DC tube amplifier working in Class B

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

The electron tubes V2 and V3 form a push-pull power amplifier stage, and V1 is the driving stage. When there is no signal input, the screen grid voltage of V2 and V3 is 0, and the screen electrode is far away from the cathode, so the attraction to the electrons emitted by the cathode is small, and the number of electrons flying to the screen electrode is small, so the total screen current is ≤1mA. At this time, 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 gate of V1, the audio voltage amplified by V1 is added to the gate and screen grid of the power amplifier stage electron tubes V2 and V3 through the secondary winding (A, B) of the driving transformer T1-when the positive half cycle arrives, point A is positive and point B is negative, and the gate and screen grid of V2 are both controlled by positive voltage. Because its gate is very close to the cathode, it has a strong attraction to the electrons emitted by the cathode, causing these electrons to fly to the screen electrode at high speed, forming a screen current, and a small number of electrons are absorbed by the gate and screen grid to form a gate current. At this time, the screen current of V2 increases rapidly, while V3 is cut off. When the negative half cycle arrives, point B is positive, point A is negative, the screen current of V3 increases, and V2 is cut off. This cycle repeats over and over again. The drastically changing screen current induces a corresponding audio voltage at the secondary 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 grid current.

Since the power amplifier stage works in the state of grid current, the driving tube V1 using ordinary 1B2 and other voltage amplifier tubes can no longer meet the working requirements of this circuit. Therefore, V2 uses the power amplifier tube 2P2 for driving, and its power can reach 200mw, which is enough to drive the power amplifier stage circuit to work normally.

The power supply system is composed of three independent voltage-stabilized power supply circuits to generate power A, power B and power C, as shown in the figure.

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

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

The +90V voltage is supplied to the screen and screen grid circuits. R1, R2 and Q1 form an electronic filter circuit to minimize the ripple factor of the output voltage.

Power C: The AC10V voltage of the transformer secondary is rectified by D5, filtered by C3 and C5, and then stabilized by the three-terminal voltage regulator integrated block Ic1 (7906), D7 and D8 to output a DC voltage of -7V to provide a 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 in the forward direction at its ground terminal. The voltage drop across D7 is 0.3V, and the voltage drop across D8 is 0.7V, which is 1V in total, so the output voltage of Ic1 is raised from -6V to -7V.

Indicator light: LED1 is connected in series with R4 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 the gate of v1 to the ground.

Components production

Power transformer B1: The primary L1 is wound with φ0.15 enameled wire for 2640 turns on an iron core with a cross-sectional area of ​​1.6cm×3.2cm, the secondary winding L2 is wound with φ0.15 enameled wire for 1200 turns, L3 is wound with φ0.25 enameled wire for 120 turns, and L4 is wound with φ0.1 enameled wire for 120 turns. The head end of L4 is connected to the tail end of L3 and then grounded.

Pushing transformer T1: It is modified from the voltage coil with iron core removed from the DD27 watt-hour meter. The original coil (φ0.1 enameled wire 19000 turns) is disassembled and rewound with the enameled wire: the primary coil (P, B) is wound with 4000 turns, the secondary coil is wound with 6000 turns, and the center tap E is grounded at 3000 turns. The two short-circuit copper sheets in the original iron core are removed and replaced with paper sheets, and the iron core is reassembled.

Output transformer T2: It is also modified using the above voltage coil. Untie the original 9000-turn voltage coil, draw a center tap at 4500 turns and connect it to +B. Rewind the entire coil, connect its head and tail to P1 and P2 respectively, and wind the secondary winding (c, D) with 95 turns of φ0.31 enameled wire and connect it to speaker Y.

DD27 type electricity meter can be found in scrap collection stations.

Machine indicators:

Output power: 0.5W.

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

Cost: about 55 yuan.