Radio Broadcast (May 1928-Apr 1929)

22 RADIO BROADCAST AUGUST, 1928 August, 1928. Radio Broadcast's Service Data Sheets on Manufactured Receivers The Federal Ortho-sonic Seven-Tube Receiver THIS set is a good example of a receiver designed for a.c. operation using standard 201A type tubes in a series filament arrangement and supplied with filament current from a rectifier filter system employing a Raytheon type BA highcurrent rectifier. The seven tubes in the receiver are arranged as follows: four of them are used as radio-frequency amplifiers, one is used as the detector and the other two are used asj audio amplifiers. A 171 A type tube is used as the power tube. The order of tubes in the series filament arrangement is: First r.f., second r.f., third r.f., fourth r.f., first a.f., detector, and second a.f. In an efficient four-stage radio-frequency amplifier careful design is essential if the receiver is to be stable over the entire broadcast band. To promote stability in this receiver all the r.f. stages are completely shielded, radio-frequency choke coils are placed in each leg of the filament of each of the tubes used in the r.f. amplifier, each plate supply lead to the r.f. tubes is filtered by the use of resistances, and bypass condensers, and finally each stage is carefully neutralized, the small inductance coils, L, in the filament circuits forming part of the neutralizing system. The five variable tuning condensers are ganged to a single drum dial control, a small vernier condenser being used to obtain fine adjustment of the tuning system. The audio amplifier is of standard design. It is interesting to note that the first audio-frequency tube has about 70 volts applied to its plate and that a 5-volt negative bias for the grid is obtained by connecting the grid return from the transformer to the detector tube filament, so that the 5-volt drop in the filament of this latter tube is impressed on the grid of the first audio tube. Bias for the second audio tube is obtained from the voltage drop across resistance Rs, in the power unit. The cores of the two audio transformers are grounded to prevent any possibility of audio-frequency oscillations due to extraneous coupling between the two transformers. Bypass condensers are also connected at various points in the audio amplifier to prevent common coupling in the power supply. The detector and audio amplifier apparatus is housed in a single compartment, entirely separate from the radio-frequency amplifier. THE RECEIVER IN ITS CONSOLE Only two values of plate voltage are supplied to the receiver from the power unit, 180 volts and 70 volts. The latter is reduced to 45 volts for the operation of the detector by means of a 160,000-ohm resistance connected in series with the detector plate circuit. Volume control is accomplished by varying the potentiometer, P, which controls the amount of voltage applied to the plates of the r.f. tubes. A grid leak condenser type of detector is used. The grid leak has a value of one megohm and the grid con denser has a value of 0.0002 mfd. Because the input capacity of a tube when used as a detector is somewhat less than when it is used as an amplifier, it is necessary to connect a small midget condenser, across the detector input so that the last tuning condenser will gang properly with the other tuning condensers. The output of the detector contains a fixed resistance in series with the plate circuit to keep the r.f. currents out of the audio amplifier. In addition, a bypass condenser is connected between the plate of the detector tube and the negative side of the detector tube filament. The loud speaker is insulated from the direct current in the plate circuit of the 171 A type power tube by means of a choke-condenser combination. The condenser, C2, has a capacity of 4 mfds. The output choke, Li, is located in the power unit. The power unit supplies all the A, B, and C voltages required for the operation of the receiver. The power transformer, T, is arranged with three taps on the primary so that the installation may be adjusted foY low, medium, or high line voltages. The high-voltage secondary of the transformer supplies the Raytheon BA rectifier. The output of the rectifier feeds into the filter system which consists of the usual two filter choke coils, L2 and L.3, and three main filter condensers, C3, Gi, and C5. The output of the filter goes to the resistance bank from which taps are taken for the A, B, and C voltages required for the operation of the receiver. Bypass condensers are not necessary across these resistances, as all the necessary bypassing is placed in the receiver itself. An additional 5-volt winding is placed on the transformer and is used in this model to supply current to the pilot lamp located on the panel of the receiver. An adjustable resistance, Ri, is connected in the primary side of the power transformer so that small variations in line voltage may be compensated. The equipment is also protected by the inclusion of two 5-ampere fuses in the primary circuit. This receiver is designed for operation on a loop type antenna. The advantages of loop operation, i.e., convenience, directional properties, better signal-to-static ratio, are well known. In the model illustrated in the photograph the loop is located on one of the doors of the cabinet. THE CIRCUIT OF THE FEDERAL ORTHO-SONIC RECEIVER