US2272060A

US2272060A - Ultra high frequency amplifier system

Info

Publication number: US2272060A
Application number: US282117A
Authority: US
Inventors: Orville E Dow
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1939-06-30
Filing date: 1939-06-30
Publication date: 1942-02-03
Anticipated expiration: 1959-02-03

Description

Feb 3, 1942. o. E. DOW

ULTRA HIGH FREQUENCY AMPLIFIER SYSTEM Filed June 30, 1939 5 Sheets-Sheet l FILAMENT TRANSFORMER F/LAMENT -"227 TRANSFOMER L "0 INV EN TOR.

0 V/LLE E. 00W BY wu'zH/x ATTORNEY.

Feb. 3, 1942.

O. E. DOW

ULTRA HIGH FREQUENCY AMPLIFIER SYSTEM Filed June 30, 1939 5 Sheets-Sheet 2 INVENTOR. V/LLE E. DOW

win/01km,

ATTORN E Y.

Feb. 3, 1942. o. E. DOW 2,272,050

ULTRA HIGH FREQUENCY AMPLIFIER SYSTEM Filed June 30, 1939 5 Sheets-Sheet 3 107 Qxf 333 INVENTOR. ORV LE E. DOW

7 n/v-u-M/ ATTORNEY.

Feb. 3, 1942; 5:, ow 2,272,060

ULTRA HIGH FREQUENCY AMPLIFIER SYSTEM Filed June 30, '1939 5 Sheets-Sheet 4 WWW . Ml/l I l 1 1 l I 1 I l i INVENTOR. ORV/LLE E. DOW

BY /w-vw A TTORNE Y.

Feb. 3, 1942. o. E. DOW 2,272,060

ULTRA HIGH FREQUENCY AMPLIFIER SYSTEM Filed June 30, 1939 5 Sheets-Sheet 5 .FZ'g. .9 as 137 I NV EN TOR.

, 7710 1??? E. now I 4 BY z A TTORN Patented Feb. 3, 1942 ULTRA HIGH FREQUENCY AMPLIFIER SYSTEM Orville E. Dow, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application June 30, 1939, Serial No. 282,117

16 Claims.

This invention relates to an ultra high frequency amplifier suitable for use on frequencies of the order of 500 megacycles, though not limited thereto, and particularly concerns the constructional and mechanical details of the circuit elements which go to make up such an amplifier.

The invention has for one of its objects to provide a construction for ultra high frequenc operation which allows. the interchange of units or the inspection thereof to be easily effected, and in which the lengths of connections are reduced to a minimum.

The invention also relatesto novel tunin arrangements for tuning the filament leads of electron discharge devices; also to novel neutralizing condensers for the amplifier vacuum tubes; and to novel adjustable condensers movable over the input and output transmission line circuits.

A feature of the invention lies in the use of a novel mechanical construction for the tuning condensers for the input and output circuit leads, wherein sliding contacts between elements of the condenser are wholly avoided. By means of this type of condenser, there is obtained a wide range of tuning with a fine adjustment of the circuit.

Another feature resides in a particular type of concentric neutralizing condenser which has an absolute minimum of lead inductance and small capacity, and which is adjustable over a considerable range. This novel construction is easily removable and facilitates changing of amplifier electron discharge devices.

A still further feature is the transposition block arrangement employed for transposing the grid and anode leads from a vertical plane in the vacuum tube compartment to a horizontal plane required in the shielded rear compartment which accommodates the input and output leads.

Still further features of the invention relate to the condenser arrangement used to series tune the filament leads so that the active part of the filament has an extremely low impedance to ground; to the filament concentri line circuit for offering a high impedance to radio frequency currents flowing from the filament to ground and low impedance for filament heating current; and to the arrangement of panels and shields which enable the prongs of the vacuum tubes to be mounted through the walls of the panels, thus minimizing lead lengths and insurin perfect shielding.

The novel constructional arrangement of the invention, as herein described, has been successfully used in a relay transmitter for amplifying 500 megacycle signals, although it will be under stood that the various elements thereof as well as the. circuit as a whole may be used in a receiver and in other arrangements.

A more detailed description of the invention follows in conjunction with the drawings, wherein:

Fig. 1 shows a schematic diagram of an ultra high frequency amplifier comprising a pair of electron discharge device amplifiers connected in push-pull relationship and mounted generall in accordance with theprinciples of the invention;

Fig. 2 is a front view of the vacuum tube compartment showing some of the novel constructional details involved in mounting the pair of tubes;

Fig. 3 is a rear view of the ultra high frequency amplifier construction embodying the principles of the present invention, showin the input and,output leads as they are mounted in their shielded respective compartments;

Fig. 4 shows a side section of Fig. 3 and illustrates in. part a side view of the exterior of the vacuum tube compartment;

Fig. 5 is a detail showing the transposition block which is employed in the construction of the circuit arrangement;

Fig. 6 is a section on line 6--6 of Fig. 5;

Fig. '7 shows a front view of the transposition block of Fig. 5 with the vacuum tube compartment and the shielded paneling between this compartment and the rear compartment removed, in order to show very generally how the input and output leads pass through th transposition block;

Fig. 8 is a cross-sectional view of one of the neutralizing condensers employed in the present invention between the anode of one vacuum tube and the grid of the other vacuum tube;

Fig. 9 is a section of the filament concentric line showing the details thereof and the manner in which tuning. of the vacuum tube filament leads is accomplished; and

Fig. 10 is an enlarged view of a portion of the filament concentric line of Fig. 9 showing in more detail the manner in which the filament heater leads are disposed within the interior of the inner conductor of the filament concentric line and the way they connect to the elements of the filament tuning condenser.

Throughout the drawings, the same reference numerals indicate the same parts.

Referring to the drawings and to Fig. 1 in particular, which shows schematically the entire circuit arrangement of the ultra high frequency amplifier of the invention, there are shown a pair of amplifier triode vacuum tubes IOI, IOI mounted inversely with respect to each other. The two terminals for each of the anode and grid electrodes of these electron discharge devices extend from the envelope in opposite directions, so as to minimize lead connections, while the filament leads of each vacuum tube extend through one end of the envelope, as shown. The general configuration of these vacuum tubes are shown in more detail in Fig. 2, which will be described later. The anode and grid terminals of th vacuum tubes IOI, IOI extending in one direction make direct contact with terminals I09, which extend through transposition block I09 shown and described in more detail later in connection with Figs. 3, 4, 5 and 6, while the anode and grid terminals which extend in an opposite direction make contact with neutralizing condensers H0, H0. Condensers H0, H eifect cross-neutralization of the amplifier circuit and are of concentric line type of construction, each of which is tunable by means of a knob 221. The input energy is supplied from a loop I05, in turn coupled to input leads I03 which are short circuited at one end by a strip I20 and connected at the other end to the grids of the vacuum tubes IOI, IOI. Output energy from the vacuum tubes is taken by the output leads I04 which are connected at one end to the anodes of the vacuum tubes IOI, MI and short circuited at their other ends by a short circuiting strap I2I. Energ is derived from the output leads I04 by means of a loop I06 which is arranged parallel to and spaced from leads I04. The conductors I03, I04, I05 and I06 are preferably made of copper rod or tubing. Both the input and output leads I03 and I04, respectively, are each electrically sub stantially three-quarters of a wavelength long at the carrier or operatin frequency (when above 300 megacycles), as measured from the short circuiting strap to the vacuum tube grid and anode electrodes. Across the input and output leads I03 and I04 there are provided balanced type tuning condensers I01 which are movable by means of a dial arrangement II3 over the lengths of these input and output leads. Condensers I01 are variable and serve to tune the input and output circuits. Across the first voltage nodal point on the output circuit, which is immediately outside the transposition block, there is provided a shunt inductance I08 to discourage oscillations at a lower frequency than the operatin frequency. Puttin it another way, the shunt inductance strap I08 prevents the amplifier from oscillating at a lower frequency than the carrier or operating frequency. Suitable grid bias for the vacuum tube amplifiers is obtained by means of grid bias resistors I22 connected to the lower end of the input leads I03. The short circuiting strap I provides a path of extremely low impedance for energ Of the operating frequency between the legs of the input leads I03. By-pass condensers I23 serve to provide a blocking path for direct current between the legs of the input leads. A suitable positive potential is applied to the anodes of the vacuum tubes by means of a lead I24 which extends from the positive terminal of the source of potential, here indicated by the positive sign to the mid-point of strap I2I across the output leads I04. The filament circuit for each of the vacuum tubes IOI, IOI comprises a concentric conductor transmission line having an outer grounded conductor I25 and an inner conductor I20, and a two-plate variable condenser II4, one plate of which comprises the plurality of by-pass condensers II6, while the other plate is an adjustable stud or disc I29 mounted on the side of the vacuum tube compartment as shown in more detail in Fig. 2. The filaments of the vacuum tubes are adjusted to be at zero radio frequency potential by means of variable condenser II4. Thus all the R. F. current which flows in the filament leads flows to ground through condenser H4. The inner conductor I26 of the filament concentric conductor line is approximately one-quarter of a wavelength long and shorted for R. F. at its far end and offers a high impedance to the radio frequency currents flowing from the filament to ground. The heater leads for the filament extend inside of the inner conductor I26. The condenser II4 serves mainly to series tune th filament internal leads (i. e., internal in the vacuum tube) so that the active part of the filament has a low impedance path to ground. In effect, the filament concentric line is a choke which serves not only as a high impedance for the radio frequency energy, but also supplies heating power to the filament and a path to ground for the D. C. emission current.

The neutralizing condensers H0, H0 are adjusted so that the capacitive resistance from the anode of one vacuum tube to the grid of the other vacuum tube is equal to the reactance of the grid to anode capacitance of the tube. This adjustment takes into account the series inductance of the leads to the neutralizin condensers. Due to the appreciable inductive reactance of the leads connecting the neutralizing condenser to the tube electrodes, the capacitance of the neutralizing condenser will be considerably less than the grid to anode capacitance of the vacuum tube. This requires that the neutralizing condensers have an absolute minimum of lead inductance and that the capacity be small but adjustable over a considerable range. These neutralizing condensers, as shown in more detail in Fig. 8, meet these desired requirements, and in addition have the advantage of being easily removable from the vacuum tube compartment, thereby facilitating the change of vacuum tubes.

From what has been said above, it is believed that the general operation of the push-pull vacuum tube amplifier arrangement of Fig. 1 will be obvious to those skilled in the art from a mere inspection of the circuit diagram.

Fig. 2 shows in more detail the mechanical constructional details of the vacuum tube mounting with the neutralizing condensers and certain of the filament tuning condensers. Each vacuum tube IOI has an anode pron and a grid prong, designated respectively A and G, extending from opposite sides of the envelope, although it will be appreciated that in Fig. 2 there is shown only one side of each of these vacuum tubes. The tubes IOI, IOI are inverted with respect to each other so that the lengths of the leads to the neutralizing condensers will be a minimum. These neutralizing condensers which have been designated H0 in Fig. 1 are here designated by the

reference numerals

22I, 222, 223, and 224, which are the same references employed in the more detailed description of these neutralizing condensers given in connection with Fig. 8. At this time it is believed to be suiiicient for an understanding of this Fig. 2 to point out that elements HI and 222 are metal cylinders which are mounted on a quartz rod 224 and that the capacity is adjustable by means of a rotatabledisc 221 fastened to a shaft 229 which controls the adjustment of the condenser.

The tuned heater leads for each filament, as well as the central lead from the mid-point of the active element of the filament, are brought out at one end into small metallic plates I21 to which they are securely attached by means of screws I28. These plates I21 are insulated from each other. The two outer leads or heater leads are connected through their associated plates I21 toleads in theinterior of the'inner conductor I26 of the filament concentric line. The central plate I21 is connected directly to the nearest end of the inner conductor I20 and to the two encasing plates of H6, as shown in more detail in Fig. 10. These three plates I21, I21 form bypass condensers labeled generally I I6, which bypass condensers form a capacity with an adjustable disc or stud I29 which is movable by means of a screw and knob I30 from the exterior of the vacuum tube compartment, as shown in more detail in Fig. 9. It should be observed that the plates I21, I21 extend through an aperture in the bottom (or top) of the vacuum tube compartment and through a hole in the outer conductor I25 of the filament concentric line to the inner conductor. By adjusting knob I30 to vary the tuning condenser II4, we are thus able to series tune the filament internal leads, so as to place the active part of the filament in the vacuum tube at ground potential. Similar constructions of filament concentric lines and tuning condensers are employed for both vacuum tubes.

Fig. 3 shows the mechanical details of the rear compartment of the ultra high frequency amplifi'er, and illustrates the manner in which the input and output leads and their associated elements are all mounted in a grounded metallic box I02. It should be observed that the walls of all compartments are made of metal, and that the input and output leads are shielded from each other"by a metallic U-shaped bracket I3I mounted externally of the transposition block I09. It should be noted that inductance I03 takes the form of a short section of concentric line which passes through the U-shaped shield I3I. The concentric conductor line serves to shield I08 from the input circuit, and passes through I 3'! as the most direct path between the voltage nodal points on the conductors. I04, I04. The short circuiting straps I20 and I2I are actually metallic plates bridged across the legs of the input and output leads, it being evident, of course, that short circuiting strap I20 is separated from each of the legs I03 by means of mica spacers I32 in order to form the radio frequency by-pass direct current blocking condensers I23. At each end of box I02 is a small metallic box II5 for accommodating the input coupling loop I05 or the output coupling loop I06. If desired, ventilating or cooling air can be blown through one of the boxes I I5 and up through the box I02 and out through the other box H5 in the direction of the arrows, as shown in Fig. 4.

The construction of thecondenser I01 is clearly shown in Fig. 3 taken in conjunction with Fig. 4 and will now be described in detail. The condenser I01 comprises. a metallic block 333 which has two apertures extending therein through which the leads I03 or I04 extend (note Fig. 4). The block 333 has the function of the rotor of the conventional balanced condenser and the size of the holes therein permit it to pass freely over sleeves 332 without touching them. Sleeves 332 each have one slot extending the whole length thereof. The block 333 is supported by insulators 334, the latter being mounted on'a plate 336 to which is secured the rack 335. A pinion 331 on a shaft 338 of a tuning dial II3 engages the rack 335 and causes the plate 336 with insulators 334 and block 333 to move-up or down, and thereby increase or decrease the resonant frequency of the circuit I03 or I04. The ratio of diameters of the holes in block 333 to the outside diameter of the sleeves 332 and the length of the sleeves 332 and the block 333 determines the maximum capacity. The condenser can be used without the sleeves 33 2 but their use results in a greater change in the resonant frequency for a given movement of block 333. Generally, the sleeve 332 will be placed near the voltage maximum point on the input or output leads. These sleeves 332 in effect increase the cross section of the conductors of the input and output circuits at or near a voltage loop, thus increasing the maximum capacity obtainable. Since the sleeves are movable, this maximum capacity may always be located at the voltage loop. This type of condenser has the advantage of totally avoiding the use of sliding contacts, and of giving a Wide range of adjustment. This wide range of tuning will be evident whenv it is considered that with the block at the extreme position at the input or output conductors, that is at a voltage nodal point, the condenser acts to increase the resonant frequency above that of the circuit alone. As the block 333 is moved toward the free ends of the input or output leads, that is, away from the short circuited ends, the resonant frequency decreases and is a minimum when the block 333 is at the voltage maximum point. The block 333 has been supported on insulators 334 merely for convenience, but these insulators are not necessary since the block is at ground or zero radio frequency potential. If the gap between the block 333 and the sleeve 332 is large enough to withstand the peak radio frequency voltage on the input or output leads, the block may be supported by metal posts. It will also be understood that other mechanical means of moving block 333 may be used than that shown and described.

Figs. 5 and 6 show the transposition block I 09 in more detail. This transposition block is made of metal and contains,in effect, four slant ing apertures, each of which has a half ledge I33. The holes which extend through the transposition block are on a straight line through the center of the largest face of the block. This transposition block serves to transpose the relative position of grid leads and anode leads from the vertical plane in the vacuum tube compartment to the horizontal plane required in the rear circuit compartment for connecting to the input and output circuits I03 and I04, respectively. This transposition block is shown also in Fig. 7 with the tube compartments broken away and with the wall between the vacuum tube compartment and therear compartment also removed, in order to show how the prong holding the clips on the transposition block connect with the input and output leads. The vacuum tubes are shown in dot and dash lines in order to indicate the relative positions they take when mounted in the tube compartment, The R. F. circuit is not connected to the metal transposition block electrically or supported from it by insulators. The transposition block shields the four leads which gothrough it from each other. The sockets or clips for the anode and grid tube prongs are supported from the circuits I04 and I63. The rod attached to I04 (or I03) which goes through the hole in the transposition block supports rigidly two spaced flat spring strips (of good electrical conductivity such as beryllium copper). On the other end of these spring strips is supported the drilled and slotted bronze rod which receives the tube prong. The screw which is tapped into this tube prong receptacle has generous clearance in the spring strips so that the receptacle is free to move slightly to prevent putting a strain on the tube prong seal. The spring strips are normally sprung apart. The screw tapped into the tube prong receptacle draws the strips nearly together thereby assuring good electrical contact at this point.

Fig. 8 shows, in detail, the construction of the neutralizing condensers. Each neutralizing condenser comprises metal cylinders 22I and 222, to which are connected clips for accommodating the anode and grid prongs, respectively, from the vacuum tube. These anode and grid prongs are labeled A and G. Both metal cylinders 22I and 222 are mounted on a rod 224 of quartz or other suitable insulating material. Telescoping over cylinder 22I is a thin metallic sleeve 223 which is movable by means of a rod 225 of isolantite or other suitable material. Sleeve 223 is slotted so as to make good electrical contact with cylinder 22 I. As the sleeve 223 is moved toward cylinder 222, the capacity of the neutralizing condenser is increased. The quartz rod 224 is supported from a metallic disc 226 which in turn is fitted with a breech lock which secures it to the side wall of the vacuum tube compartment. A rotatable disc 221 is provided which is fastened to the internally threaded sleeve 228 by means of a set screw 230. Sleeve 228 is prevented from moving lengthwise by a flange 23I. Sleeve 228 engages the threaded shaft 229 which is cemented to the isolantite rod 225. Thus by moving the disc 221, the isolantite rod 225 is made to move lengthwise and vary the capacitance of the neutralizing condenser. Although movement of the rod 225 may cause metallic sleeve 223 to telescope sleeve 222, so as to cause an increased capacitance between 222 and 223, in the particular arrangement successfully tried out in practice for 500 megacycles, it was not necessary to have cylinder 223 extend over cylinder 222, there being sufiicient capacity obtained for the purpose of the embodiment merely by having the end of cylinder 223 come close to the end of cylinder 222 without any overlapping therebetween.

Figs. 9 and 10 illustrate in some detail the construction of the filament concentric line I25, I26 and the manner in which filament leads extend to the inner conductor. The filament leads are shown terminated at their ends with a small metallic terminal I34 which extends into holes in the plates I21. All three plates I21, I21 have an aperture therein for permitting the inner cnductor I 26 of the concentric line to pass therethrough. The outer plates I21, however, to which the heater leads from the vacuum tube are connected, have apertures which are larger than the inner conductor I26 of the filament concentric line. The central metallic plate I 21, however, provides a close fit for the inner conductor and makes contact therewith, thus being directly connected to the inner conductor. The heater wires which extend to a source of external heater supply energy (not shown) are positioned within the interior of the inner conductor I26 and make contact with the two outer plates I21 for supplying heater energy to the filament of the vacuum tube. Since all three plates I21, I21 are insulated from each other and are sandwiched between other metallic plates from which the plates connected to the heater leads are insulated, it will be obvious that the plates I21 form by-pass condensers which are low impedance paths to radio frequency potential. The heater leads in the interior of the inner conductor I26 extend from the inner conductor to an enlarged metallic box I36 at the opposite end of the filament concentric line, in which box are terminals and by-pass condensers for the heater supply energy. The end of the inner conductor which is at the filament box I 36 is provided with flanges I31 which are insulated by means of mica spacers I38 from the filament box and the outer grounded conductor of the line. A connection from the flange I31 which is the D. C. path to ground for the filament center tap is indicated as I40. Thus the D. 0. ground path is through the meters shown in Fig. 1. The by-pass condensers in box I36 for the heater leads act as additional filtering means on the heater supply leads to reduce stray radio frequency fields outside the amplifier. These flanges form by-pass condensers and are in effect a short circuiting path for radio frequency energy. The inner conductor I26 of the filament concentric line I25, I26 is made to be approximately one-quarter wavelength long so as to offer a high impedance to radio frequency currents flowing from the filament to ground through the branch circuit for the filament heating current and emission current. The filament terminal block comprising the by-pass condensers II6, including the plates I21, are so made that they can be elevated to clear the filament leads I34, thus enabling the change of the vacuum tubes when necessary. A threaded screw end-plate I35 serves to provide access to the end of the filament concentric line adjacent the filament terminal block.

In the operation of the filament concentric line circuit, it will be evident from what has been said above that the D. 0. ground for the filaments is through the meters shown in Fig, 1. The R. F. currents in the filament leads (heater and center tap) flow to ground through the condenser H4 and are prevented from flowing to ground by Way of the concentric line I26 because it is one-quarter Wavelength long and grounded at the far end by condenser I31. The capacity of I I4 is between stud I29 and the outside plate of I I6. Both outside plates of H6 are conductively connected to the middle plate (filament center tap) I21 at the inner conductor I 26 and also by screws which hold the assembly IIB together. Thus, the R. F. current in the filament center tap lead flows directly to the outside plates of H6 and then to ground through H4. The R. F. currents in the heater leads fiow through by-pass condensers of II 6 to the outside plates and then to ground through II4. Thus all the filament leads are grounded by means of by-pass condensers in I16 and series tuning condenser I I4. The purpose of by-pass condenser I31 is to tie one end of the inner conductor I26 to ground so that the other end offers high impedance to R. F. currents.

It should be understood that the invention is not limited to the precise arrangements indicated since modifications may be made Without departing from the spirit and scope of the invention.

For example, the amplifier system may be operated'without the filament center tap connection. The center tap offers a low loss path for the R. F. currents in the filament. However, if the vacuum tubes had no filament center tap the middle plate I21 would be omitted. The path from filament to ground for D. C. currents would then be obtained by a connection from ground to the center-tap of the filament heating transformer or to a center tapped resistor connected across the heater supply terminals.

What is claimed is:

'1. An ultra high frequency amplifier comprising a vacuum tube having a filament, a connection from the center of said filament to a point of zero radio frequency potential, and means associated with said connection for series tuning the reactance between the center of said filament and said point to provide a path of low impedance to radio frequency energy.

2. An ultra high frequency amplifier comprising a vacuum tube having a filament, a connection from the center of said filament to a point of zero radio frequency potential, and means associated with said connection for series tuning the reactance between the center of said filament and said point to provide a low impedance path to radio frequency energy, a concentric line having inner and outer conductors and effectively one-quarter of a wavelength long at the operating frequency, and heater leads for said filament located within the inner conductor of said line and extending substantially the entire length of said inner conductor.

3. An ultra high frequency amplifier comprising a vacuum tube having a filament, heater leads for said filament and a lead for the middle point of said filament, all of said leads extending out from one end of said vacuum tube, individual metallic plates for said leads, said plates being located externally of said tube and insulated from each other for providing a low impedance capacitive coupling therebetween, and adjustable means for capacitively coupling said plates to a point of zero radio frequency potential.

4. An ultra high frequency amplifier comprising a vacuum tube having a filament, a connection from the center of said filament to a point of zero radio frequency potential, and means associated with said connection for series tuning the reactance between the center of said filament and said point to provide a path of low impedance to radio frequency energy, a concentric line having inner andouter conductors and effectively one-quarter of a wavelength long at the operating frequency, and heater leads for said filament located within the inner conductor of said line and extending substantially the entire length of said inner conductor, said heater leads being capacitively coupled to the outer conductor of said line at the end of said line farthest away from said filament.

5. An electron discharge device having a filament, leads for said filament, a direct current connection from the center of said filament to a point of fixed alternating current potential, means external of said device for capacitively coupling said leads and connection together, and additional means capacitively coupling said first means to said point of fixed alternating current potential.

6. An electron discharge device having a filament, leads for said filament, a connection from the center of said filament to a point of fixed alternating current potential, terminal plates cated externally of said device for said leads and connection, and a metallic element spaced from said plates for capacitively coupling said plates to said point of fixed alternating current potential, said element being adjustable in position for varying the spacing between it and said plates.

7. An electron discharge device having a filament, a lead for said filament, means for overcoming the effect of the reactance of said lead comprising a variable capacitor located externally of said device and coupling said lead to a point of fixed alternatingcurrent potential, the effective value of said capacitor being such that there is obtained a series resonant condition thereby providing a low impedance path to said point from the active part of said filament.

8. An electron discharge device having a filament, heater leads for said filament extending externally of said device, means associated with said heater leads ofiering a high impedance to radio frequency currents flowing from said filament and a low impedance to heating current, a connection from said means to the middle point of said filament, and means associated with said leads and connection for tuning the same so that the active part of said filament has a low impedance path to ground.

9. An electron discharge device having a filament, heater leads for said filament and a lead for the middle point of said filament, all of said leads extending out from one end of said vacuum tube, individual metallic plates for said leads, said plates being located externally of said tube and insulated from each other for providing a low impedance capacitive coupling therebetween, adjustable means for capacitively coupling said plates to a point of Zero radio frequency potential, a concentric line having hallow inner and outer conductors effectively an odd multiple of one quarter of the length of the operating wave and coupled together at one end through a low impedance path, connections extending through the interior of said inner conductor and connecting a source of heating energy to those plates associated with said heater leads, a direct current connection from said inner conductor to ground at said one end of said concentric line, and a direct current connection from the free end of said inner conductor to that plate which is in circuit with the middle point of said filament.

10. An electron discharge device having a filament, heater leads for said filament, a concentric line having hollow inner and outer conductors and effectively an odd multiple of one-quarter of the length of the operating wave, said inner and outer conductors being coupled together at one end through a path of low impedance to radio frequency energy, and a direct current connection from said outer conductor to a point of fixed alternating current potential, said heater leads extending the entire length of said inner conductor within the interior thereof.

11. High frequency apparatus having, in combination, an electron discharge device, a filament for said device, a source of heating current for said filament, leads extending from both legs of said filament to said source, a tubular conductor surrounding said leads, a lead extending from the mid point of said filament to the nearest end of said tubular conductor, means for capacitively coupling all of said leads together at said end, and a direct current connection from the other end of said tubular conductor to ground.

12. High frequency apparatus having, in combination, an electron discharge device, a filament for said device, a source of heating current for said filament, leads extending from both legs of said filament to said source, a tubular conductor surrounding said leads, a direct current connection extending from the mid point of said filament to the nearest end of said tubular conductor, means at said end of said tubular conductor for capacitively coupling said filament leads and said direct current connection together, a direct current connection from the other end of said tubular conductor to ground, and means at said last end for capacitively coupling said filament leads to ground.

13. High frequency apparatus having, in combination, an electron discharge device, a filament for said device, a source of heating current for said filament, leads extending from both legs of said filament to said source, a tubular conductor surrounding said leads, a lead extending from the midpoint of said filament to the nearest end of said tubular conductor, means for capacitively coupling all of said leads together at said end, a variable capacitor comprising a grounded metallic element spaced from said last means and adjustably positioned with respect thereto, and a direct current connection from the other end of said tubular conductor to ground.

14. An electron discharge device having a filament and heating leads therefor, said leads having inductive reactance at the operating frequency, and capacitive means connected between a surface of zero radio frequency potential and a point on said leads, said capacitive means having such value as to form a series resonant circuit between said filament and said surface for energy of the operating frequency.

15. An electron discharge device having a filament and heating leads therefor, said leads having inductive reactance at the operating frequency, capacitive means connected between a surface of zero radio frequency potential and a point on said leads, said capacitive means having such value as to form a series resonant circuit between said filament and said surface for energy of the operating frequency, and means offering a high impedance for radio frequency currents connected between said point on said leads and said surface.

16. An electron discharge device having a filament, a lead for said filament, means for overcoming the effect of the reactance of said lead comprising a capacitor located externally of said device and coupling said lead to a point of fixed alternating current potential, the effective value of said capacitor being such that there is obtained a series resonant condition thereby pro- Viding a low impedance path to said point from the active part of said filament.

ORVILLE E. DOW.