The AA8V 6x2 Superheterodyne Receiver
by Greg Latta, AA8V

B+ Power Supply Circuit Schematic Diagram and Circuit Description

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Schematic Diagram and Circuit Descriptions Page

Introduction:
The B+ power supply uses the standard, full wave center-tapped configuration. A 5Y3-GT (or a pair of 1N1007 silicon diodes, if desired) is used as the full-wave rectifier. The output is smoothed by a pair of filter capacitors and a filter choke. A series dropping resistor drops the output of the supply to 250V.

The 5Y3-GT rectifier filament is powered by a separate 5V AC winding on the power transformer, and the tube filaments and dial lamps are powered by another 6.3V AC winding.

B+ Power Supply Circuit
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B+ Power Supply Circuit:

Power Transformer Secondary: The power transformer is a Chicago Stancor PC-8405. The transformer cost $10.30 as listed in a 1967 Allied parts catalog. The transformer ratings are as follows: Primary: 117V at 60Hz HV Secondary: 540V CT at 120mA Filament Secondary #1: 6.3V at 3.5A Filament Secondary #2: 5V at 3A

5Y3-GT Vacuum Tube: The transformer steps up the 120 volts AC in the primary to 540 volts AC in the secondary. When the top connection to the secondary is positive with respect to the bottom connection and center tap, the upper diode of the 5Y3-GT rectifier tube (pins 4 and 2/8) conducts, allowing current to pass to the output. During this time the bottom diode (pins 6 and 2/8) is cut off. When the bottom of the secondary is positive with respect to the top and center tap the opposite is true: the upper diode is cut off and the bottom diode conducts. The result is a stream of DC pulses at pin 8, the output of the rectifier. The output is then fed to the power supply filter, which consists of two 47uf filter capacitors and a 10H filter choke. The 5 volt secondary provides the 5 volts needed to heat the filament/cathode of the rectifier tube. The 5Y3-GT is one of the most popular full wave rectifiers. With two diodes in a single envelope, and an output capability of about 125mA, it is perfectly suited for use in receivers and other equipment with moderate power requirements. You can click here for a 5Y3-GT tube data sheet .

Filter Capacitors: The output from the 5Y3-GT vacuum tube is direct current (DC) but with a large alternating current (AC) component superimposed. The two 47uf filter capacitors are essentially an open circuit to the DC, but they effectively short circuit the AC component to ground. Any remaining AC component is blocked by the filter choke. The result is that little of the AC component reaches the output.

Filter Choke: The two 47uf filter capacitors remove most of the AC component in the output of the rectifier, but very good filtering is needed for a receiver, so a 10H, 100mA filter choke (Hammond type 158M) is also used. The filter choke allows the DC component to flow through, while offering a very high impedance to the AC component. The result is that very little of the AC component reaches the output.

Voltage Dropping Resistor: The circuits in the 6x2 receiver were designed and tested at 250V. Since the output of the plate supply was above this, a dropping resistor was chosen to bring the voltage back down to 250V.

Tube Filaments: 6.3V tubes are used throughout the 6x2 receiver. The filaments of the 12AX7 first audio tube, which can be wired for either 6.3V or 12.6V operation, are wired in parallel for 6.3V operation. A single 6.3V winding on the transformer supplies the filaments and the dial lamps.

Dial Lamps: The National ICN dial is illuminated by two dial lamps. These operate on 6.3V and are powered by the 6.3V secondary on the power transformer. Either a #47 or #44 lamp can be used. The #44 lamp is brighter.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu