3.6.1    Fixed bias for vacuum tubes

The fixed bias for vacuum tubes schema is given in Figure 11. Two different power supplies are used. PS₁ gives the high-tension V+ to the anode of the vacuum tube, through the load. The negative of PS₁ is connected to ground. PS₂ produces the needed grid bias voltage V_g. The negative grid bias -V_g is obtained by connecting the positive of PS₂ to ground. In this way, the positive of PS₂ is at ground level and the negative is at –V_g with respect to ground. The grid receives the -V_g bias voltage using a resistor R_l. Given that no current goes through the grid, in normal operations, the resistor R_l does not affect the voltage seen by the grid. The cathode is also connected to ground so that the grid is at –V_g with respect to the cathode, as needed.

The usage of resistor R_l will be better discussed in Section 4.1.1. For the moment, we just mention that one of the purposes of R_l (also called the grid leak) is to provide the input signal, received from previous stage, with a high impedance path to ground.

The capacitor C_d, from the grid leak resistor to ground, decouples the residual input signal, that traverses R_l, from the bias voltage supply. Consider that, generally the bias power supply provides bias voltage to several vacuum tubes in the amplifier. For instance, in a stereo amplifier, both left and right channels are sometimes biased by the same power supply. The residual input signal, which traverses R_l, is added to the bias voltage and goes also to the other channels, where it is amplified by the other vacuum tubes, creating problems of cross-talk. In order to avoid that, the capacitor C_dforms, with the resistor R_l, a low-pass filter that shorts to ground the residual input signal. The value of this capacitor should be large, so that even very low frequencies are shorted to ground and do not go to the grid of the other vacuum tubes.

Example 3: Determining the decoupling capacitor for fixed bias Suppose, for simplicity, that Rl is the only resistance seen by the capacitor Cd, and suppose its value is 200K Ohm. In order to have a low-pass filter with a very low cut-off frequency, for instance 1 Hz, we use the low-pass filter equation and we obtain the following capacitance: .