DC tube amplifier working in Class B

This article introduces a push-pull power amplifier made of DC tubes. Its characteristic is that the power amplification stage works in a Class B amplification state, with low static power consumption, high efficiency, power saving, and high output power. The circuit is shown in the attached picture.

Vacuum tubes V2 and V3 form a push-pull power amplification stage, and v1 is a push stage. When there is no signal input, the V2 and V3 curtain grid voltages are 0, and the screen electrode is far away from the cathode, so the electrons emitted by the cathode have little attraction, and the number of electrons flying to the screen electrode is very small, so the screen electrode is always The current is ≤1mA, and the total screen consumption of the two tubes is ≤0.09W, so the static power consumption is extremely small. When an audio signal is input from the v1 gate, the audio voltage amplified by v1 is added to the power amplifier stage tube V2, V3 gate and curtain gate by pushing the secondary winding (A, B) of the transformer T1 - when the positive half cycle When it arrives, point A is positive, point B is negative, and both the gate and curtain grid of V2 are controlled by positive voltages. Because the grid is very close to the cathode, it has a strong attraction for the electrons emitted by the cathode, causing these electrons to fly to the screen at high speed, forming a screen current. A small number of electrons are absorbed by the grid and curtain grid to form a gate current. . At this time, the screen flow of V2 increases rapidly, and V3 is cut off. When the negative half cycle arrives, point B is positive, point A is negative, the screen flow of V3 increases, and V2 is cut off. What goes around comes around. The drastically changing screen current induces a corresponding audio voltage on the secondary side of the output transformer T2 to drive the speaker Y to work. R6 and R7 are current-limiting resistors to prevent damage to the power amplifier tube due to excessive gate current.

Since the power amplifier stage works in a gate current state, the ordinary 1B2 and other voltage amplifier tubes used for the push tube v1 cannot meet the working requirements of this circuit. Therefore, V2 uses power amplifier tube 2P2 to drive, and its power can reach 200mw. It is enough to drive the power amplifier stage circuit to work normally.

The power supply system consists of three independent voltage-stabilized power supply circuits to generate electricity A, electricity B and electricity C. as the picture shows.

The secondary Ac10V voltage of the A power transformer is rectified by D6, filtered by C4 and C6, and stabilized by IC2 (LM317) and then outputs +1.2V voltage through R5 to supply the filament circuit.

B electricity: The secondary AC10V voltage of the transformer is rectified by D1~D4, filtered by c1, c2, c7, and stabilized by Q1, w1, R1 and R2, and then output from the emitter of transistor Q1.

The +90v voltage is supplied to the screen and curtain grid circuits for operation. Among them, R1, R2 and Q1 form an electronic filter circuit to minimize the ripple coefficient of the output voltage.

C Electric: The secondary AC10V voltage of the transformer is rectified by D5, filtered by c3 and C5, and the three-terminal voltage stabilizing integrated block Ic1 (7906), D7 and D8 stabilize the voltage and output a DC voltage of -7V to provide the negative gate bias voltage to V2. Since the output voltage of Ic1 is only -6V, a germanium diode D7 and a silicon diode D8 are connected in series to its ground terminal in the forward direction. The voltage drop across D7 is 0.3v, and the voltage drop across D8 is 0.7v, totaling 1v. Therefore, the output of Ic1 The voltage is raised from -6V to -7V.

Indicator light: LED1 and R4 are connected in series and connected to both ends of the -7V power supply. When the circuit is working, LED1 lights up as an indicator light and provides a DC path from v1 gate to ground.

Component production

Power transformer B1: The primary L1 is wound with 2640 turns on a core with a cross-sectional area of ​​1.6 cm Use φ0.1 enameled wire to wind 120 turns, connect the first end of L4 to the tail end of L3 and then ground it.

Push transformer T1: It is modified from the voltage coil with iron core removed from the DD27 electric meter. Disassemble the original coil (19,000 turns of φ0.1 enameled wire) and rewind it with the enameled wire: the primary coil (P, B) is wound with 4,000 turns, the secondary coil is wound with 6,000 turns, and the center tap E of 3,000 turns is grounded. Remove the two short-circuiting copper pieces in the original iron core, replace them with pieces of paper, and reassemble the iron core.

Output transformer T2: Also use the above-mentioned voltage coil to modify it. Unwind the original 9000-turn voltage coil, take a center tap at 4500 turns and connect it to +B, re-wind the entire coil, connect its head and tail to P1 and P2 respectively, and the secondary winding. (c, D) Use φ0.31 enameled wire to wind 95 turns and connect it to speaker Y.

The DD27 watt-hour meter can be found in scrap collection stations.

Overall machine indicators:

Output power: 0.5W.

Power consumption: 3.4W when there is no signal, 5.7W when there is maximum signal.

Cost: about 55 yuan.