Patent GB-278750-A (GB)
Title: Improvements in vacuum tubes
IPC: H01J1/20
Filed: 1927-10-08
Abstract: 278,750. Arcturus Radio Co., (Assig nees of Krahl, W. L.). Oct. 8, 1920, [Convention date]. Vacuum tubes.- thermionic discharge tubes such as amplifiers, detectors, rectifiers, &c. are provided with tubular cathodes 3Fig. 1, heated by radiation only from a single, straight, carbon filament 4 shielded by the cathode throughout its length and heated by alternating current. The tubes fit standard valve-holders and avoid disturbing noises when low filament current cf 15 volts is used. The narrow tube 3 is of magnetic or other metal, for example, nickel, and is connected in series with the filament 4 between leads 17<a>, 22<b>, the parallel position of the tube 3 and filament 4 reducing inductive effects. The tube 3 and filament 4 are rigidly spaced apart at their lower ends on short supports 17, 22<a> and are ccnnected together at their upper end by a bar 50, a support 20 and a tungsten or other spring 19 tensioning the filament. The upper end of the carbon filament is electroplated with copper and then with nickel to allow welding to the tungsten spring; the lower end may be similarly treated to allow welding to the wire 17 but the nickel coating may be omitted if the wire 17 is of nickel. The tube 3 is held in collars 2, 23 on the parts 22a, 50 and extends axially at both ends just beyond the filament and beyond the surrounding anode 1 and grid 2 to shield the filament 3 electrostatically. A shield may extend the cathode. to the pinch 5. The part of the tube 3 within the anode 1 is coated with an electron-emitting oxide, for example, by applying a mixture of 1 part of barium carbonate and 3 parts of paraffin wax and a similar mixture of strontium hydroxide and paraffin, wax. In applying the mixtures, the tube 3 is heated and six coatings of the strontium mixture and one of the barium mixture are applied, the wax being burnt off after each coating, and the tube being finally heated to red heat in hydrogen. The electrodes are all supported by support wires in the single press 5, the cylindrical anode 1 and helical grid 2 being steadied by wires 51, 69 in a glass bead 30 on the support 20. The cap 8 is provided with four pins 9 - - 12, two 9, 12 being connected to the grid and anode and the usual two filament pins 10, 11 being connected together at 65 and connected to the lead 22<b>. The lead 17<a> is connected to a spring terminal 13, Fig. 2, at the side of the cap, the terminal 13 being held in place by a rivet 15<a> and a fibre strip 14 beneath the connector 65. The valve may be plugged into a standard holder of a radio set, the secondary of a voltage-reducing transformer 31 connected to the mains being connected between the terminal 13 and the lowtension trminal of the set which includes the filament rheostat in circuit. The inner and outer surfaces 67, 60 of the valve envelope 7 may be connected to earth or to a point of low potential, metal deposits or strips being applied to the glass. The outside surface may be earthed at 70. In a modification, the transformer is connected across two terminals such as 13 at the side of the cap. The Specification as open to inspection under Sect. 91 (3) (a) comprises also modifications with a metal heating filament and with the terminal 13 and connector 65 omitted, the transformer being connected across the filament pins, and refers to heating the filament by direct current. This subject-matter does not appear in the Specification as accepted.