Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST DECEMBER, 1928 FIG. 3. SCHEMATIC DIAGRAM OF THE WRITER S AMPLIFIER necessary A, and B voltages to both the amplifier and the radio-tuner unit. A large number of special amplifiers which, in some cases, may be adapted to phonographradio combinations, are also made by many manufacturers, including those mentioned in Table 1. As an example we might consider the installation of power-amplifier equipment in a hotel. In such a case one would require one or more amplifiers arranged in the form of a group, each amplifier supplying a certain portion of the power for the loud speakers. The utility of such an installation is, of course, increased if it is arranged so that music from either radio stations or phonograph records can be transmitted throughout the system. When this arrangement is employed it is necessary to connect the input posts of all the amplifiers to the same set of terminals, and a switching system is needed to connect either the output of a radio receiver or the output of a phonograph pick-up to these terminals. Any readers interested in the details of such amplifiers will do well to write the manufacturers mentioned in this article for complete descriptions of this equipment. An excellent power amplifier which, unfortunately, is not illustrated in this article, is the Amertran type 2-ap. This is a complete twostage transformer-coupled amplifier, the output stage being push-pull. Either 171or 210-type tubes may be used in the push-pull stage and the first audio amplifier tube may be either a 227 or a standard 20iA-type tube. Many such amplifiers are made — several are illustrated here — and a list of a few of the best units, with their characteristics, is given in Table 1. COMMERCIAL POWER AMPLIFIERS IN FIG. 5 are illustrated two Silver-Marshall * "Unipacs;" both of them satisfactory for use in a phonograph-radio combination. Data on these and other S-M. amplifiers is given in Table 1. As indicated, these amplifiers may be obtained either completely wired or in kit form. The 678-PD amplifier is especially interesting in connection with this article since the circuit is arranged so that the field of the dynamic loud speaker acts as the filter choke and is energized by the d.c. current flowing through the filter circuit. This amplifier, with its 250-type output tube is capable of supplying up to 4.5 watts of undistorted audio-frequency power, to the loud speaker. In Fig. 5 are illustrated also some of the power amplifiers made by the Samson Electric Company, which may be used in constructing a phonograph-radio combination, or any other unit from which high-quality reproduction may be desired. Data on the various models are given in Table 1. It will be noted that the amplifier PAM-17 is similar to the PAM-16 except that it supplies field current for a dynamic loud speaker. The type pac-2 amplifier should be used where the unit is also to supply B voltages for the radio tuner. This amplifier, it wil! be noted, also supplies a C voltage of minus 4§ volts which may be used to bias the grids of the r.f. tubes in the receiving set. The National Company also makes a power amplifier that may be used. This amplifier is also illustrated in Fig. 5, and Table 1 gives complete data on the various models. Model 8110 is a complete power amplifier and B-supply unit, but the Push-Pull amplifier does not contain any power supply and, therefore, must be used with a separate power unit designed to supply the Table I — Data on Power Amplifier Units and Kits Type Prices Voltages Available for Receiver Manufacturer Description of Amplifier No. Kit Wired B Voltages A. C. Voltages Silver-Marshall 682-250 $96.50 $111.50 45, 90, 135 1.5, 2.25 Two-stage transformer-coupled. 226 in first stage and one 210 or 250 in output stage. Rectifiers are two 281 tubes. An 874 glow tube is used. Silver-Marshall 682-210 $102 $117 45, 90, 135 1.5, 2.25 Same as 682-250 except that output stage is push-pull. Either 210 or 250 tubes may be used. Silver-Marshall 678-PD $66 $73 None None Two-stage transformer-coupled. 226 in first stage and one 250 tube in output stage. Rectifier is one 281 tube. This model supplies current for field excitation of a dynamic loud speaker. Samson PAM-17 $125 None None Two-stage transformer-coupled. 227 in first stage and 210's in push pull in output stage. Rectifier is one 281 tube. This model supplies current to the field winding of a dynamic loud speaker. Samson PAM-16 $125 None None Same as pam-17 except that no provision is made for supplying the field of a dynamic loud speaker. Samson PAC-2 $175 45, 90, 135 1.5, 2.5 Two-stage transformer-coupled. 227 in first stage and 210's in push pull in output stage. Rectifier is one 281 tube. An 874 glow tube is used. This model also supplies a — 4.5 volt C bias. National 8110 $85 45, 67 None Three-stage resistance-coupled. (adjustable) Uses one 210 power tube in output stage. Rectifier is one 281 tube. One 201a and one 240 are used in the first two stages. National 8050 $75 45, 67 None Same as type 8110 except that (adjustable) the output tube is a 250 and the rectifier is a type 280 tube. National PushPull Am $40 None None This unit is a two-stage transformer-coupled push-pull amplifier using type 210 or 250 plifier tubes in the output. It does not contain a power supply and must therefore be used with a separate unit such as the National Type 250. American Trans 2-AP $60 None None Twostage transformer-coupled. former Co. Either a227-or20lA-type tube in the first stage and 171a or 210-type tubes in the output stage which is push-pull. It does not contain a power supply