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

The negative voltage of the class B amplifier is set at the approximate cut-off of the screen current. Only in the positive half of the signal, the screen current begins 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. A complete cycle. Due to the power amplification, the screen current of the upper and lower tubes changes with the amplitude of the signal, and the average current of the screen is small, which is the reason for the high efficiency.

Since the 6N7PB type power amplifier must have a certain driving power, it is difficult to meet the requirements with a RC-coupled voltage amplifier.

One day in the electronic flea market, I found an audio amplifier in the 1950s. I used two 6N7Ps for audio power amplification. The pusher used a transformer. The push tube was 6N9P. After I bought it, I carefully studied the amplifier. Measurement, on the basis of which a circuit is combined and experimentally produced, the circuit is shown below.

The first stage voltage amplification uses the pentode 6J4P. The first is to take the higher screen voltage and the power supply is relatively simple. The second is that the 6J4P is also a large 8-pin GT tube, which is matched with the 6N7P.

The key of this machine is to push the transformer. Because the B-type power tube has a gate current at the maximum signal, it is required that the DC resistance of the gate circuit is as small as possible. Otherwise, the voltage drop generated by the gate current will affect the stability of the gate negative voltage. Thus, the distortion is increased and the transformer is pushed to meet the above requirements. In addition, the DC resistance of the gate loop has a decisive influence on the output power. It is pointed out that in theory, if the gate loop resistance is zero, it can output 20W of power. Of course, this is impossible in practice, but it can be Take measures to minimize the DC resistance of the gate loop, and also consider the frequency response, so the push transformer should be a step-down ratio transformer.

Pushing the transformer: The iron core is the disassembled American instrument core with a tongue width of 22 mm and a stack thickness of 26 mm. See Figure 3. Using the original wire frame, the primary winding is wound around the primary and the secondary DC resistance is reduced. The secondary uses φ0.33mm high-strength enameled wire to wrap 12 layers, each layer 90匝, a total of 1180匝, the primary use φ0.08mm high-strength enameled wire winding, around the full, the number of turns is about 6000 ~ 7000 ,, each After 1000 匝, the pad capacitor paper can be insulated, and the initial/secondary is insulated with 0.2 mm thick kraft paper. No matter how many defects the primary can end, two cattle are required to have the same number and symmetry. After the winding is completed, the measured secondary DC resistance is about 60Ω, and the primary DC resistance is about 5kΩ. The inductance is about 100H in the primary and about 0.5H in the secondary. First dipped the wire bag to dry the paint, then insert the iron core. Since the primary has a direct current, the silicon steel sheet is inserted in parallel, and a 0.06 mm thick kraft paper is interposed between the E and I sheets as an air gap to avoid magnetic saturation.

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

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

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

Some enthusiasts think that Class B power amplifiers are thick and unpleasant. Through this production and audition, compared with single-ended class A and Class A and B-type push-pull, the subjective feeling is full and powerful, and the sound orientation is quite different, but each has Features.

Some people may say that it is not necessary to make such a big effort to make this cheap tube type B amplifier. It is too much effort to push the cow.

I think that since I have a fever, I have to deal with all aspects. I don’t know the taste of pears when I don’t eat pears. I don’t know the characteristics of the pears, and the sound is pretty good! It’s like the people on the Loess Plateau. The singing voice, although lacking in some delicate, is shocking! Some aspects can show a special charm, so for various types of amplifiers can not simply conclude that it is not good. Just like piano and keyboard, guzheng and guqin, horn and cymbal, can you simply say who is nice and who is not good?

Finally, it is said that the promotion of cattle, without accurate calculations, only draws on the data requirements and experience, using the existing materials on hand, there is no layered winding, the main purpose is to minimize secondary DC resistance, such as enthusiasts It is possible to find iron cores with higher magnetic flux density, such as permalloy, which can be calculated and wound in strict accordance with relevant formulas, and can certainly produce high-performance, high-index, wide-frequency HI-FI class B power amplifiers.

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