The principle method of tube amplifier to suppress 50 Hz hum
1. The influence of the filament voltage amplifier
There is a capacitor Cf between the grid and the filament of the tube. The AC filament voltage of 50 Hz will cause a leakage current IA between the filament and the grid through Cf. IA flows through the input terminal to cause interference. This interference generated at the input stage Although the voltage is very small, after several stages of amplification, the impact on the amplifier is still very large, it increases the internal interference and causes the amplifier's zero drift.
Suppression method: as shown in Figure D-2, ground the center voltage of the filament voltage to make the voltage at both ends of the filament and the ground voltage symmetrical and equal. Inside the electron tube, there are two capacitors between the filament and the grid. Cf1, the other end is Cf2. In this way, the center tap of the AC filament voltage is grounded, and the two ends of the ground potential are exactly inverted, so the currents flowing through Cf1 and Cf2 also exactly cancel each other out, thereby eliminating the impact on the amplifier.
Another method is to ground the sliding arm of a potentiometer at both ends of the filament, and adjust the sliding arm 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 capacitance, another reason is that the filament of the electron tube emits electrons when it is hot. These electrons are absorbed by the cathode or grid to form an interference current. Drop, thereby forming AC interference.
Suppression method: In order to eliminate this AC interference, not only the center tap of the filament winding should be grounded, but also the DC potential of the filament winding to ground should be increased so that the filament potential is higher than the cathode and grid potentials as shown in Figure D-4 The electrons are emitted, and the electrons will not reach the cathode or the grid. How to achieve it? We can use two resistors and a capacitor to form a voltage divider circuit in the anode high voltage of the electron tube. This voltage can also be taken from the cathode resistance of the power tube (the cathode of the power tube often has a DC potential of about 10 volts to ground).
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