Radio age (Jan 1927-Jan 1928)

RADIO AGE for November, 1927 The Aero-Seven Receiver THE design of the Aero-Seven receiver is one of distinction in the radio kit field. It claims several new features as follows : A special feature eliminating the objectionable detuning of the first stage of radio frequency amplification by the antenna, a common defect in many so-called single dial sets; the use of Aero Universal Coils, which are tapped in such a way as to allow the use of the new high-mu tubes for radio frequency amplifiers, with a considerable gain in both the sensitivity and the selectivity of the receiver; the use of these tubes in a perfectly balanced resistance coupled amplifier to produce exceptional tone quality and volume; and the embodyment of a special foundation unit consisting of drilled and decorated panels, with mounting brackets so that the construction of the receiver is simplified to an assembly operation. A front view of the completed receiver, Figure 1, shows the workmanlike appearance of the receiver that may be constructed by the home builder. Many of the present-day single control receivers have the first radio frequency transformer tuned by one of the units of the gang condenser. In spite of any care in matching of the coils and condensers there is no provision made against the detuning of the first stage by antennas of different electrical characteristics. This effect is not noted in the succeeding stages as the other transformers each operate out of the plate circuits of similar tubes and hence similar impedances. A very simple means has been resorted to in the design of the Aero-Seven in order to eliminate this defect. As will be seen in the circuit diagram, Figure 2, the antenna is connected across the 1,000-ohm resistor in the grid circuit of the first radio frequency tube, thereby allowing the first R. F. transformer to operate out of the impedance of a tube in similar manner to the rest of the transformers in the receiver. The size of this resistor has been carefully chosen so as to permit the voltages set up in the antenna to be effectively transferred to the grid of the first amplifier tube. As will be seen from the circuit diagram, the remainder of the radio frequency features are the tried and tested standards of the past, with the exception of the employment of high-mu tubes as radio frequency amplifiers. This feature is made possible by the design of the Aero Universal Coil. This coil is an arrangement of an exceptionally efficient, secondary construction that has been on the market for several years, with a primary arranged so as to get the most effective coupling coefficient to the secondary and tapped so as to produce primary impedances of widely varying values on the different taps. The highest of these values is secured by the use of tap No. 1 as the plate and the No. 4 tap as the B battery connection, as shown in the circuit diagram. When employed in this connection the impedance is of such a value as to perform very effectively with the high-mu tubes now on the market, such as the UX240 and the CX340. At the plate potential used (90 volts), these tubes on the average show a figure of merit, as generally accepted in engineering work, of 1.6 broadcast band. It has been found after considerable investigation of the subject that if two or more coils match at one wavelength that they will not necessarily match at another wavelength at the other end of the broadcast band. In order to prevent the receiver from falling out of tune at one end of the dial and in at other points of the dials, the Aero coils are matched at the factory at 250 meters and also at 500 meters. It has been found that by matching the coils in this manner that they will be sure of maintaining the same inductance over the whole broadcast band. The Amsco gang condenser used to tune these coils is one of exceptional accuracy and is provided with small compensating capacities to adjust for small differences in the wiring and tube capacities of the various stages. This adjustment will be described later. Preliminary to constructing the AEROSEVEN, a word about the Aero Noskip No. 60 choke coil used in the set. It has Figure 1 — Front panel view of the Aero-Seven Receiver times the figure of merit for the usual 201A type of construction. This figure of merit may be realized in the design of a practical receiver either in selectivity or in amplification, or partially in both. In the design of the Aero Universal Coil the primary impedance was so proportioned as to conform to the latter possibility. The result is an extremely sensitive receiver of extreme selectivity. An innovation in the matching of the radio frequency stages has been employed by matching the Aero coils in kits of three at two widely separated frequencies in the two nuts at the mounting end, the outer one of which is the only one supposed to be removed by the set constructor. The remaining one is intended to hold the engraved bakelite disc at the same end in place, and should not be removed. The Aero Noskip No. 60 choke coil is made of very fine wire, of especial design, and we caution you against substituting other chokes in circuits where it is recommended. Furthermore, we like to tell you not to tamper with it on the inside because of the very fine wire. Each one is tested at the factory, and is mechanically and elec A. 240 ■THREE GANG .0005 MFD CON. <? 240 / 240 A BAT. BBfll Figure 2 — Schematic diagram of the Aero-Seven.