Self-made and debugged electron tube amplifier

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Self-made and debugged electron tube amplifier [Copy link]

When making a tube amplifier, the following issues should be focused on.

1. Power supply system

The quality of the power supply system directly affects the stability of the system and the performance of the various performances of the power amplifier circuit.

1. Power transformer. In the tube amplifier, the power amplifier tube (a large power consumer) requires high voltage and low current from the power transformer. Under normal circumstances, its power is more than twice the power output of the power amplifier at full power. The shielding measures for the power transformer should be strengthened, because the radiation interference capability of the power transformer with high voltage output is much greater than that of the power transformer with low voltage output.

2. Rectification and filtering circuit. In the tube amplifier circuit, the general practice is that the entire circuit shares a set of DC power supply. The withstand voltage and rectification current of the rectifier should be selected to be higher. The withstand voltage should generally be more than 1 times the power supply voltage, and the current should be more than 2 times the output current of the whole machine at full power. For DC power supply filtering, π-type, LC-type and parallel resonant filters with excellent filtering performance should be selected as much as possible, or electronic voltage stabilization circuits should be set. Because the main voltage in the tube amplifier is usually designed to be high (generally above 200V), it is necessary to connect a resistor (discharge resistor) of appropriate power and resistance in parallel between the rectified and filtered DC power supply and the ground, so that the charge stored in the filter capacitor can be discharged in time when the rectifier is adjusted or repaired in the off state. In addition, the power isolation and decoupling work between each level should be done well.

3. Filament power supply. In the tube amplifier, the 50Hz interference of the filament has long been judged to be the culprit for the low signal-to-noise ratio of the whole machine. There are three ways to solve the problem: one is to use DC voltage to power the filament; the other is to use AC voltage to power the filament, but it must be suspended. The method is to ground the center tap of the secondary transformer filament power supply winding, add an AC noise balance circuit between the filament voltage output terminals, that is, connect a wire-wound potentiometer with appropriate resistance across the two terminals, and ground its center sliding piece. By adjusting this AC noise balance potentiometer, the purpose of reducing or eliminating the AC noise can be achieved; the third is to ground any end of the tube filament and connect the other end to one end of the transformer filament winding (assuming that this end is A and the other end is grounded). By changing the direction of the AC plug inserted into the power socket and the exchange between points A and B, the interference of 50Hz AC noise in the monitoring speaker will change significantly. When it is determined that the AC noise of a certain connection method is the smallest, this connection method can be fixed, and the direction of the AC power plug inserted into the power socket can be remembered. Then, by carefully changing the grounding position of the grounding end of the filament winding, the ultimate goal is to reduce the 50Hz AC noise interference to the limit.

2. Power amplifier circuit

1. Electronic tube. The input stage, pre-stage or driving stage can use the common 6N series tubes, such as 6N1, 6N2, 6N3, 6N11, 6N8P, etc. The power amplifier stage can use the 6P1, 6P14, 6P15, 6P3P and FU7, FU-25 and other models of electron tubes with higher performance indicators.

2. Coupling capacitor. The coupling capacitor should try to choose a CBB capacitor or other high-quality capacitor with high withstand voltage, low leakage current and non-polarity, and its capacity is generally selected between 0.1μF and 1μF.

3. Anode resistor. The resistance value of the electron tube anode resistor is determined after its anode voltage is determined. Under the premise of ensuring that the resistance value meets the normal operation of the tube, its power should also leave a considerable margin to reduce the heat generated during operation.

4. Grid resistor. The function of the grid resistor is to transfer bias to the target tube. In order to improve the signal-to-noise ratio of the whole machine, this resistor should use a higher quality metal film resistor, and its power can be smaller.

5. Cathode resistor. After the anode voltage and grid bias of the electron tube are determined, the resistance of its cathode resistor determines the size of the anode current of the tube. When selecting, the recommended value should be used as much as possible, and the power of the cathode resistor should be selected to be larger.

6. Output transformer. The performance has a great impact on the whole machine. It is recommended to purchase a well-known finished output transformer (with a shielding cover attached), which should be carefully made by layering, segmentation and cross winding.


III. Manufacturing process

1. Structure and layout. Under normal circumstances, electron tube machines are generally designed with a metal base and an open structure. That is, the larger devices such as the electron tube, power transformer, output transformer and filter capacitor are placed on the upper part of the base, while other small resistors and capacitors and connecting wires are set inside the base.

When arranging the positions of various components on the base (inside), the order of audio signals from small to large should be followed for reasonable arrangement. Components of large and small signals should not be set crosswise to avoid "cross infection". Generally, the signal input circuit is placed at the front of the base (the signal input base can also be installed at the back of the machine with a shielded wire), and then the components are arranged backwards in sequence according to the signal flow.

2. Welding and wiring. When welding the components, the traditional and popular scaffolding welding process is adopted. That is, the components and the leads of each part are directly welded to the corresponding pins of the electronic tube holder. When the span between the two welding points is large, the corresponding support (such as plastic bracket or double-sided tape) and transition (such as wiring board) objects should be added. Before the components are installed on the machine, the components should be strictly tested and screened with a digital multimeter to ensure that the parameters of the selected components are consistent with the design values. Then, the oxide layer on the pins of the selected components should be scraped clean with a sharp blade and tinned. It should also be noted that the pins of the components should be shortened as much as possible to prevent interference. When connecting components with wires, pay attention to the fact that the power leads should be kept away from the audio signal channel as much as possible to prevent the degradation of the audio signal caused by power radiation. In addition, the AC power cord in the machine (including the filament power cord when AC power is supplied) should be twisted. The power leads with different AC and DC voltages and voltage and current should not be set in parallel or twisted together, but should be kept as far away as possible or cross-set. In addition, the leads connecting the poles of the electron tubes in the machine should be distinguished by different colors (yang-red, yellow-grid, green-yin, black-ground) to facilitate identification and should be short and thick. The wires in the machine should be tightened and reasonably fixed with nylon buckles without affecting each other.

3. Shielding measures. The power transformer and output transformer are the largest sources of electromagnetic interference in the whole machine. Metal objects should be used to isolate or shield them from other devices. At the same time, when the distance between two points in the audio signal transmission in the machine exceeds 20mm, high-quality double-core shielded wires should be used as inter-stage connections, and the shielding layer should be grounded at one end. The tubes used for signal input and pre-amplification should be covered with special metal shielding covers to prevent attacks from external stray electric fields.

It is best to adopt a single-point grounding method for the whole machine. Specifically, except for a lead with zero potential in the circuit that is connected to the housing, other parts of the circuit must not be connected to the housing or have too low resistance (including signal input and power output terminals). And this grounding wire is collected from each unit board at a point with a thick black wire at the shortest distance and grounded nearby. Experience has shown that the grounding point of the tube amplifier is generally selected near the rectifier filter circuit or the signal input socket.

4. System debugging

First, turn on the AC power without inserting the tube and turn the power switch to the on position. Check (pay attention to the method, do not get electric shock) whether there is any smoke or high temperature in the machine, and use a multimeter to measure the AC and DC no-load voltage values of each key point in the circuit, and compare them with the designed voltage values. If there is no abnormality in the above inspection, the power supply of the whole machine can be cut off. After a few minutes, insert each electron tube into the corresponding position and connect a dummy load (a high-power inductive load that matches the output impedance of the output transformer secondary) to the two output terminals of the power amplifier to turn on the machine (no no-load startup is allowed to avoid breakdown of the expensive output transformer). After turning on the machine (do not leave the power switch), check whether there is any abnormality in the machine in time, and observe whether the electron tube filament is lit normally. At the same time, use a multimeter to measure whether the whole machine current and the anode voltage, current and grid negative voltage of each electron tube are within the designed value.

After completing the above steps, the next step is to remove the dummy load and connect the speaker (power off) for the following simpler debugging. If you hear AC noise or high-frequency noise interference in the speaker after powering on, it is generally due to poor power filtering and decoupling, poor circuit grounding or incorrect position, and ineffective shielding measures. This can be overcome by improving the power filtering and decoupling capabilities, improving the grounding condition of the ground wire or changing its grounding position, and strengthening electromagnetic shielding.

All the work is done, and the last step is to connect the sound source for audition.

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