International projectionist (Jan-Dec 1935)

May 1935 INTERNATIONAL PROJECTIONIST 25 tions and supports between base pins and electrode structure. The familiar metal shield which is necessary with the glass tube in r. f. portions of a circuit is not required with the new tube. The metal envelope itself serves as a shield. And, since closer proximity of shield to elements can be realized, the shielding is more effective. Whereas, in certain types of glass enclosed screen-grid tubes the anode is shielded first by an internal structure, next by a coating on the inside of the glass bulb, and finally, when in use, by an external "can," in the new metal tube all these functions are performed by the shell. A further favorable feature of the metal shell, G. E. believes, is that it contributes to residual gas cleanup, thus promoting continued efficient operation. New Pin Arrangement The new tubes have one more base pin than comparable glass tubes, since the metal envelope has become the shield, and provision must therefore be made to ground this envelope. Designers have even taken into consideration greater ease of inserting it in the socket. In the present conventional glass tube, two of the base pins are of larger diameter than the others, necessitating alignment of these larger pins with corresponding socket holes. In the base of the new tube, all the pins are of the same diameter, and in the center is a longer insulated keyed pin. By placing this insulated pin in a hole centrally located in the socket, and rotating the tube until the key slips into its groove, the tube is quickly and easily inserted. The metal construction has been applied both to existing types of glass tubes having indirectly heated cathodes, and to other newly-developed tubes. Included in these is a duo-diode, which is only about five-eighths of an inch high above the base, and a hexode, which is an improved pentagrid converter. The engineering contributions cited by G. E. as having helped make the metal tubes possible, are: A new, sturdy seal to replace the troublesome and expensive feather-edge type of glass-to-metal seal. Accurately controlled seam, spot and projection welding for the fabrication of the envelope or shell. A vacuum-tight means of sealing off a metal exhaust tabulation. Philco Opposes New Tubes Promptly upon announcement by G. E. of the new metal tubes, the Philco Radio & Television Corp. used full-page advertisements in leading newspapers to register its opposition thereto. Excerpts from the Philco blast follow: Metal radio tubes in England were a dismal failure. They are now defunct. Progress continues on glass tubes; experimentation on metal tubes should be encouraged. Metal may possibly some day take its place alongside glass for radio tubes. In Philco's opinion that day has not arrived as yet. INCREASE YOUR WINNINGS WITH THE RCA VICTOR PHOTOPHONE GIVES GREATEST SOUND SATISFACTION AND OFFERS YOU: • A Sound Box-Office Attraction • Complete Ownership • A Self-Liquidating Investment • RCA SuperService PHOTOPHONE DIVISION RCA MANUFACTURING CO., INC. Camden, New Jersey A Radio Corporation of America Subsidiary Here are some disadvantages of metal radio tubes: 1. With the proposed American metal tube construction, bulb size is reduced. With an equal amount of heat to dissipate, the smaller metal tubes must operate at a higher surface temperature than the larger glass tubes. High temperature is not only detrimental to tube life, but changes the characteristics of nearby coils, resistors, etc., thus impairing the delicate balance of all the component parts, which is absolutely necessary for fine performance. This could be avoided by spacing the coils farther away from the metal tubes, but this requires more space, not less. 2. Production difficulties in the proposed metal tubes restrict the manufacturer of certain highly desirable multiple function types which are in general use in glass. The proposed metal tubes are limited to a few types — principally single function types. 3. Thus, to achieve a given performance, more metal tubes than glass tubes are required. This again requires more, not less, space in a radio. Cites Costs, Other Defects 4. Also, the additional metal tubes add to cost and electric current consumption without adding to performance. 5. The transparency of the glass tube often allows the user to determine when