Radio Broadcast (May 1928-Apr 1929)

JULY, 1928 KAD1U tiKUAJJUAbl 15 Radio Broadcast's Service Data Sheets on Manufactured Receivers Al* C* Six THE Grebe A C Six is a six-tube receiver designed for a.c. light socket operation. By light socket operation is meant the use of a.c. tubes and the use of A B and C power units. The receiver employs three stages of tuned radiofrequency amplification, a non-regenerative detector and two stages of transformer-coupled audiofrequency amplification. In many respects, particularly in the tuning system, this receiver is very similar to the design of the standard Synchrophase receiver. The antenna circuit is arranged for use with two lengths of antennas. The long antenna connection includes in series a capacity of .000225 mfds. The short antenna connection leads directly to1 the coil proper. The secondary circuit of the antenna coupler, that is, the input circuit of the first radiofrequency tube, is arranged for a tapped inductor permitting variation of the inductance tuned by the first radio-frequency condenser. In this manner it is possible to compensate the effects of various lengths of antennas and still permit accurate operation of the ganged condensers, which provide single tuning control. The tuning system employed in the other stages is conventional and employs the peculiar method used by the engineers of the Grebe organization. Type 326 a.c. tubes are employed for the three radio-frequency stages and the first audio-frequency amplifier. A 327 type tube is employed as the detector and a 171 as the output audio tube. A novel "local" and "distance" switch is incorporated into the set. This shunts a resistance of 1 70 ohms across the plate coil of the second radio-frequency transformer, thus broadening the tuning and permitting better sideband response, with consequent improvement in tone quality. For distant reception this fixed resistance is disconnected from the circuit with accompanying increase of sensitivity and selectivity. The electrical balance in the filament circuit of the four 326s is obtained by means of a variable midtapped resistance across the filament circuit. All the grid return leads in the receiver with the exception of those which convey the bias voltages are connected to ground) and the midtap of this (resistance is also connected to ground. Its value is 27 ohms. The volume control employed in the receiver is a variable resistance of 2500 ohms placed in shunt with the primary winding of the fourth tuned radio THE RECEIVER IN ITS CABINET HOW THE PARTS ARE ARRANGED frequency transformer. By manipulating this resistance it is possible to vary the amount of radiofrequency voltage generated across this coil and transferred to the secondary, thence onto the grid of the detector tube. By employing this arrangement, the filaments of the various tubes are not disturbed and annoyance from "hum" due to an unbalanced filament circuit is avoided. The power pack consists of two separate transformers, one to supply the filament voltages and the other to supply the plate voltages of the tubes by means of the rectifier tube. The electrical balance of the various filament circuits, of which there are three, a 1.5 for the 326s, a 2.25-volt for the 227, and a 5-volt winding for the 171, is obtained by the use of a midtapped resistance across each filament circuit. Particular notice should be made of the fact that the 327 is now rated at 2.25 volts rather than 2.5 volts, the previous rating. This permits much longer life. The B and C supply is obtained from a fullwave rectifying system employing a 380 tube with a single section " brute force" filter. The condensers employed in the filter are of 4 and 5 mfd. each. The voltage dividing resistance consists of six sections. One resistance E of 750 ohms reduces the eliminator voltage to the required 180 volts. Another resistance F of 2800 ohms reduces the voltage to 100 for the first audio and the three radio-frequency tube plates. Another resistance G of 5900 ohms reduces the voltage to the prescribed 45 volts. The 4.5 volts negative C bias applied to the grids of the three radiofrequency and the first audio tube is obtained by means of the voltage drop across the resistance I of 95 ohms. The gridlbias for the 171 is obtained by means of the additional drop across the resistance J of 695 ohms. The receiver is a single tuning control affair with an antenna compensating control. The primary power circuit is arranged for 110 or 120 volt 60 cycle a.c. supply. The "on" and "off" switch controlling the operation of the complete receiver is located on the face of the panel, between the tuning and the volume control drums. The tuning dial is graduated in kilocycles. No method of coupling the speaker to the output tube is provided, that is to say, binding posts are provided but one should; not permit the current of 20 mils to flow through the speaker. Any transformer designed for use with the 171 or any chokecondenser combination of suitable electrical values will be sufficient. The power unit is contained within the cabinet which houses the receiver proper, but all units inclusive of the rectifier tube are contained within shields. CIRCUIT OF THE GREBE A. C. SIX. THE POWER UNIT IS INCLUDED