Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST JUNE, 1928 June, 1928, Radio Broadcast's Service Data Sheets on Manufactured Receivers The-Pfansteihl A. C. 34 and 50 CIMPLICITY of design marks the develop^ ment of the Pfanstiehl line of radio receivers. The photograph and wiring diagram shown herewith apply to both the Nos. 34 and 50 a.c. electric receivers. The 34 is the console model, whereas the 50 is the table model of the same receiver. This receiving system employs 6 tubes, apportioned as three stages of tuned radio-frequency amplification, non-regenerative detector, and two stages of transformer-coupled audio amplification. The wiring diagram of the system is shown herewith. As is evident from the drawing, 226 type a.c. lubes are used for the three radio-frequency amplifiers and for the first stage of audio. A 227 type detector is employed and a 171 is the output audio tube, arranged in conventional transformer-coupled fashion. The four 226's are wired in parallel and obtain their filament potential from a 1.5-volt winding on the power transformer. The tuning system used in the tuned radio-frequency stages is conventional, consisting of fixed inductances and variable capacities. The method of stabilization employed makes use of grid resistances, commonly known as grid "suppressors." In order to attain utmost simplicity, only two values of plate voltage are applied to the receiver. The three-radio-frequency stages and the two audio-frequency tubes obtain their plate voltage from the same voltage tap, while there is another tap for the detector. The lirst radio-frequency tube's plate voltage is governed by a potentiometer type of resistance which shunts the plate coil of the first radio-frequency tube. The standard grid-leak condenser system of detection is employed. The audio system is conventional in every way. The volume control is a voltage divider shunting the secondary of the first audiofrequency transformer, with the center tap of this control connected to the grid of the amplifying tube. THE RECEIVER WITH ITS POWER UNIT The secondary of the second audio-frequency transformer is shunted with a fixed capacity. The C bias for the output audio tube is obtained by causing a voltage drop across a resistance in the grid return lead. The receiver is divided into two parts, the radio and audio systems being contained in one can, while the power unit is in another can. The wiring system employed in the power unit is standard. Two transformers are employed. One is the power transformer supplying the filament and plate voltages required for the full-wave filament type rectifying tube, and the other is the filament transformer supplying the 1.5 -, 2.5-, and 5-volt windings for the tube filaments. The primaries of these two transformers are connected in parallel and are designed for a 115volt a.c. line. The filament windings supplying the 1.5 and the 2.5. voltages are equipped with voltage control resistances thus safeguarding the tube filaments in the event of an excessive line surge or increase in line voltage. The |filter system consists of a two-section filter, with a single distributing resistance across the output. The "high" side of this resistance supplies the plate voltage for all tubes other than the detector tube. A tap supplies the detector plate voltage. The electrical balance and the eleccrostutic balance in the filament circuit is obtained by means of midtapped resistances placed in parallel with the tube systems, rather than by tapping the filament voltage winding. The loud speaker coupling to the output audio tube is direct, without any transformer or chokecondenser system. Two output binding posts are provided for the loud speaker terminals. If desired, a loud speaker coupling unit can be added to the receiver. Tuning is accomplished by means of single-dial control, the second knob on the front panel being for volume control. THE PFANSTEIHL MODEL 50 A.C. RECEIVER 5 V. Red CIRCUIT OF THE PFANSTEIHL MODELS 34 AND 50. THE POWER UNIT IS SEPARATE