## Cathode Follower

 Recommended to read: Using Tube Curves - Load Line Calculations Tube CAD Journal - Extrapolation from Plate Curves Tube CAD Journal - The Grounded-Cathode Amplifier The Valve Wizard - Heater / Filament Supplies

## Cathode Follower ( calculate Load & Bias Resistor )

 Plate voltage ( Va ) volts * Plate current ( Ia ) mA * Bias voltage ( Vg1 ) - volts * Idle current ( use Tube datasheet Plate Characteristics ) mA * Cathode load resistor ( Rk ) kOhms Cathode bias resistor ( Rb ) Ohms * = required values

Heater elevation:
It is very important not to exceed the maximum heater-to-cathode voltage. Since the cathode is at a high voltage it is often necessary to elevate the heater supply to ensure safe operation and long valve life.

More about Heater / Filament Supplies

 The design of the cathode follower is similar to the gain stage. I have chosen a steeper loadline as I want to have the cathode to run at higher output current. Cathode load resistor ( Rk ) = 300V / 3mA = 100k ohm. Cathode bias resistor ( Rb ) = Bias voltage / Idle current = 1V/1.35mA = 740 ohm At idle point, the tube will drop about 163 V as seen from the graph. So the cathode idle voltage is equal to 300V - 163V = 137 V. The grid will be at 1 V below the cathode so it will be running at 136 V.

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