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 Vg. The negative grid bias -Vg 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 –Vg with respect to ground. The grid receives the -Vg bias voltage using a resistor Rl. Given that no current goes through the grid, in normal operations, the resistor Rl does not affect the voltage seen by the grid. The cathode is also connected to ground so that the grid is at –Vg with respect to the cathode, as needed.

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

The capacitor Cd, from the grid leak resistor to ground, decouples the residual input signal, that traverses Rl, 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 Rl, 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 Cdforms, with the resistor Rl, 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:

image022.

image023
Figure 11: Fixed bias schema.
Fixed bias can be obtained by using two separate power supplies. PS1 produce the high-tension V+ to be given to the vacuum tube anode. PS2 produces the voltage Vg for the grid. The negative voltage -Vg is obtained by connecting the positive of PS2 to ground. Also the negative of PS1 is connected to ground. In this way, the negative of PS1 and the positive of PS2 are at ground voltage, the negative of PS2 is at -Vg and is connected to the grid.

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