US2354585A - Low capacity filament transformer system - Google Patents

Description

July 25, 1944. J. L. FINCH LOW CAPACITY FILAMENT TRANSFORMER SYSTEM Original Filed March 15, 1940 2 Sheets-Sheet l INVENTOR Jarrzeo Llihclv BY wvt ATTORNEY July 25, 1944. J F H 2,354,585

Low CAPACITY FILAMENT TRANSFORMER SYSTEM Original Filed March 15, 1940 2 Sheets-Sheet 2 INVENTO R' WL.M-

ATTORN EY Patented July 25, 1944 .UNl'lED STATES PATENT. OFFICE LOW CAPACITY FILAMENT TRANS- FORMER SYSTEM James Leslie Finch, am Rockaway, is. Y., assignor to Radio Corporation of America, a corporation of Delaware '3 Claims. ('01. 175-356) ferring to the accompanying drawings, in which:

This invention relates to a new anduseful transformer device which is particularly adapted to short wave radio frequency generators and amplifiers which employ thermionic devices and particularly to such devices wherein the cathodes are heated by alternating current and wherein the anodes are maintained at an --appreciably generated or amplified wave voltage.

This application is a division of my copending application Serial No. 324,035, filed March 15, 1940, Patent No. 2,314,083 granted March 16,

1943, the claims in this application beingdirected to the transformer construction.

A system according to this invention will be found particularly advantageous when anodes of the thermionic devices are fitted with bulky metallic fin devices, over which air is blown for the purpose of cooling them. In such devices, the anodes of the thermionic devices can be gr unded with respect to the electric waves which are being generated or amplified.

1n the prior art, it has been the usual practice to ground the cathode and to subject the anodes to the electric wave voltages. The high capacity of the anodes to ground and to other objects when bulky metallic cooling found objectionable. The resultant high capacity may limit the maximum usable frequency to a materially lower value than would be otherwise possible. Further, when the anode was grounded and accordingly both the grid and the cathode of the thermionic devices subjected to high voltages, it was necessary to supply heating current to the cathode through chokes or' tuning coils. Such a method proved to be a handicap in many cases. One detrimental effect was that it made it more diflicult to change the frequency of the tuned circuits involved. Another detrimental factor was that the chokes, etc., introduced losses both in the high frequency wave voltage circuit and in the heater circuit. This method of supplying cathode heater power has iinsareusedis' generally been found impractical at radio fr'equency waves below twenty megacycles.

It is an object of my invention to improve such a system by employing a transformer inwhich the windings are isolated from ground and by the introduction of an improved radio frequency circuit which employs tuning means between the grid and the anode of the thermionic device for .the purpose of preventing regeneration due to grid-anode capacity and for the purpose of preventing excitation power from feeding through to the output due to the last mentioned capacity.

Fig. 1 is a schematic circuit arrangement of the system of this invention;

Fig. 2 is a section of the improved filament transformer;

Fig. 3 is a plan view of Fig. 2: and

Fig. 4 is a modified sectional view similar to that of Fig. 2, except that both the primary and secondary are insulated from the core by an air gap.

Referring now in detail to the drawings-Fig. 1 shows a circuit diagram of a radio frequency amplifier having a range up to twenty megaoycles in frequency. The thermionic triode device is of a type similar to an RCA 893 tube and is indicated as i having an anode 2, grid I and filament 4. The filament is shown as consisting of three separate strands connected together at their lower ends. The RCA 893 thermionic triode actually has six such strands connected together at their lower ends, so that in the figure each line is representative of two strands in parallel. A metallic fin assembly, preferably of copper, surrounds the anode 2 for cooling the same. This cooling assembly consists of a hub 0 of solid copper into which the anode is soldered. A-plurality of fins I extend radially out from hub 0. A blast of air is blown between the ilns, as indicated by the arrows. The radio frequency circuit employs a tuning means between the grid and anode for the purpose of preventing regeneration due to the grid-anode capacity and for the purpose of preventing excitation power from feeding through to the output'due to the grid-anode capacity. This tuningmeans consists essentially of and inductance coil 0 and a trimming condensed 9, which also includes bias bypass condenser II and a plate bypass condenser ll having a common ground connection between the two. The input circuit is disposed between the grid and filament of the thermionic tube. This circuit is tuned by making the reactance of coil II equal to the combined reactance of the grid-to-filament capacity, and the tuning condenser II. The source of excitation-power is represented by a generator l4. Generator l4 supplies current to coil ll which is inductively coupled to coil l2 and I have found it advisable to shield against capacitive coupling between these two circuits by This invention will best be understood by remade equal to the reactance of the filament-toground capacity, the fllament-to-anode capacity, and the capacity of tuning condenser ll. The output circuit is coupled in some suitable manner to the load. In this case, this tcoupling is obtained by means of output coil is, which is inductively coupled to tuning coil l1. Thespecial low capacity filament transformer is indicated at and comprises an iron core assembly, primary coils 22, 23 and 24, and secondary coils 25, 26 and 21. The description of this transformer will be more fully described with reference to Figs. 2 and 3. The plate voltage source is indicated as +3 and is connected to the anode as sembly. Thus, the anode is subjected to the full voltage of the plate supply. However, it is subjected to negligible radio frequency voltage, since any such voltage is bypassed to ground through condenser i I. The anode current flows to the illament, through the individual transformer windings and through tuning coil I], thus returning to the negative side of the anode supply through the common ground connection. Bias voltage for the amplifier system is introduced from the source indicated as C through grid-anode tuning inductance I to the grid 3. The return circuit for the grid current is through the filaments, through tuning coil l1 and the common ground connection to the positive end of the grid bias source. The radio frequency connections are made to the filament by means of bypass condensers 28, shown located adjacent to the low capacity transformer assembly 20.

Referring now in detail to Figs. 2 and 3, it will be noted that this construction differs from the ordinary filament transformer, as known in the prior art, in that the secondary windings are isolated from the iron core and from the primary windings by an air gap, that is, there is provided a space between the core and the coil winding in which there is no metallic substance or solid dielectric material. Such a construction insures a low capacity between the secondary windings and other adjacent objects. The secondary windings 25, 2| and 21 are shown supported on angles 28 and 30 which in turn are supported by four insulators 3i, lid, 32 and 32a. The iron core 2| is indicated as being substantially circular in section. This allows the minimum spacing for a given impressed voltage. The circular core is constructed by having the central laminations larger than the outermost laminations, that is,

each lamination on each side of the center line decreases proportionally in size on each side of the central line, thus providing a core essentially circular in cross-section. The secondary windings are armored with a non-magnetic metal 35 which protects the insulation of the individual conductor turns from the radio frequency field. It also protects the insulation from damage in case of an are between the secondary windings and the core. This armor 3! is connected to the common point of the three secondary windings as indicated in the circuit diagram of Fig. 1.

Located directly below each individual secondary winding is a primary winding 22, 23 and 24. Core 2| is mechanically secured to a base plate 36 by means of a plurality of strap angles 31 secured to plate I by means of screws 2!. Bypass condensers 28 are mounted on angle 2! adjacent to each secondary winding and connected thereto by leads 3! and ll.

Fig. 4 shows a further modification from that of Figs. 2 and 3 in which the core is isolated by an air gap from the primary in the same manner as shown and previously described in which the secondary was isolated. The core in this case will also be supported by a strap II secured to insulators 3| and 82. This construction will further decrease the capacity.

While I have described my invention as related to a system using three-phase filaments and have shown the same as connected with threephase transformers, this invention applies equally well to single phase filament supply and three single phase transformers can be used for a three-phase supply. Also, the transformers can be arranged in any other of the forms known in the prior art.

What is claimed is:

1. A low capacity transformer especially adapted for use with the filament system of a vacuum tube to be used in high frequency circuits comprising a closed laminated core, a primary winding wound on said core, and a secondary winding through which said core passes, the secondary being so spaced from said core and primary that there is relatively small capacity between said secondary and said core and primary, said primary and secondary being ar ranged in the same plane and having spaced substantially parallel axes, said primary and secondary moreover being arranged to physically encircle different portions of said core.

2. Apparatus as claimed in the preceding claim characterized by the fact that said secondary winding is shielded by being enclosed within a non-magnetic metallic casing.

3. A transformer especially adapted for the illament circuit of a vacuum tube to be used in high frequency circuits comprising a laminated metallic core substantially circular in cross-section, a primary winding linked with and spaced from said core and a secondary winding linked with and spaced from said core, said secondary winding being encased within a metallic shield, the spacings of said windings. with respect to the core and with respect to each other being such that relatively low capacity exists betwen the secondary and both the core and primary winding, said windings moreover being arranged in substantially the same plane and about spaced substantially parallel axes, said windings being arranged furthermore to encircle different portions of said core.

JAMES LESLIE FINCH.

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