The Johnson Viking Ranger
by Greg Latta, AA8V

Clamper Circuit Schematic Diagram and Circuit Description

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Introduction:
The clamper circuit protects the final amplifier tube when a signal is not present at the input to the amplifier, for example, during key up periods. The circuit samples the bias on the final amplifier tube, and if the bias is absent it lowers the screen voltage on the final amplifier to a safe value, protecting the screen grid and plate of the final amplifier tube.

Clamper Circuit
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Clamper Circuit:

Input From Final Amplifier Grid: The grid of the final amplifier tube is connected through an RF choke to the grid leak resistor R37. Grid current through the grid leak resistor develops grid leak bias, which is also applied to the grid of the clamper tube. When the grid leak bias is present, the clamper tube is cutoff and draws no additional current through the final screen dropping resistor.

Grid Leak Resistor: When a signal is present on the grid of the final amplifier, 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 R37, providing operating bias for the tube. The value of the resistor must equal the desired operating bias divided by the grid current of the tube, as obtained from the tube data sheets. For the 6146A, nominal bias is -66V and nominal grid current is 2.5mA. Thus, the resistor should have a value of 66V/2.5mA=26.4kohms. The closest standard value available is 27kohms.

Grid Metering Resistor: The grid current of the final amplifier tube V5 flows through metering resistor SH3, creating a voltage drop across the resistor. When the front panel meter is set to the "Grid" position this voltage drop is read and is used by the meter to indicate the final amplifier grid current.

Connection To Operate Switch: One section of the "Operate" switch is connected to the plate of the clamper tube. When the "Operate" switch is placed in the "TUNE" or "STDBY" modes, the clamper tube plate and screen grid of the final amplifier are grounded, disabling the final amplifier.

Connection To Final Amplifier Screen Grid: The plate of the clamper tube is connected to the screen grid of the final amplifier tube. When grid leak bias is present on the final amplifier, indicating that a signal is present, the clamper tube is cutoff and has no effect. However, if grid leak bias is absent, the clamper tube turns on and pulls extra current through the final amplifier screen dropping resistor, increasing the voltage drop across the resistor. This lowers the screen voltage far below its normal value and protects the final amplifier tube from excessive current.

6AQ5 Tube: The clamper tube V6 is either cutoff or fully on. When turned on it must be able to draw enough current through the final screen dropping resistor to lower the final screen voltage to a safe value during zero signal conditions. The plate dissipation of the tube must be high enough to handle the heat produced when the tube is turned on. The 6AQ5 is an inexpensive 7-pin beam power tube with a plate dissipation of 12 watts. It can easily handle the necessary current and heat when turned on and its relatively high transconductance means that it can be easily cutoff. It is ideal for this application. You can click here for a 6AQ5 data sheet.

Connection To Final Amplifier Plate: The final amplifier plate voltage is obtained from the high B+ power supply. This connection has no bearing on the operation of the clamper circuit and is shown here only for completeness.

Final Amplifier Screen Dropping Resistor: The screen voltage for the 6146 is obtained from the high B+ supply through dropping resistor R15. This resistor is chosen so that the screen voltage is the proper value when the final amplifier tube is drawing rated screen current. The value of this resistor is fairly critical, since the screen voltage has a big effect on the operation of the amplifier. When a signal is absent from the input of the final amplifier, as during key up periods in CW mode, the clamper tube turns on and pulls extra current through the dropping resistor, increasing the voltage drop across the resistor, lowering the screen voltage far below its normal value. This protects the final amplifier tube from excessive current. This resistor must have a power rating sufficient to take the entire high B+ supply since it is connected directly across the high B+ supply when the Operate Switch is in "Standby" mode.

Clamper Screen Dropping Resistor: The screen voltage for the 6AQ5 could be obtained from the high B+ supply, but because of the smaller current requirement of the clamper tube the current is obtained from a tap on modulator screen resistor R35 through dropping resistor R38. R38 is chosen so that the clamper screen voltage is the proper value when the clamper tube is drawing rated current. R38 actually consists of three 33kohm resistors in parallel for an equivalent resistance of 11kohms. Obtaining the clamper screen current from the tap on resistor R35 also has the advantage of dropping the screen voltage on the modulator tubes during key up conditions. The extra current drawn by the clamper screen increases the voltage drop through R35, lowering the voltage on the modulator screens. Lowering the modulator screen voltage during key up conditions lowers the standby plate current through the modulator tubes, decreasing the standby dissipation of the modulator tubes.

Clamper Screen Bypass Capacitor: The screen grid of the clamper tube must be kept at ground potential for RF. The screen bypass capacitor C85 short circuits any RF on the screen grid to ground while preventing the screen DC supply from being short circuited.

Connection To R35 Screen Tap: The screen voltage for the 6AQ5 could be obtained from the high B+ supply, but because of the smaller current requirement of the clamper tube the current is obtained from a tap on modulator screen resistor R35. Obtaining the clamper screen current from the tap on resistor R35 also has the advantage of dropping the screen voltage on the modulator tubes during key up conditions. The extra current drawn by the clamper screen increases the voltage drop through R35, lowering the voltage on the modulator screens. Lowering the modulator screen voltage during key up conditions lowers the standby plate current through the modulator tubes, decreasing the standby dissipation of the modulator tubes

Connection To High B+: The high B+ power supply provides the plate voltages for the final amplifier and the modulator tubes. The screen voltage for the final amplifier tube and is obtained from the high B+ through final amplifier screen dropping resistor R15. The screen voltages for the modulator tubes and the clamper tube are obtained from the high B+ supply through voltage divider/bleeder resistor R35.

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