Essentials of making a tube amplifier

What is a tube amplifier? How to make it? The following is an introduction to the production process and debugging content of a tube amplifier. I think the following four aspects should be focused on when making a tube amplifier in daily work.

1. Power supply system

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

1. Power transformer In electronic amplifiers, the power transformer of the power amplifier tube (a large consumer of power) requires high voltage and small current. Normally, its power is more than twice the power amplifier's full power output. We should focus on strengthening shielding measures for power transformers, because power transformers with high voltage output have much greater radiation interference capabilities than power transformers with low voltage output.

2. Rectifier and filter 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 rectified current of the rectifier should be selected higher. The withstand voltage should generally be above l level of the power supply voltage, and the through current should be more than twice the output current of the complete machine at full power. DC power supply filtering should try to choose 1T type, IC type and parallel resonance type filters with excellent filtering performance or set up electronic voltage stabilizing circuits. Because the main voltage in a tube power amplifier is usually designed to be relatively high (generally above 200V), attention must be paid to connecting a resistor (bleeder resistor) with appropriate power and resistance in parallel between the rectified and filtered DC power supply and the ground, so that When adjusting and inspecting the rectifier in the shutdown state, discharge the charge stored in the filter capacitor in a timely manner. In addition, the power supply isolation and decoupling between various levels should be fully done.

3. Filament power supply In tube amplifiers, the 50Hz interference of the filament has long been regarded as the culprit causing 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 of the filament. Power supply, but it must be suspended power supply.

The method is to ground the center tap of the transformer's secondary power winding for the filament, and add a hum balance circuit between the filament voltage output terminals, that is, connect a wirewound potentiometer with appropriate resistance between the two terminals and slide its center The chip is grounded, and the purpose of reducing or eliminating the hum is achieved by adjusting the hum balance potentiometer; the third is to ground either end of the tube filament, and the other end is connected to one end of the transformer filament winding (assuming this end is A and the other end is B Grounding), by changing the direction of inserting the mains AC plug into the power socket and exchanging the sounds at points A and B. At this time, the interference of the 50Hz AC sound in the monitoring speaker will change significantly. When you determine that a certain connection method produces the smallest hum, you can fix this connection method and remember the direction in which the AC power plug is inserted into the power outlet. Then, by carefully changing the grounding position of the grounding end of the filament winding, the ultimate goal is to reduce the 50Hz hum interference to the limit.

2. Power amplifier circuit

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

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

3. Anode resistance The resistance value of the anode resistor of the electron tube is also determined after the anode voltage is determined. On the premise of ensuring that the resistance value meets the normal operation of the tube, considerable leeway should be left for its power to reduce the amount of heat generated during operation.

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

5. Cathode resistance: After the anode voltage and grid bias of the electron tube are determined, the resistance of the 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 larger.

6. The performance of the output transformer has a great impact on the entire machine. It is recommended to purchase a well-known finished output transformer (with a shielding cover included for sale), which should be carefully made using layering, segmentation and cross-winding methods.

3. Production process

1. Structure and layout Normally, electronic tube machines are generally designed with a metal base and an open structure. That is, larger devices such as electron tubes, power transformers, output transformers, and filter capacitors are placed on the upper part of the base, while other small-sized resistive and capacitive components and connecting wires are set inside the base.

When arranging the position of each component on the upper (inner) part of the base, the audio signals should be arranged in the order from small to large. The components of large and small signals should not be placed cross-wise to avoid "cross-infection". The general approach is that the front of the base is the signal input circuit (the signal input base can also be installed behind the machine with a shielded wire), and then the components are arranged backwards according to the signal flow.

2. Welding and wiring When welding various components, the traditional and feverish shed welding process is adopted. That is, directly weld the components and leads to the corresponding pins of the electronic tube holder. When the span between the two welding points is large, corresponding supports (such as plastic brackets or double-sided tape) and transition (such as wiring boards) objects should be added. Before the components are put on the machine, the components should be strictly tested and screened using a digital three-meter to ensure that the parameters of the selected components are consistent with the design values. Then use a sharp blade to scrape off the oxide layer on the pins of the selected components and perform tin enamel treatment. Care should also be taken to keep component leads as short as possible to prevent interference. When using wires to connect components, be sure to keep the power leads away from the audio signal channel to prevent audio signal degradation caused by power radiation. In addition, the AC power cord inside the machine (including the filament power cord when using AC power supply) should be twisted. Power supply leads with different AC and DC voltages and voltages and currents cannot be set up in parallel or twisted together. Instead, they should be kept as far apart as possible or crossed. Furthermore, the leads connecting each pole of the electron tube in the machine should be distinguished by different colors (red for positive, yellow for grid, green for negative, and black for ground) to facilitate identification and should be short and thick. The internal connections within the machine should be tightened with nylon buckles and properly fixed 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 them from other devices or perform shielding treatment. At the same time, when the distance between two points in the audio signal transmission in the machine exceeds 20mm, a high-quality two-core shielded wire should be used as the inter-stage connection, and the shielding layer should be grounded at one end. The tubes responsible for signal input and pre-amplification should be covered with a special metal shield to prevent attacks from external stray electric fields.

It is best to use a one-point grounding method for the whole machine. The specific method is that except for one lead with zero potential in the circuit that is connected to the case, other parts of the circuit must not have any connection with the case or have a resistance that is too small (including signal input and power output terminal). And this grounding wire is gathered from each unit board with thick black wires at the shortest distance to one point and grounded nearby. Experience tells us that the grounding point of a tube amplifier is generally chosen near the rectifier filter circuit or the signal input socket.

4. System debugging

First plug in AC power without plugging in the tube and flip the power switch to the on position. Detect (pay attention to the method to avoid electric shock) whether there is any smoke or excessive temperature inside the machine. At the same time, use a multimeter to measure the AC and DC no-load voltage values ​​at key points in the circuit and compare it with the design voltage value. If the above If there is no abnormality, you can cut off the power supply of the whole machine. After a few minutes, insert each electronic tube into the corresponding position and connect a dummy load (a high-power inductive load that matches the secondary output impedance of the output transformer) to the two output ends of the power amplifier. ) Start up (no-load startup is prohibited to avoid breakdown of the expensive output transformer). After turning on the machine (without taking your hands off the power switch), promptly check whether there are any abnormalities inside the machine, and observe whether the electronic tube filament lights up normally. At the same time, use a multimeter to measure the current of the whole machine and the anode voltage, current and grid negative voltage of each electron tube to see if they are within the design value.

After completing the above steps, the next step is to remove the dummy load and connect the speaker (shutdown operation) for the following simpler debugging. If you hear hum or high-frequency noise interference in the speaker after turning it on, it is usually due to poor power filtering and decoupling, poor circuit grounding or incorrect positioning, and inadequate shielding measures. It 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. The above is how to make a tube amplifier, I hope it can be a reference for everyone.