6N5P tube direct-coupled single-ended amplifier

Circuit Description

6N5P is a dual triode electron tube, commonly used in electronic stabilization circuits. If it is used in audio power amplifier circuits, its negative gate voltage generally needs to be 60~80 volts to work in the linear region. If the circuit is composed in the traditional form with such a high negative gate voltage, some problems will arise. First, when using a self-biased circuit, in order to obtain such a high negative gate voltage, the cathode potential must also be raised very high, and the power consumption of the cathode resistor will also increase accordingly. The power of the resistor can sometimes be 10-15 watts, and this part of the power is wasted just to establish the negative gate voltage. If other methods are used to obtain negative gate voltage, generally no power is consumed. Second, if a fixed bias circuit is used, it is necessary to add a set of bias circuits that can generate negative voltage separately, which complicates the circuit. To solve these contradictions, we can only work hard to improve the circuit. After a period of research on traditional circuits, the author designed this direct-coupled single-ended power amplifier circuit (see Figure 1).

This circuit is designed for power tubes with high negative gate voltage, and is basically suitable for all power tubes with high negative gate voltage. It is composed of direct coupling between the driver stage and the power amplifier stage. In traditional circuits, this connection method is rare, especially for power tubes with high negative gate voltage. As can be seen from the circuit, the two amplifiers are powered by two sets of power supplies, and then the two circuits are cleverly combined to form a complete direct coupling circuit. This circuit not only plays the role of fixed bias, but also realizes direct coupling with the power tube, killing two birds with one stone, and completely solving the above-mentioned defects.

Circuit principle and debugging

The working principle of this circuit is not much different from the traditional circuit, but the coupling circuit is different. Due to the direct coupling method, the frequency band is widened. The DC load resistance of the driving stage in the circuit is the same as the AC load resistance, which is basically the gate-drain resistance of the power tube. The anode current of the front stage 6N2 generates a negative voltage on the gate-drain resistance of the power tube. Adjusting the size of this voltage just establishes a suitable negative gate voltage for the power tube, and also establishes a normal working state for the front and rear stages of the circuit. This is the characteristic of this circuit.

The debugging of the circuit is not complicated. Although it is directly coupled and the working points are related to each other, it is easy to debug as long as the rules are found. In this circuit, you only need to adjust the cathode resistance value of the driving stage to make the voltage across the gate-drain resistance of the power tube the required value.

Precautions

This circuit is quite different from the traditional circuit. It is also different from the traditional circuit during assembly, debugging and use. The following points should be paid special attention to.

1. During debugging and use, it is strictly forbidden to unplug the tube of the driving stage. Because it is directly coupled, the working point of the power tube is established by the front-stage tube and circuit. Unplugging the front-stage tube will cause the power tube to enter a serious conduction state, which will damage the power tube or rectifier tube.

2 This circuit has two potential reference points. Each group of power supplies uses its own "ground" as a reference point. This should be noted when assembling or testing the circuit.

3. When assembling the circuit, the interconnection points of the 6N5P gate-drain resistor and the power isolation resistor should be as close as possible and welded as close to the front stage as possible. The cathode resistor of the power tube should be welded as close to the power supply side as possible.

4. The preamplifier tube should use high-U tube or pentathlon tube. The gain cannot be achieved by using low-U tube. Two preamplifiers are required.

Production of output transformer

When I made the transformer, I used a common 30-watt power transformer core from the market. The production parameters and winding structure are shown in Figure 2.

Performance and listening experience

This circuit has only two stages of amplification. When the input is 1.2 volts, the output power can reach 7.5 watts. When the input is 0.8 volts, the output power can reach 4.5 watts. When observed on the oscilloscope, the waveform distortion is very small and basically invisible. At 7.5 watts, there is a little clipping in the upper half of the waveform. The frequency response is: 30Hz~22kHz.

Listening experience: strong resolution, clear layers, and good vocals, which means the mid-range is good, the bass is powerful and imposing, the treble is crisp, and the triangle and copper bell sounds pleasant.