Cathode Follower

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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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