The Johnson Viking Ranger
by Greg Latta, AA8V

Audio Driver Circuit Schematic Diagram and Circuit Description

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Introduction:
The audio driver is a small power amplifier that amplifies the signal from the second audio amplifier sufficiently to drive the modulator. Triodes are best used for this purpose, and the parallel connected 12AU7 is ideally suited for this purpose. The output of the audio driver is coupled to the modulator via the audio driver transformer.

The audio driver and modulator are connected in a negative feedback loop to lower the distortion of the system. A tertiary audio feedback winding on the modulation transformer feeds some of the modulator output out of phase back to the input of the audio driver. The negative feedback greatly lowers the distortion in the modulator, improves the frequency response, and helps to stabilize the system and prevent spurious oscillations.

Audio Driver Circuit
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Audio Driver Circuit:

Input Coupling Capacitor: Capacitor C57 allows the signal from the second audio amplifier to pass through to the grid of the audio driver tube while blocking the DC on the plate of the second audio amplifier tube. The reactance of this capacitor is small enough even at the lowest audio frequencies that is has no effect on the frequency response of the audio system.

Feedback Control/Grid Resistor: The grid resistor R27 allows the DC bias developed by the cathode bias resistor to pass through to the grid of the tube. It also couples the feedback voltage from the audio feedback winding of the modulation transformer to the grid of the audio driver tube.

Audio Feedback Winding: The tertiary audio feedback winding on the modulation transformer is one of the most important parts of the audio system. This winding picks up the same signal as the modulation transformer secondary, but at a lower voltage. This signal is fed out of phase back into the input of the audio driver. This negative feedback slightly lowers the overall gain of the audio driver/modulator combination, but also cancels out most of the distortion produced by the system, improves the frequency response, and helps to stabilize the system and prevent spurious oscillations. An additional winding on the modulation transformer raises the cost, but the improvement in performance is well worth the expense. It is one of the reasons that the Viking Ranger is such a good sounding transmitter.

Cathode Bypass Capacitor: Without the cathode bypass capacitor, the voltage developed by the cathode bias resistor would fluctuate with the varying plate current and produce negative feedback, lowering the gain of the stage. The cathode bypass capacitor C59A smooths out the variations and provides a steady bias for the tube. Another way to look at it is that the plate/cathode current contains both a DC component and an AC component (the output signal). The bypass capacitor shunts the AC component around the cathode bias resistor so that only the DC component contributes to the cathode bias.

Cathode Bias Resistor: The grid of the tube must be kept negative to operate properly. Plate/cathode current flowing through the cathode bias resistor produces a voltage that is applied through the grid resistor to the grid of the tube. Variations in this voltage are smoothed out by the cathode bypass capacitor to provide a steady grid bias on the tube.

12AU7 Tube: The audio driver is a small power (not voltage) amplifier. The audio driver must generate enough power to properly drive the modulator through the audio driver transformer. Voltage gain is not needed here, but power gain is. Triodes are best suited as modulator drivers because they have a lower plate resistance than tetrodes or pentodes. Among the small, 9-pin, dual triode family (12AX7, 12AU7, etc.) the 12AU7 has the lowest gain and highest plate dissipation (2.75 watts per section). If both sections are run in parallel, the 12AU7 can produce more than enough power to properly drive the modulator. You can click here for a 12AU7A data sheet.

Filament Bypass Capacitor: Capacitor C70 is connected between the hot filament lead and ground to eliminate any stray RF that may have gotten onto the filament supply line. The capacitor prevents the RF from getting into the tube filament and into the audio chain through the cathode to filament capacitance of the audio driver tube.

Audio Driver Transformer: The output of the audio driver tube is fed to the audio driver transformer. The transformer matches the output impedance of the audio driver tube to the input impedance of the modulator. Since the modulator tubes must be driven out of phase, the transformer secondary is center tapped. The center tap is grounded for audio frequencies, and the modulator tube grids are connected to opposite ends of the secondary, which are out of phase with each other.

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