Preamplifier made of 6P15 tube

In the early years, 6P15 was mainly used for video voltage amplification of black-and-white tube televisions. The linearity and distortion of video voltage amplification directly affect the quality of TV images. Although video amplifiers are similar to audio amplifiers, their requirements are relatively high. It is of unique significance to learn from the concept of video amplification and try to make audio amplifiers with video amplification components.

6P15 is one of the professional electronic tubes designed for high-frequency circuits, especially with extremely strict requirements on amplitude-frequency characteristics FR and noise. It has an anti-microphonic effect in technology, which is reflected in the TV image to avoid noise points caused by vibration. Some "players" boldly "transplanted" it to audio equipment, which produced unexpected superior effects.

The pre-stage circuit made by the author is shown in the attached figure. It combines the characteristics of video amplifier circuit and integrates the characteristics of modern audio. The 6P15 cathode resistor in the figure is composed of two resistors in series, and a resistor at the ground end is connected to a bypass electrolytic capacitor.

It is a pure DC negative feedback bias. The resistor connected to one end of the 6P15 cathode is not connected in parallel with the capacitor. It is an AC and DC composite negative feedback bias. It can not only obtain a linear operating point, but also ensure the stability of the circuit.

Considering the impact of the inductive reactance of electrolytic capacitors at high frequencies on the clarity of high-frequency audio, a 0.0068μF mica capacitor is connected to the cathode of 6P15 to improve the transparency and airiness of the high-frequency sound range.

The 6P15 anode load resistor is based on the frequency equalization circuit used in classical record players. The series network resistors are connected in series at a ratio of 1:10. A "frequency correction" network is connected in parallel to the 6P15 anode resistor, which not only has a good effect on improving the low frequency, but also has the functions of suppressing high-frequency noise, preventing howling, and eliminating self-excitation, making the whole circuit amplification linear and balanced, with flat frequency response and low distortion.

The output coupling capacitor of 6P15 should not be too large. If the capacitance is too large and the volume is also large, the distributed capacitance will increase. The value of the gate-drain resistance should not be too large. If it is too large, the electron tube will work unstably.

In order to obtain pure DC power, this pre-stage adopts a double power transformer to form a non-fully coupled power transformer, so that the clutter signal in the power grid can be attenuated and not transmitted.

1. Component Selection

The power transformer and choke are Shanghai Radio 27Y-1 products: the power transformer power is about 100W: the choke specification is 10H/100mA. Choose a suitable chassis. Add shielding cover to the power transformer and choke. The components should be reasonably arranged and the welding should be as compact as possible. The signal seat uses a gold-plated insulating seat.

2. Assembly and debugging

In order to bring out the performance characteristics of 6P15, let 6P15 work in a typical working state. By adjusting the cathode resistance and screen grid voltage of 6P15, the cathode voltage of 6P15 is 3V, the cathode current is about 30mA, and the screen grid voltage is about 150V. The 6P15 of this machine works at about 30mA, which is much larger than the general pre-stage working current. It is conducive to improving the working stability and the quality of the music.

3. Burn-in

Burn-in, which is called Run In by foreign players, refers to the long-term continuous use of the audio system to allow the audio equipment to fully "age", making the sound mellow, sweet, delicate and smooth.

4. Online audition

Matching equipment: Philips DVD player, Italian Orbacas speakers, American Music Ribbon Super F1a-tine Cab1e speaker cable; the signal cable is Ortofon AC-5000 8N oxygen-free copper wire, the power cable is Hitachi 4N single crystal copper 3×3.5mm2 silicone rubber power cable; the audio-specific power purifier is G&WTW-05D.

When playing with genuine Hugo CD, the sound field is wide, the dynamics are particularly good, the audio and video positioning and resolution are good, the low frequency is solid, the control is good, the air sense is strong, and the sound quality is very pure. It is unforgettable! Compared with several homemade preamplifiers, it feels that its adaptability is wider and its expressiveness is stronger. It can vividly portray various instruments such as brass music and symphony. It is neat and the music is magnificent, all thanks to the unexpected charm of the video amplifier tube 6P15!