The AA8V 6x2 Superheterodyne Receiver
by Greg Latta, AA8V

IF Amplifier Schematic Diagram and Circuit Description

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Schematic Diagram and Circuit Descriptions Page

Introduction:
The IF amplifier takes the signal from the crystal filter and amplifies it by an amount determined by the setting of the IF gain control. Signal levels can vary over a very wide range. With very strong signals, the IF amplifier may not even be needed, but for weak QRP signals a substantial amount of gain is required. To handle this wide range of signal levels, the IF amplifier gain must be smoothly adjustable over an equally wide range.

In audio amplifiers, simple voltage dividers (potentiometers or "volume controls") are used to control the gain, but these can't be used at RF because of capacitive leakage around them. For RF voltage amplifiers, the usual method is to use a vacuum tube known as a remote cutoff pentode. The gain of such a tube is controlled by varying the grid bias on the tube. More advanced receivers automatically control the grid bias to provide automatic gain control (AGC) but in a simpler receiver such as the 6x2 the gain is controlled manually.

The standard tube used in IF amplifiers is the 12BA6. The 12BA6 was used in the popular "All American Five" AM table radios of the 50s and 60s. The 6x2 power transformer has only a 6.3V filament winding, so the 6BA6 (the 6V version) was used instead. The IF gain control varies the bias on the tube so that the gain of the IF amplifier can be varied from about 6dB to 31dB. This is plenty of gain for even the weakest of signals. However, very strong signals can overload the receiver, even with the IF gain at a minimum. In such situations, the input network can be detuned to prevent overload.

IF Amplifier Circuit
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IF Amplifier Circuit:

Input from Crystal Filter: Input to the IF amplifier is from the crystal filter. The crystal filter is placed at the input of the IF amplifier rather than the output to avoid amplifying unwanted signals that could otherwise cause overload.

Grid Resistors: Grid bias for the 6U8A is developed across the cathode bias resistor and gain control circuit. The bias passes through the grid resistors to get to the grid of the tube. A single resistor is not used because the grid resistors also function as the output load for the crystal. Since the RF load for the crystal must be 68k, a capacitor shorts out the 470k resistor for RF, allowing the 68k resistor to function as the RF load for the crystal filter. The 470k resistor provides the additional DC resistance needed for the bias on the tube.

Crystal Filter Load Bypass Capacitor: A single grid resistor cannot be used because the RF load for the crystal must be 68k, yet the DC grid resistance must be much higher, around 500k. The 0.001 bypass capacitor across the 470k resistor shorts it out for RF, allowing the 68k resistor to function as the RF load for the crystal filter. However, the capacitor has no effect at DC, so the 470k resistor provides the additional DC resistance needed for proper operation of the tube.

Cathode Bias Resistor: The 68 ohm cathode resistor provides for a minimum bias on the tube and controls the maximum gain of the stage. The voltage drop developed across this resistor and the IF gain control is the operating bias on the tube. When the IF gain control is set for maximum gain (fully CW), only the 68 ohm resistor is in the circuit, the bias is a minimum, and the gain is a maximum.

Cathode Bypass Capacitor: The cathode must be kept at ground potential for RF, otherwise the gain of the tube will be reduced due to negative feedback. The cathode bypass capacitor shunts RF around the cathode bias resistor and IF gain control keeping the cathode at ground potential for RF.

IF Gain Control And Dropping Resistor: Effective gain control is important in a receiver. The gain must be variable to permit reception of weak signals, yet prevent overload on strong signals. In the 6x2, a remote cutoff pentode, the 6BA6, is used in the IF stage. The gain of a remote cutoff pentode is controlled by the bias on the tube. Higher bias reduces the gain of the tube. The IF gain control and dropping resistor are used to control the bias on the tube. Maximum gain is controlled by the 68 ohm cathode bias resistor. When the IF gain control is fully clockwise, the wiper is at the grounded end of the control and only the 68 ohm cathode bias resistor is in the circuit. This produces minimum bias, and a maximum gain of 31db. When the control is fully CCW, the wiper is at the end connected to the 330k resistor. Current through the tube and the 330k resistor then passes through the IF gain control resistance, developing a much larger voltage drop. This reduces the gain to approximately 6db. The 330k resistor controls the minimum gain. Without the 330k resistor, the maximum voltage drop across the IF gain control would be insufficient to reduce the gain to the desired amount. The 330k resistor increases the maximum voltage drop across the IF gain control resistance. Decreasing the value of the 330k increases the maximum voltage drop, lowering the minimum gain. Increasing the value of the 330k resistor decreases the maximum voltage drop, increasing the minimum gain.

IF Gain Control Decoupling Capacitor: Current flowing through the IF gain control and 330k dropping resistor affects the gain of the IF amplifier. Because it is connected to the B+ supply, rapid changes changes in the B+ supply caused by other circuits connected to the B+ can affect the IF gain, setting up an undesired feedback loop known as "motorboating". The IF decoupling capacitor smooths out such changes and prevents the feedback.

Mute Circuit: In normal operation, one end of the IF gain control is grounded through the mute switch or external mute jack. If the mute switch and external mute jack are open, a 180k resistor is connected in series with the IF gain control. This greatly increases the bias on the tube, cutting it off, and muting the receiver. For normal mute operation, the mute switch is open and the external mute jack is connected to a normally closed set of contacts on the transmit/receive relay. When the relay is switch to transmit, the contacts open, muting the receiver.

6BA6 Vacuum Tube: The 6BA6 vacuum tube is a remote cutoff pentode that is often used in IF amplifiers. By varying the bias on the tube with the IF Gain Control the gain can be smoothly controlled. The 12V version of the 6BA6, the 12BA6, was used in the popular "`All American Five" AM receivers. You can click on this link for a 6BA6 data sheet.

Output Resonant Circuit: The output of the IF amplifier appears across a parallel resonant circuit. Coil L7 is adjusted for maximum signal during receiver alignment.

Plate Dropping/Isolation Resistor: The plate of the IF amplifier is powered from the B+ supply. To isolate the IF amplifier from variations in the B+ supply and prevent undesired feedback through the B+ supply, a 2200 ohm resistor is connected between the resonant circuit and the B+ supply. The resistor does drop the voltage slightly, but its main purpose is to provide isolation.

Screen Bypass Capacitor: The screen of the IF amplifier must be kept at ground potential for RF. A 0.01uf capacitor allows RF to pass through to ground, while blocking the passage of the DC screen voltage

Screen Dropping Resistor: Proper screen voltage is obtained by powering the screen through a voltage dropping resistor connected to the B+ supply. The resistance is selected so that proper screen voltage is obtained when the tube draws normal screen current.

Plate Bypass Capacitor: The B+ end of the resonant circuit must be connected to ground for RF. A 0.01uf capacitor allows RF to pass to ground, while blocking the flow of any DC.

Plate Decoupling Capacitor: The plate of the IF amplifier is powered from the B+ supply. To isolate the IF amplifier from variations in the B+ supply and prevent undesired feedback through the B+ supply, a 12uf capacitor is connected to ground to smooth out any variations that may occur. Without the capacitor, a type of feedback called "motorboating" can occur.

B+: The IF amplifier is powered from the B+ supply, which is nominally 250V. The IF amplifier is decoupled and bypassed to isolate it from other circuits that are powered by the B+ supply.

Output Coupling Capacitor: Output of the IF amplifier is fed to the detector stage through a 100pf coupling capacitor. The capacitor allows the RF to pass through, while blocking the flow of DC.

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