The AA8V 6x2 Superheterodyne Receiver
by Greg Latta, AA8V

Voltage Regulator Circuit Schematic Diagram and Circuit Description

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Schematic Diagram and Circuit Descriptions Page

Introduction:
The 6x2 local oscillator, mixer screen grid, and BFO are sensitive to voltage fluctuations in their power source. Another circuit, the crystal calibrator, needs to run off of a voltage lower than the 250V B+ supply. The voltage regulator circuit uses an 0C3 regulator tube to provide a source of regulated voltage at 108V to power these circuits.

Voltage Regulator Circuit Circuit
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Voltage Regulator Circuit:

Voltage Regulator Tube: A 0C3 gaseous regulator tube is used to provide a source of regulated voltage for the crystal calibrator, local oscillator, mixer screen grid, and BFO. Gaseous regulator tubes such as the 0C3 have the property that as long as the current through them is between about 5 mA and 40 mA the voltage across the tube is constant. For the 0C3, the voltage is 108 volts. The solid state equivalent of a gaseous regulator tube is the zener diode. You can click here for an 0C3 tube data sheet.

Regulator Dropping Resistor: The regulator dropping resistor is selected so that the current through the tube is at least equal to the maximum current draw plus about 5 mA. Having the idle current higher is OK, as long as it isn't over 40 mA. (Higher idling current does, however, cause more heating in the regulator tube and power supply.) When the current draw to the load changes, the regulator tube simply draw less current, automatically keeping the voltage across the tube constant at 108 volts.

Input From 250V Plate Supply: The 250V plate supply provides power for the regulator circuit. The 250V supply is well filtered, but the output changes with line voltage fluctuations. These are eliminated by the voltage regulator circuit so they don't affect critical circuits, such as the local oscillator. .

Output To BFO: One of the circuits fed by the voltage regulator circuit is the beat frequency oscillator (BFO). The BFO is fed through the mode switch, so it is activated only in the CW mode. The BFO is very sensitive to plate voltage changes, and the voltage regulator circuit provides a source of regulated voltage to prevent such changes.

Output To Other Circuits: There are other circuits in the 6x2 receiver that are sensitive to supply voltage changes, particularly the local oscillator and the mixer screen grid. Other circuits, such as the crystal calibrator, simply require a lower supply voltage, the the voltage regulator is a convenient source.

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