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JULY, 1928
A COMPLETE A-B-OPOWER UNIT
147
A POWER UNIT
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FIG. 2. HOW THE APPARATUS IS ARRANGED AND WIRED This picture-diagram shows clearly how to place the apparatus on the baseboard and how it should be wired. The power lead from the power transformer used in the B-supply is plugged into the socket on top of the A-power unit. The entire unit is then controlled by the small toggle switch on the
A-power unit
across the C bias end of resistor, as shown in Fig. 1.
Acx-371-A tube is employed in the output stage. Its filament is operated by means of a.c. supplied directly from the Thordarson R-171 transformer. A standard choke coil and condenser is also required for keeping the high voltage d.c. out of the loud speaker. A Thordarson R-196 choke coil together with two Dubilier type 907 i-mfd. condensers make up this arrangement.
To the grid terminal of the power stage tube socket connect a Thordarson R-172 power input plug. This plug is merely inserted in the second audio stage of the receiver proper and in the same manner as a tube; it transfers the first stage audio output directly to the power amplifier in this unit.
The illustrations show just where each component is placed. Fig. 2 combines suggestions on how to dispose the various parts and a wiring diagram in picture form for those who prefer to follow this kind of diagram.
THE LIST OF PARTS
THE following parts were used in the unit assembled by the writer. Parts mechanically and electrically equivalent to those mentioned below may of course be used.
Li, U [Filter Choke Coils.
T < Power Transformer.
C9, Cm (.0. i-Mfd. Buffer Condensers.
All contained in the Thordarson Power Compact R-171.
L3 Output Choke, Thordarson Type 196. Ci /2-Mfd. 600-Volt Filter Condenser. C2 1 2-Mfd. 400-Volt Filter Condenser. Cs <T8-Mfd. 400-Volt Filter Condenser. C4 I i-Mfd. 400-Volt Bypass Condenser. Cs ' i-Mfd. 400-Volt Bypass Condenser.
All contained in the Dubilier Condenser Block Type PL-574.
Ce i-Mfd. 160-Volt Bypass Condenser,
Dubilier Type 907. Rt Voltage-Dividing Resistor, Thordarson
Type R-508-1.
C7, Cs i-Mfd. 160-Volt Bypass Condenser,
Dubilier Type 907. Two Benjamin Sockets.
One Roll Belden Colorubber Hook-up Wire. One Power Input Plug, Thordarson Type
R-172. Five Binding Posts.
One Kit of Parts, or Completely Assembled
Knapp A-Power Unit. One Baseboard 12X17 inches. The following tubes are required: One Raytheon bh Type Rectifier One cx-37 1 -a Power Amplifier
The writer feels that this A-B-C-Power unit and amplifier possesses many excellent features. Its total cost is quite small considering its general utility and large improvement over batteryoperated equipment; further, it is thoroughly dependable in operation and the unit makes it possible for any radio constructor to realize full set electrification of all existing batteryoperated receivers.