Filtering for the Voltage Supplies

Author: J.B. Hoag

When separate A, B, and C batteries are used for each of the tubes in a multistage amplifier, the fluctuating and the direct currents both flow in the same circuits. It is difficult to design the same circuit for efficient handling of two different kinds of current. For one thing, the a.c. would have to flow far away from the tubes into the batteries and back again, much energy would be lost in the internal resistances of the batteries, the stray electric and magnetic fields of these currents would induce voltages in other parts of the circuit, and the capacities of the batteries to the ground would tend to shunt away some of the alternating currents.

Fig. 13 K. Bypass condensers C1 and C2 and decoupling resistor R2 are used to separate or " filter " the a.c. from the d.c.

Figure 13 K shows how partially to overcome these difficulties by the use of suitably located bypass condensers: C₁ across the C-bias resistor, and C₂ across the plate battery.

It would be uneconomical to use separate B-batteries or plate-voltage supply systems for each tube in a multistage amplifier; nor is it necessary except in certain special cases.

Fig. 13 L. The same B-voltage supply unit may be used in common for several tubes, provided the circuits are carefully filtered

Figure 13 L shows a filtered circuit wherein the same B-voltage is used with two transformer-coupled stages.

It will be noticed in Figs. 13 K and 13 L, that additional high resistances R₂ and R₃ (called " de-coupling resistors ") are used in series with the supply line. These assist the bypass condensers to keep the a.c. out of the d.c. supply circuits and also to drop the B-voltage to the correct value for each tube.