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Converter Tube Design Features TECHNICAL SECTION Copyright 1943, Sylvania Electric Products Inc. (Formerly Hygrade Sylvania Corporation) MARCH, 1943 EMPORIUM, PENNA. Vol. 10, No. 3 Converter Tube Design Features Service problems related to converter tubes may often be clarified through knowl- edge of the tube design of the particular type in question.Numerous converter tube types have been employed in superheterodyne receivers.Five principal de- signs are in general use and these will be briefly described as to their constructional features and the functions associated with the various grid structures. Several typical performance curves are also discussed. TRIODE-HEPTODE CONVERTERS gram is shown wherein plate -tuning of the oscillator is indicated. Types 7J7, 7S7 14.J7, 14S7, 6J8G Grid -circuit tuning is also widely employed with triode-heptode The superheterodyne receiver requires a converter tube whose tubes, in which case the oscillator circuit resembles that shown function is to mix or beat (heterodyne) the incoming signal fre- for the pentagrid converter described below. quency with a locally generated frequency to obtain an inter- mediate frequency.If the output from a conventional triode oscillator is suitably coupled to a tuned r -f voltage amplifier, the pentode plate circuit will contain four frequencies: the signal OUTPUT frequency, the oscillator frequency, and the sum and the differ- A.T. SIGNAL dP ence frequencies of these two. The latter are obtained by beat- IINIUTI PO ing the first two frequencies mentioned.Since more gain is -) secured at lower frequencies in the i -f amplifier stages, the differ- ence frequency (called the i -f frequency) is the one desired. This d can be obtained when the tuned circuits in the pentode plate +100r +4 0r are designed to resonate at the i -f frequency, in which case they will reject the other three frequencies. THE PENTAGRID CONVERTER T.IOD! Types ILA6, IA7GT/G, 7A8, 7B8, 6A8G, and others nAT[ NOTOOt PLAN Pentagrid Converter is the name applied to a tube having five grids in addition to the cathode and plate and intended for frequency conversion in superheterodyne radio receivers. Such tubes combine the functions of oscillator, mixer and amplifier within one structure. The No. 1 and No. 2 gridsare used as the oscillator grid and oscillator plate respectively.Electrons passing through grids No. 1 and No. 2 are further controlled by the signal input grid No. 4.This grid is shielded from the oscillator section by grid No. 3 and from interaction with the plate by grid No. 5.Screen grids No. 3 and No. 5 are connected together internally. The circuit on the nextpage illustrates the use of the pentagrid converter, such as Type 7B8.Voltages of signal and oscillator frequency reaching the plateare bypassed to ground, since the tuned circuit in the plate is resonant only to the beat or intermediate frequency (I.F.) which is to be fur- ther amplified. The pentagrid converter may be consideredas operating very much like a conventional variable-mu tetrode first detector with Such a circuit can be simplified considerably by combining the an associated triode oscillator, except that the oscillator triode triode oscillator with the mixer tube in one bulb.Furthermore, grid is located next to the cathode which iscommon to both the improved efficiency and stability can be secured by adding an in- first detector variable -mu tetrode and the oscillatortriode. jector grid in the mixer section which is connected directly to the Electrons emitted from the cathode surfaceare influenced by the grid of the oscillator section, thereby permitting both the mixer various grid and plate voltages and divideup so that grid No. 1 control grid and the injector grid to control the electron stream. receives about 3 to 6 per cent of the electrons,the oscillator This provides true electron coupling. An illustration of such a anode receives about 40 per cent of electrons,grids 3 and 5 tube is the triode-heptode Type 7J7 shown above.The dia- (screen grid) receive about 25 to 30per cent of the electrons, and gram indicates that the cathode is common to both units, the the plate receives the remaining electronsemitted. upper portion being associated with the triode oscillator and the Because of the oscillator grid's strategic 'positionnext to the cathode,.any lower section with the heptode mixer. A typical circuit dia- (Continued on following page).
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