The Johnson Viking Ranger
by Greg Latta, AA8V

Buffer Schematic Diagram and Circuit Description

Click here for a higher resolution (larger) schematic.

This circuit can function as either a buffer amplifier or as a crystal oscillator,
depending on the setting of the Crystal/VFO switch. This page discusses the operation as a buffer amplifier.

Click Here To Read About The Circuit As A Crystal Oscillator

Back to the Johnson Viking Ranger Schematic Diagrams And Circuit Descriptions Page

Introduction:
When the internal VFO is used it must be amplified before reaching the buffer/multiplier stage. When the Crystal/VFO switch is set to the VFO or ZERO position the crystal oscillator/buffer stage is configured as a buffer amplifier by disconnecting the grid coupling capacitor and shorting out the cathode RF choke and cathode feedback capacitor. In the schematic diagrams on this page, it is assumed that the Crystal/VFO switch is in the VFO or ZERO position.

Buffer Circuit
Click On A Section of the Schematic
Below for Information on That Part of the Circuit:

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Buffer Circuit:

Input Coupling Capacitor: The signal from the VFO passes to the grid of the buffer stage through the input coupling capacitor. This capacitor also functions as the plate coupling capacitor for the VFO stage. The capacitor allows the RF to flow through while blocking the DC on the plate of the VFO tube.

Grid Leak Resistor: When a signal is present on the grid of the buffer stage, some of the RF is rectified by the diode action of the grid and cathode. This causes a voltage to develope across the grid leak resistor (R7 and R44 in series), providing operating bias for the tube. Resistor R7 also allows the grid-block keying voltage to reach the grid of the tube while keeping the RF on the grid from flowing back to the keying circuit.

Grid Leak Capacitor: The .005uf grid leak capacitor C87 smooths out any voltage variations across the grid leak resistor keeping the tube bias steady. This capacitor also bypasses any RF leaking through the resistor to ground, preventing it from getting to the keying circuit.

Keying Resistors: Grid block keying is used in the Johnson Viking Ranger. While the key is up the keying circuit applies a negative voltage across resistors R45 and R44 which function as a voltage divider, and also isolate the buffer somewhat from the output of the keying circuit. About 3/4 of the applied voltage is passed on through the grid leak resistor to the grid of the tube, shutting it off. When the key is pressed, the blocking voltage is removed and the circuit begins amplifying.

6CL6 Tube: In 1950, an important article in QST magazine, "Crystal-Controlled Oscillators, A Review of Modern Crystals, Circuits and Tubes" (QST, March 1950, C. Vernon Chambers, W1JEQ) addressed several points concerning crystal oscillators, including which tube to use. In that article, various electron-coupled circuits were tried along with a variety of tubes: the 6AG7, 6F6, 6V6GT, and 6L6. Among the many conclusions in the article, one came through loud and clear, which I quote here: "Of the four tubes tested the 6AG7 is by far the best from every standpoint." As a result of that article, virtually all crystal oscillator circuits in the ARRL handbook for the next 15 years featured or recommended the use of the 6AG7. The 6CL6 is the 9-pin miniature equivalent of the 6AG7, so it will give exactly the same performance as the 6AG7 in a smaller package. You can click here for a 6CL6 data sheet. Though the 6CL6 was selected for its performance as a crystal oscillator, it also functions very well as a buffer amplifier.

Meter Bypass Capacitor: The meter bypass capacitor prevents any RF that may be present from getting through to the front panel meter, causing erroneous readings.

Screen Bypass Capacitor: The screen grid of the tube must be kept at ground potential for RF. The screen bypass capacitor short circuits any RF on the screen grid to ground while preventing the screen DC supply from being short circuited.

Output Tank Circuit: The output of V3 is fed to a tank circuit controlled by the band switch. The bandswitch switches in and out R9 and various portions of L5, the tank coil. Resistor R10 loads the tank coil and lowers the overall Q of the circuit. The effect is to select the proper output frequency without requiring an additional tuning control on the front of the transmitter.

Band Switch: The band switch shorts out various resistors and coil sections in the output tank circuit to select the correct output frequency from the oscillator and to keep the output as uniform as possible.

Plate Bypass Capacitor: The plate bypass capacitor grounds the bottom of the output tank circuit for RF while blocking the DC plate voltage on the tube. It also prevents any RF from reaching the low B+ supply.

Plate Coupling Capacitor: RF appearing at the plate must be fed to the next stage for amplification. However, the DC plate voltage must not be allowed to pass through to the next stage. The plate coupling capacitor allows the RF to pass through while blocking the DC.

Filament Bypass Capacitor: When used as a crystal oscillator , the cathode of the 6CL6 is not grounded for RF. Even though there is no direct connection between the filament and the cathode, there is a capacitive connection, and it is possible for RF to get on the filament through the filament/cathode capacitance. Filament bypass capacitor C25 shorts any of this residual RF to ground making sure that none of it escapes.

Screen Dropping Resistor: Proper screen voltage must be applied to the buffer amplifier. The correct voltage for the screen of the tube is obtained by using a dropping resistor connected to the low B+ supply. The value of the resistor is chosen so that the screen voltage is correct when the tube is drawing normal screen current.

Metering Resistor: When the "Crystal/VFO" switch is set to VFO or ZERO both the oscillator (V2) and buffer (V3) cathode currents flow through the metering resistor, creating a voltage drop across the resistor. (The connection to V2 is not shown in the schematic diagram above.) When the front panel meter is set to the "Osc" position this voltage drop is read and is used by the meter to indicate oscillator/buffer cathode current. When using crystal control, the cathode of V2, the VFO oscillator, is disconnected, and the meter indicates only the crystal oscillator (V3) current.

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