Design of a tube amplifier that uses the mains electricity to directly provide screen working power

When making tube amplifiers as an amateur, the production of output transformers and power transformers is very tedious. Moreover, the efficiency, volume and weight of the power transformer are a headache for welders. Here we introduce a method for making a tube amplifier that uses mains electricity (220V lighting electricity) to directly supply the tube panel after rectification and filtering. For your reference.

To use the mains electricity to directly provide the working power supply for the electron tube screen, two problems must be solved: 1. The problem of safety in use. 2. The problem of isolation between the front stage and the speaker. Let's talk about the isolation problem between the front stage and the speaker first. The circuit is shown in Figure 1.
From the simplified diagram of Figure 1, we can see that C1, R1, R2 and the photocoupler GD form a linear photoelectric coupling system. It plays the role of isolating the front stage and the rear stage power amplifier, and is an independent power supply system and an independent grounding loop, which can prevent the noise introduced by the potential difference between the ground wires from entering the power amplifier loop, thereby improving the stability and fidelity of the entire power amplifier system. The output transformer of the final stage of the power amplifier completes the isolation between the power amplifier and the speaker system. Its electron tube filament and photoelectric coupling system are supplied by a small-capacity power transformer, which not only reduces the weight of the whole machine, but also reduces the production cost and saves the trouble of winding high-voltage coils. Because it is directly powered by the mains electricity, its power supply internal resistance is much lower than that of the power transformer, making the speed of reproducing sound faster and more in line with the requirements of modern sound sources. The method of using AC power supply is not suitable for every type of electron tube. Here are some electron tube power amplifier circuits that use AC power supply. Interested readers can try them. You will definitely succeed if you assemble them according to the diagram.

1. Single-ended Class A amplifier

The single-ended Class A power amplifier is the most popular circuit form among the tube power amplifiers. This circuit has rich even harmonics and is particularly musical. According to the data that the DC voltage of the 220V mains power is 310V after full-wave rectification and filtering, two single-ended Class A power amplifier circuits are designed, as shown in Figure 2 and Figure 3. From the circuit in Figure 2, we can see that this is a typical low-power single-ended Class A power amplifier circuit. The signal is sent to the input isolation circuit composed of the photocoupler 4N36 through a 4.7 μ F capacitor and a 100 Ω resistor. The 2.2k resistor provides the working current for the light-emitting diode in the photocoupler. As the input signal changes, the luminous intensity of the light-emitting diode also changes. After being received by the phototransistor in the photocoupler, the collector current of the phototransistor changes. In this way, the audio signal is coupled to the 100 Ω resistor of the phototransistor collector for power amplification by the tube power amplifier. In this way, the input signal and the power amplifier part are completely electrically isolated. The final power amplifier tube in the circuit of Figure 2 uses the common 6P1 or 6P14 tube. The pins of these two tubes are exactly the same and can be directly interchanged. The biggest difference between the two is the difference in the optimal load impedance, which is 5.5k for 6P1 and 4.5k for 6P14. The DC voltage value of the two tubes for the plate operation in the tube manual is 250V DC. In this case, the DC voltage is 310V, which theoretically exceeds the plate voltage value and the maximum plate power dissipation of the tube provided in the tube manual. However, after a long period of practice, and deducting the voltage drop caused by the output transformer voltage primary coil (about 500 Ω). The actual DC voltage added to the tube plate is about 280 volts. After hundreds of hours of use, there is no adverse effect, and the output power is higher than the data provided in the manual. The 6P1 is 5W at full power and the 6P14 is 6W at full power.

The circuit in Figure 3 is basically the same as that in Figure 2, except that the final power amplifier uses a more powerful 6P3P tube. Its input isolation circuit is exactly the same as that in Figure 2 and will not be described in detail. Using 6P3P as the power amplifier tube, the output power can reach 10W at a screen voltage of 310V, which is enough to drive some large-caliber speakers in slightly larger areas. Moreover, the optimal load impedance of 6P3P is relatively low at 3.5k, which provides convenience for homemade output transformers.

Figure 3 The measured technical indicators of the circuit are as follows:

Maximum undistorted output power: 10W (RL=8Ω)

Frequency response: 30Hz-22KHz (±3dB)

Total harmonic distortion: 1.5% (at 1W) Signal-to-noise ratio: 96dB Output transformer winding data: Core size: tongue width 26mm, stack thickness 32mm, core cross-sectional area approximately 7.6mm2 Primary winding turns: The total number of turns is 2400 turns, divided into 4 sections, 600 turns in each section Primary wire diameter: Use high-strength enameled wire with a diameter of 0.17mm. The secondary is wound with 140 turns of high-strength enameled wire with a diameter of 0.7mm, sandwiched between the 4 layers of primary coils.

After the coil is wound, dip in varnish and dry it before inserting the iron core. The iron core should be inserted in the same direction. Use 0.5mm insulating paper to separate the air gap between E. After inserting, dip in varnish and dry it. You can use AC to simply test the impedance ratio. Pass 220V AC on the primary side and there should be 11V AC on the secondary side. The output isolation of the above two circuits is achieved by using output transformers. When making, all components in the red dotted box in the figure cannot contact the casing. Wear insulating gloves during debugging to prevent electric shock. In the above two circuits, you only need to adjust the resistance of the collector resistor of the photosensitive transistor to change the input amplitude to meet the needs of full power output. Other models of optocouplers can also be selected, as long as the transmission ratio is 100% and the transmission speed is greater than 4ms. It is best to use finished products for the output transformers of 6P1 and 6P14, which are slightly more difficult to make by yourself. Other components can be selected according to the conditions of the enthusiasts themselves. The three-terminal regulator should be equipped with a heat sink with a large enough area.

2. Class AB1 push-pull amplifier

When the power requirement is higher, you can choose the circuit form of the tube push-pull amplifier. The tube power amplifier uses the 310V DC directly rectified and filtered by the mains as the screen power supply. The maximum output power can reach 20W, which is fully able to meet the family's needs for replaying music. There are many kinds of push-pull amplifier circuits. The more classic and mature one is the Williamson amplifier circuit. However, the power tubes used for push-pull amplification in the final stage of the Williamson amplifier circuit all use the triode connection method, so the output power is relatively small. I combined the Williamson circuit with the super-linear connection method of the final push-pull power tube and designed the following power amplifier circuit. It has the excellent performance of the Williamson amplifier circuit and can greatly increase the output power of the power amplifier. After several years of actual use, the effect is very good. The power amplifier circuit diagram, the three connection methods of the final push-pull power tube are shown.

The final push-pull power amplifier tube in the circuit of Figure 4 can also use 6P3P electron tube, and the maximum output power can be increased to 22W. The working status of the two power amplifier tubes under 310V screen voltage and Class A, B and 1 is shown in the attached table.

The output transformers of the above circuits all use 25W push-pull output transformers. The production data is shown in Figure 6. The winding method and assembly are the same as those of the single-ended output transformer, so they will not be described in detail.

In the circuit of Figure 4, except for the optocoupler system and the output transformer, all components are connected to the mains. During the production and debugging process, attention must be paid to safety.

After assembling the above two types of tube amplifier circuits, use hot melt adhesive to seal all solder joints and other conductive parts connected to the mains to avoid electric shock accidents.