Principle method for suppressing 50 Hz hum by tube amplifier

First, the influence of the filament voltage amplifier

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There is a capacitance Cf between the grid gate and the filament. The AC filament voltage of 50 Hz will cause leakage current IA between the filament and the gate through Cf. IA flows through the input terminal and causes interference. This interference occurs in the input stage. Although the voltage is very small, after several stages of amplification, the effect on the amplifier is still very large, which increases the internal interference and causes the zero drift of the amplifier.
The method of suppression: as shown in Figure D--2, ground the filament voltage center to make the output of the filament at both ends and the voltage of the ground symmetrically equal. Inside the tube, there are two capacitors between the filament and the gate, and one end of the filament is Cf1, the other end is Cf2. Thus, since the center tap of the AC filament voltage is grounded, the ground potential at both ends is exactly inverted, so the current flowing through Cf1 and Cf2 also cancels each other, thereby eliminating the influence on the amplifier.
Another method is to ground the wiper arm at both ends of the filament and a potentiometer, and adjust the slider of the potentiometer to make IA1 and IA2 equal. The principle is the same as described above.

Second, the influence of the filament on the amplifier.

In addition to the interference caused by the two-pole capacitor, another reason is that the filament of the tube emits electrons in the hot state. These electrons are absorbed by the cathode or the gate to form an interference current, which will generate a voltage drop on the RK and a pressure on the RG. Drop, resulting in AC interference.
Suppression method: In order to eliminate this kind of AC interference, not only the center winding of the filament winding is grounded, but also the DC potential of the filament winding to the ground is raised, so that the filament potential is higher than the cathode and the gate potential as shown in Figure D--4, so that even if the filament has The electrons are emitted and the electrons do not reach the cathode or the gate. How to achieve it? We can do this by using a resistor and a capacitor to form a voltage divider circuit in the anode high voltage of the tube. It is also possible to take this voltage on the cathode resistance of the power tube (the cathode of the power tube tends to have a DC potential of about 10 volts to ground potential).

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