Ceramic Boc., Vol. 15, pp 234-251,1932.4. Russell, A. S. et al, "Alumina Properties" Tech. Paper No,

10, Alcoa Research Lab., Aluminum Co. of America, Pittsburgh,Pa., 1966.5~Goetsel, C. G. Treatise on POWder M etaZlurgYJ VOl. II~t1950; Intersolence Publtshers, Inc., New YOrk.

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Vacuum Tube Heaters