Making a 6N7P Class B power amplifier circuit

Recommended parameters for 6N7P in Class B power amplification: screen supply voltage (Uaa): 300V; screen current (la) at zero signal and maximum signal: 35mA/70mA; grid negative voltage (Ug1): 0V; two-tube grid Inter-electrode driving voltage (Ug-grms): 58V.

The negative voltage of the gate of the Class B amplifier is set at the approximate cutoff of the screen current. Only in the positive half cycle of the signal does the screen current begin to increase with the signal amplitude. The upper and lower tubes respectively amplify the positive and negative half cycles of the signal, and are synthesized in the output transformer as A complete cycle. Since during power amplification, the screen currents of the upper and lower tubes change with the amplitude of the signal, the average screen current is smaller, which is the reason for the high efficiency.

Since the 6N7PB class power amplifier must drive a certain amount of power, it is difficult to meet the requirements with a resistor-capacitor coupling voltage amplifier.

One day, I found an audio amplifier from the 1950s at an electronics flea market. It used two 6N7Ps for audio power amplification. The drive used a transformer and the drive tube was 6N9P. I bought it home and carefully studied and analyzed the amplifier. Measurement, based on this, a circuit was assembled and experimentally produced. The circuit is shown in the figure below.

The first-stage voltage amplification uses the pentode 6J4P. Firstly, the screen electrode voltage is higher and the power supply is relatively simple. Secondly, 6J4P is also a large 8-pin GT tube, which matches the 6N7P and has a more coordinated appearance.

The key to this machine is to drive the transformer. Because the Class B power tube generates gate current when the maximum signal is reached, the DC resistance of the gate circuit is required to be as small as possible. Otherwise, the voltage drop generated by the gate current will affect the stability of the gate negative voltage. , thereby increasing the distortion and pushing the transformer to meet the above requirements. In addition, the DC resistance of the gate loop has a decisive influence on the output power. Some articles point out that theoretically if the gate loop resistance is zero, a power of 20W can be output. Of course, this is impossible to achieve in practice, but it can Measures should be taken to reduce the DC resistance of the gate loop as much as possible, and frequency response should also be taken into consideration, so the driving transformer should be a step-down ratio transformer.

Push transformer: The iron core is a disassembled American Instrument core, with a tongue width of 22mm and a stack thickness of 26mm, as shown in Figure 3. Using the original wire frame, winding the secondary first and then the primary will help reduce the secondary DC resistance. The secondary is wound with 12 layers of φ0.33mm high-strength enameled wire, 90 turns per layer, 1180 turns in total. The primary is wound randomly with φ0.08mm high-strength enameled wire until it is fully wound. The number of turns is approximately between 6000 and 7000. Winding 1000 turns, capacitor paper can be placed for insulation, and 0.2mm thick kraft paper should be wrapped with 4 layers of insulation between the primary and secondary. No matter how many turns the primary can wind around in the end, the two cows must have the same number of turns and be symmetrical. After the winding is completed, the measured secondary DC resistance is about 60Ω and the primary DC resistance is about 5kΩ. The primary inductance is about 100H, and the secondary inductance is about 0.5H. First dip the wire package into paint and dry it, then insert the iron core. Since there is a DC current in the primary, the silicon steel sheets are inserted sequentially, and 0.06mm thick kraft paper is placed between the E and I sheets as an air gap to avoid magnetic saturation.

Output transformer: impedance 8kΩ/4Ω, 8Ω. High silicon wafer core, area 32mm×60mm,

Power transformer: Two 80W power transformers supply power to the two channels respectively, with one set of high-voltage 300V windings and three sets of 6.3V filament windings. The rectifier tube uses FR-3073A/1000V fast diode.

Debugging: The screen voltage is about 300V, the quiescent current at 0 signal is about 35mA, and the maximum signal (when outputting 10W) ​​is about 70mA.

Some audiophiles think that Class B power amplifiers sound loud and unpleasant. After this production and listening test, they feel that compared with single-ended Class A and Class A and B push-pull, the subjective feeling is that the sound is full and powerful, and the sound orientation is quite different, but each has its own characteristics. feature.

Some people may say that there is no need to put so much effort into making a Class B amplifier with such cheap tubes. It would take too much effort to just push the cow.

I believe that since you have a fever, you should be involved in all aspects. If you don’t eat pears, you will never know the taste of pears. If you don’t listen to the Class B amplifier, you will never know its characteristics. Besides, its sound is quite nice! It’s like the rough and high-pitched man on the Loess Plateau. Although the singing voice lacks some delicacy, it is shocking! Some aspects can express a special charm. Therefore, we cannot simply draw a conclusion about which amplifier is better or worse for various types of amplifiers. Just like piano and electronic keyboard, guzheng and guqin, French horn and suona, can you simply say who sounds better and who doesn't?

Finally, I would like to talk about the push cow. It has not been accurately calculated, but only based on data requirements and experience. It is wound using existing materials on hand. There is no layered winding. The main purpose is to minimize the secondary DC resistance, such as enthusiasts If you can find an iron core with a higher magnetic flux density, such as permalloy, and calculate and wind it in strict accordance with the relevant formulas, you will definitely be able to make a HI-FI Class B power amplifier with high performance, high indicators, and wide frequency response.

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