Designing 6922 Cathode Follower

These tutorials will provide some guidelines for designing a high quality tube amplifier based on 6922 tubes. Although the tutorials are using 6922 calculations, using the same formulas may assist you in designing an amplifier based on other triodes.

Let's design a load line of a cathode follower based on 6922 tube. Cathode follower has a voltage gain of slightly less than 1, a low output resistance of about 1 KOhm, a high output resistance of about 500 MOhm, and is a non-inverting circuit.

1. Designing the Gain Stage

The first two design considerations are the 6922's the plate current and plate voltage. The plate current is selected at 8.5mA and plate voltage is 250V. The load line of the 6922 (blue line on the graph below) is defined by two points - the plate current of 8.5mA and the plate voltage of 250V. From the 6922's average plate characteristics graph below, select a linear region as the idle operation point. The point of intersection of the curve representing -2V grid bias line and the load line represents a very linear region. The red dot is depicting the idle operation point of the gain stage. At the selected idle operating point the grid voltage is -2V, the plate voltage is about 85V, and the plate current is at 5.5mA. This voltage limit between heater and cathode has to be taken into consideration. We select the cathode voltage to be at 90V so that the 150V limit is not exceeded. 

With these numbers we can calculate the resistors values in the schematics by using Ohm's law. Let's start with plate resistor. Assuming that the 6922 is open and there is current flow through the tube, all plate voltage will drop across the plate resistor and none across the tube. Using V = IR, we have:

250V = 8.5mA x Rplate Rplate = 250V / 8.5mA = 30KOhm

Now let's calculate cathode resistor. Since the grid voltage has to be at -2V below cathode's voltage, and the grid is grounded via a 1MOhm resistor, the cathode's voltage has to be at 2V above the ground. For idle current of 5.5mA, the cathode's resistor value is:

Rcathode = R3 = 2V / 5.5mA = 363 Ohm

The gain of the Gain Stage is calculated according to the formula:

Gain = (Tube Gain x Rplate)/(Rplate + Plate Resistance + ((Tube Gain +1) * Rcathode)).

The 6922 has plate resistance of 3 KOhm. The 6922 has gain of 33. Plugging these numbers in to the formula above gives Gain Stage gain of 22. 

2. Designing the Cathode Follower Stage Load

When designing the Cathode Follower Stage, keep in mind that the 6922's maximum difference between cathode voltage to heater voltage is limited to 150V. This voltage limit between cathode and heater has to be taken into consideration. We select the cathode voltage to be 90V which is significantly below the 150V limit. A steeper load line will be chosen for the cathode follower stage in order to obtain higher output current.

Rcathode = 250V = 25mA x Rcathode Rcathode = 250V / 25mA = 10KOhm.

Cathode bias resistor is calculated according to the formula:

Rcb = R8 = Bias voltage / Idle current Rcb = 4V / 8.5mA = 470 ohm

At the idle point, 6922 will lose about 160V as seen from the graph and the cathode voltage at the idle point will be equal to 250V - 160V = 90V. The grid potential will be 4V below the cathode or 86V.