US1650232A - Thermionic tube - Google Patents

Description

G. W. PICKARD THERMIONIC TUBE Filed March '7, 1922 2 Sheets-Sheet 1 IN VEN TOR ArmRNEY G. W., PICKARD THERMIONIG TUBE Filed March 7, 1922 n 2 Sheets-Sheet 2 PLB 341' @L5 y fi,

IN VEN TOR lA TTORNE Y Patented Nov. 22, 192.7.

UNITED STA-TES PATENT OFFICE.

GREENLEAF WHITTIER PICKARD, OF NEWTON CENTER, MASSACHUSETTS, ASBIGNOR T0 WIRELESS SPECIALTY APPARATUS CQHPANY, Ol' BOSTON, MASSACHUSETTS, A

CORPORATION OF NEW YORK.

THEBHIONIC TUBE.

Application led Harsh 7, 1922. Serial N0. 541,603.

This invention relates to thermionic tubes.

The object of the invention is to provide for such tubes an emitter of electrons which will permit the employment of alternating current for heating while at the same time prevent the varying potential of such current from deleteriously alecting the action of the elements of the tube.

' The invention consists of an organization substantially as described herein, pointed out as to novelt-y in the claims, and shown in the accompanying drawings which are about four times scale, and of which Figure 1 is alongitudinal section;

Fig. 2 isa like view of a modification; Fig. 3 is'a likerview of another modification;

Fig. 4.- a like view of yet another modification; and

Fig. 5 a diagrammatic illustration of a radio receiver set employing the invention in each of a plurality of tubes.

Heretofore tube-filaments 0r emitters for receiving tubes have been heated by direct current as from storage batteries, this notwithstanding the desirability for convenience of employing alternating current, and on account of the disturbance or hum created by the potential across the legs of the filament if an alternating current be used. It is highly desirable to employ alternating current because it may be easily and cheaply transformed down to the proper voltage for the filament, and is nearly always available, as in practically all house-lighting circuits. The employment of storage batteries is much less desirable because they involve an initial expense constituting a large part of the total cost of radio-receiving apparatus; they must be frequently re'charged, necessitating either the use of an adjacent rectifier, or the transportation of the battery to a distant charging station; and this recharging becomes a greater inconvenience when an amplifier train present-day practice, which makes a large demand upon the battery. Also the storage battery is a source ot acid spray. But notwithstanding the disadvantages of the storage battery and the advantages of the, alternating current, use of the battery with relv ceiving tubes has continued up to the present time on account ofthe etfect Vof the alternating difference of potentlal across the legs of several tubes is used as in of the filament when alternatn current 1a employed. This varying potential creates a. varying electrostatic field in the vicinity of the filament which induces a varying potential on the grid or control element of the tube, thereby varying the plate-filament current so as to cause a loud hum in the telephone receivers supplied by the tube. This action of the alternating current is probably somewhat complex, but may be assumed to be due in large part to the var ing potential induced on the grid; secon arily, to the varying electrostatic eld around the filament itself; and in some part also due to thermal variations of the filament, at least when a thin filament is used. While alternating current for heating the filament has been used in high power tubes for radio transmitter use (wherein the effect of the alternating fields around the lilament is swamped by the high potentials employed on the grid and plate elements of the tube), yet in tubes employed for radio receiving purposes, the direct current as from stora e batteries has been universally employed y reason of the above.

In accordance with this invention, the source of the emission of electrons for use in the operation of the tube is not the incandescent ilament, as generally employed heretofore, although there is employed an electrical heater adapted for operation by an alternating current. Here the emitter of electrons consists of an element the surface of which is at substantially the same potential at all points and times. Also the emitting elementmay serve not only as a source of electrons but vas a shield located between the heating element and the other elements of the tube such as the grid and plate.

In Fig. 1, the emit-ter for glass tube T is shown as comprising a thin platinum tube or thimble F, permissively about one-sixteenth of an inch or less in outside diameter and, say, about one-quarteror one-fifth of an inch long. I In all the drawings, the emitter is shown in enlarged scale, and actually these arts will be made as small as possible in or er to avoid the use of a large heater for the emitter, and in order to keep down the electrostatic-capacity of the tube (i. e., the capacity between the grid and emitter and the capacity between the grid and plate), so that the'tube may be used eifec-v tivel on short wave-lengths. As to the desire small size of the heater, it is to be noted that the heat dissipated at a given temperature is directly proportional to the area. An emitter of the abo-ve general d1- mensions may have an input of, say, 2.8 Watts and an electron-emitting area of, say, 0.3 square centimeter.

The outside surface of emitter cylinder F is coated with lime or other suitable oxid, such as barium or strontium, in the manner now Well known in the art (a V or hairpin filament has been used, coated with some such oxid, for the purpose ot' emitting electrons in abundance in a tliermionic tube; but like the other filaments employed, .this could not be used with alternating current for the reasons above stated).

Inside of tube F is a heater H, consist-ing of a fine wire hairpin or coil of suitable material which will stand the. desiredteniperature Without melting, i. e., such as nichroine, tungsten and the like. Surrouinlingl heater I-I and filling the interior of tube F may be a suitable insulating lilling I such as a high melting point enamel or mica. Insulator I may be air, or any substance which is a reasonably good conductor of heat, with a meltin point suiiiciently high to Witlistand a red ieat, and a fair electrical insulator for low voltages. Also it should be of some substance, as stated, which will not interfere with the exhaustion of the therinionie tube or with the maintenance of the high vacuum during the life of the tube. (By air, I mean the gaseous fluid, whatever it may be which is within the glass tube and which is maintained at a low pressure usually referred to as a more or less perfect vacuum.) Crushed quartz may be used for this insulating filling I, packed or taniped into tube F so as to hold heater H securely in central position.

The grid element and the plate element P may be arranged in any usual manner as Shown, i. e., concentric with emitter F.

The entire emitter F, with its heater I-I, is supported on wires W of platinum or. the like, which are thin to prevent thermal conduction away from the emitter. One of these supporting wires W is electrically connected by conductor X with au electrostatic shield S which surrounds the entrance leads A, A to heater H. (But shield S may be electrically connected with emitter F in any way which may be 'found convenient in the assembly of the thermionic tube.) This shield S extends up near to the lower edge of thiinble F. but does not touch it, being there insulated from it. Shield S may be of any thin construction of metal such as nickeL-suitably supported in relation to glass tube T, and, together with thinible emitter F, shield S forms an electrically-continuous surface all permissively at the saine potential through- .not surround the leads in the sense of causing resultan't deleterious action from alternating currents through the leads.

The various leads from tube T may be taken out in any desired manner, as usual; the heating leads A, A being taken out through glass tube T as shown, from heater H, the lead G1 from grid G, and the lead P1 from plate P. The lead from emitter F (the outside circuit lead as distinguished from the heating Wires A, A) may be W1 as a continuation of one of the supporting Wires W.

The heating wires A, A may be connected to any suitable source of alternating current of the proper potential, such as the secondary Z (Fig. l) of an ordinary alternating current transformer, the primary of which, not shown, may be connected to Vthe ordinary house-lighting mains, as atv 60 cycles.

Emitting tube F may consist of platinum, alloyed with a small percentage of iridium (6%), and covered with the oxids of bariuin and strontium. These oXids may be applied alternately, andafter each oxid application the temperature of the tube is momentarily raised to about 1,0000 C. As many as sixteen such applications may be made, and after that the tube may be baked 'at about 1,200o C. for two hours. Heater H preferably inay be so designed that when operated on the Working alternating current voltage, there will be dissipated in the tube approximately eight watts per square centimeter of tube F aiea. The area of the tube F will, of

course, depend upon the desired plate-current, and for ordinary detector and amplifier work this area may be generally between 0.2 and 0.5 square centimeter, so that the heater will be designed to dissipate between .1.6 and 4.0 watts.

Such a tube as the above is adapted for heating of the emitter by alternating current, for any purpose where the potentials of the plate and grid are not sufliciently high to swamp the alternating fields around the filament. For example, the tube of this invention may be used in radio communication receiving circuits as detector or amplilier (see Fig. 5), and it may be used as a relay in telephone lilies, as theiniionic tubes now are employed: and in all such cases the step-down alternating current transformer Z (Fig. 1) or any other convenient source 0f alternating current may be substituted for the storage battery heretofore used with prior thermionic tubes, without any deletenous effect on the thermionic tube of this 1nvention or its operation.

The invention lnay be embodied in various forms, but its embodiment requires nothing out ot' the usual practice in respect of the design or exhaustion of the glass or other structure which may enclose the elements. Those skilled in the art of thermionic tubes may employ in combination any or all of the things advantageous for such tubes, in embodiments of the invention which may be specifically different from any form shown herein. I

The emitter F, in that it may consist of a metal base having the oxid coating, is to that extent like the lVehnelt cathode construction which heretofore has been employed in the form of V or hairpin filaments in thermionic tubes, where the oXid-coated filament was heated by direct current or from a storage battery; but the emitter hereof may consist equivalently of any material or materials capable of emitting electrons freely when heated, high incandescence not being requisite in this class of thermionic emitters as in the case of uncoated filament `the tube, in any case.

emitters.

In the modification of Fig. .2, heater H1 (instead of being a hairpin filament insulated from emitter F), is a fine wire electrically connected to the upper end of emitter F whereby the latter acts as; a return heating lead, thereby simplifying the construction. In Fig. 2 the emitter F is in the form of Fig. 1 with its exterior o Xid layer FO. The cross section of this emitter F is so proportioned relative to thatl of its heater H1 that the voltage drop along the emitter willbe little, if any, and substantially negligible inits effect on the action of This proportioning will result substantially without special design, when heater I-I1 consists of a fine wire having a cross section from say ten to one hundred times smaller than that of emitter F; but this proportioning is important and -should be taken into cons1deration in the design of emitter and heater. In this form (and in other forms as shown in'Fig. 3), no insulating substance need be used in the space -between the heating portion H1 and the emitter portion F; so that the heat is transferred to emitter F partly by radiation across the short vacuous space between the heating and lemitting parts, and partly by conduction via Also in the mionic tube T around the heater leads HL, HL from any electrostatic field from said leads; and conductor X connects shield S to lead FL. The grid coil G, cylindrical plate P and their respective leads GL and PL are made and brought out in the usual manner, and in Fig. 2, as stated above, one of the heater-leads HL is also the emitterlead FL to the tube-circuit. In Fig. 3 emitter F2 (having oxid coatin FO) is in the form of a. flat circular disk o platinum F (cf. Fig. 1), about one-quarter inch in diameter or less, and say two or three thousandths of an inch thick. This disk F is supported on a central ri id stalk FL2, forming a part of the lead rom theA emitter to the circuit of the tube.

Underneath disk F2 is the heater H2, H2, consisting of tun ten or other wire of high melting point. eater vH2 and disk F2 may contact at their midpoints, as shown. The heater may be supplied with alternating current by leads HL2, HL2. This heater heats the disk F2 partly by radiation and partly by conduction at the point of contact or attachment. i Themost effective form probably is one wherein a. thin layer of enamel thermally connects the coil of resistance wire H2 to'emit-ter F2, pursuant to conventional rheostat practice; thereby the enamel layer serves to support .the resistance wire close to the emitter to be heated, electricall sulates it therefrom, and as a thin ayer constitutes a. sufiiciently good thermal con- 1ductor to permit the effective transfer of eat.

Above disk F2 the grid G2vis held in posi-4 tion by the stiff grid-lead GL2; grid G2 consistino of a flat spiral of fine wire.. Above grid 2 the plate element P2 is supported on the 'plate-lead PL2 and also, if desired, by a separate support PS. A nickel shield S2 electrically shields the interior of the tube from heater leads HL, HL and is electrically connected by conductor X2 to emitter disk F2. By the above construction and arrangement of emitter and heater, the

emitter itself will have little, if any, voltage drop along it.

In the modification of Fig. 4, the heater wire H3 is embedded in a tubular insulator I3 (cf. Fig. 1), which may be a short piece are electrically connected to emitter F3 by fine wires X3, X5, this arrangement preventing excessive loss of heat by a thermal connection ot large cross-section. Grid Ga and its lead GIE, and plate Ia and its lead PL3 are as in Figs. 1 and 2. Emitter lead FL3 to the external tube circuit is taken from one of the two shields S3.

In any case, the thermionic tube may be constructed so that the emitter may be heated by current from a storage battery alternatively with alternating currents, if and whenever convenient or desired.

Fig. 5 illustrates an ordinary radio receiving set employing two thermionic tubes, each of which embodies the invention. Here, the signals are received by antenna A, passed by transformer ()'l` to the first thermionie tube 'l`1 acting (at left) as detector, and then passed by the usual transformer AF to the second thermionic tube T2 acting (at right) as amplifier, whence the amplified currents pass to the telephone receiver shown. The current for heating the heaters for the emitters F1, F2 ot the two tubes is furnished from a step-down transformer T supplied from an ordinary sixty-cycle main to the heater leads A, A. The plate element P of the first or detector tube T1 is connected via lead PL through transformer AF to the positive terminal of the plate battery PB (which may be a simple dry cell or cells), the negative pole of this battery being connected to emitter F1 of the first or detector tube T1 via lead IVI. ln the two tubes here shown, each emitter, in-

cluding its heater, is indicated as being constructed, for example, like the corresponding-parts in Fig. 1. The second or righthand tube T2, being the amplifier tube, has its grid Grl connected via lead GL to `one terminal of the secondary of transformer AF, the other terminal of AF being connected to the negative terminal of battery PB. The plate P1 of the second tube is connected to the receiving telephones and through them to the positive terminal of plate battery PB. The operation of this system of Fig. 5 is identical with that of the ordinary detector-amplifier train of tubes, with the exception of the emitter construction which-permits the use of alternating current for heating.

The forms of Figs. 2-4 may be constructed according to the preferred electrical and thermal proportions above described for the form of Fig. 1, especially as to emitter' F.

I claim 1. In an electrically heated electron emitter for low power thermionic tubes of the evacuated glass-enclosed type having grid and plate electrodes independent of the emitter, the improvement comprising a small emitter member having an external emitting surface and an internal electric heater, the emission surface constituting a shield between the heater and the grid of the thermionic tube; electric insulatin material between the internal heater an the external emitting surface and supporting the emitting surface upon the heater; and circuit leads from the heater and ex tending through the glass tube; said emitting surface and electric heater being thermally isolated within the tube save for said circuit leads.

2. In an electrically heated electron emitter for low power thermionicv tubes of the evacuated glass-enclosed type having grid and plate electrodes independent of the emitter, the improvement comprising a small emitter member having an external electron-emitting surface and an internal electric heating means; electric insulating material between the internal heating means and the emitting surface and supporting the emitter upon the heating means; and circuit leads for the heating means extending through the glass tube; said heating means and emitter being wholly supported by said circuit leads and thermally isolated within the tube save for such leads, the latter heilig substantially thin to prevent excessive thermal conduction.

3. In a low power thermionic tube of the evacuated glass-enclosed type having grid and plate electrodes independent of the the improved emitting means which comprises a small emitter member having an external emitting surface and an internal electric heating means, said external surface constituting an electrode shield between the grid and the heater; circuit leads for the electric heater; and a shield of metal located between the'grid and I portions of said leads extending outside beyond the emitter; said metal shield being connected by an electric lead to the emitting member but otherwise metallicallyT independent thereof.

4. In a low power thermionic tube of the evacuated glass-enclosed type having three independent electrodes including grid, plate and elcctron-emitter, the improved emitter which comprises a small emitter member having an external emitting surface and a neighboring electric heater. said emission surface constituting a shield between the heater and the grid: circuitv leads J[or the heater; a shield ot' metal located between the grid and parts of said leads which extend beyond the emitter member: and electric insulating material between the internal heating means and the emitter member and supporting thelatter: the emitter member and the electric heater being thermally isolated within the glass tube save for said circuit leads.

5. In a low power thermionic tube of the evacuated glass-enclosed type having three and electron-emitter, the improved emitter which comprises a small emitter member having an electron-emission surface, and a neighboring electric heater therefor, said emitter member constituting a shield between the electric heater and the grid; circuit leads for the heater and extending through and supported in the glass of the tube; a shield of metal located between the grid and parts of said leads which extend beyond the emitter' member; and electric insulating material between the emitter member and the electric heater and supporting the emitter member on the heater; said metal shield being connected by an electric lead to said emitter member butotherwise metallically independent thereof:

6. In a low power thermionictube of the evacuated glass-enclosed type havin three independent electrodes including gri plate and electron-emitter, the improved emitter which comprises a small emitter member having an electron-emitting surface and va neighboring electric heater, said emitter member constituting a shield between the electric heater and the grid.; circuit leads for the heating means and extending through and supported in the glass of the tube; a shield of metal located between the grid and parts of said leads .which extend beyond the emitter member, said metal shield being connected by an electric lead to said emitter member but otherwise metallically independent thereof; and said emitter member and electric heater being supported insidethe glass tube by said circuit leads alone independent of said meta-l shield.

7. In a low power thermionic tube of the evacuated glass-enclosed type having three independent electrodes including grid, plate and electron-emitter, the improved construction and arrangement of emitter which comprises a small emitter member having an electron-emitting surface and a neighboring electric heater, said emitter member constituting a shield between the grid and the heater; circuit leads for the heater and extending through and supported in the glass of the tube; a shield of metal supported on the glass tube and located between the grid and parts of said leads which extend bey yond the emitter member; and electric insulating material located between the emitter member and the heater and supportin the emitter member on the heater; sai metal shield being connected by an electric lead to said emitter member but otherwise metallically independent thereof; and said emitter member and heater being supported inside the glass tube by said circuit leads independently of said metal shield.

GREENLEAF WHITTIER PICKARD.

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