6P1 Bile Duct Amplifier

The 6P1 tube is disdained by some audiophiles, but I recently used 6P1 to make a small tube amplifier, the circuit is simple and classic, the production cost is low, and the sound is beautiful. The whole machine is shown in Figure 1 (omitted), the tube used is shown in Figure 2 (omitted), and the circuit is shown in Figure 3.

1. Circuit design and selection

In the 1960s and 1970s, tube radios almost always used 6P1 as power amplifier tubes. Compared with transistor radios, tube radios have insurmountable advantages in sound quality and tone. No wonder they are still collected by radio enthusiasts. The role played by 6P1 is indispensable. The author has made a systematic arrangement of the entire circuit in combination with the current fever concept, which must reach the HiFi level. The circuit is a single-ended Class A output. The first stage uses half of the American 5670 for common cathode amplification, and in order to widen the bandwidth, no electrolytic capacitor is added to the cathode to form current negative feedback. The second stage uses the other half of the 5670 as the cathode output.

The cathode output circuit originated in the 1940s and is derived from the classic and popular common cathode amplifier circuit. It directly moves the load in the original cathode loop to the cathode loop. Its characteristic is that it constitutes 100% negative feedback of the circuit, so the gain of the circuit is 1 or less than 1, but the impedance, frequency response and other characteristic indicators have been greatly improved, which fully meets the high-fidelity amplification requirements of many digital audio sources today. The first stage to the second stage adopts a direct coupling circuit, which eliminates the phase distortion and intermodulation distortion caused by the inter-stage capacitance, and also widens the frequency band, and the dynamic range is further widened.

The input stage and the driver stage have also tried to use the SRPP circuit, but the effect is not satisfactory. The SRPP circuit does have many advantages. It was popular in the 1990s, mainly because people had a superficial understanding of the SRPP circuit in theory. From the working principle, the SRPP circuit is not a single-ended circuit, it is a series adjustment push-pull circuit, so it is inevitable that there will be crossover distortion and transient distortion. Moreover, the cathode of the upper tube of this circuit is not the signal ground potential, but the output end of the signal, which cannot eliminate the noise formed by the thermal electron emission between the filament and the cathode of the electron tube. Because it is the first stage, the distortion and noise are amplified by multiple stages later, and the impact on the sound can be imagined.

From the schematic diagram, the machine does not apply large loop negative feedback, because the input stage has added current negative feedback, and the driving stage is 100% negative feedback, which lays the foundation for canceling the large loop negative feedback. Practice has proved that after canceling the cross-level negative feedback, its sound has quite good transient intermodulation characteristics, and the timbre not only has a unique charm, but also eradicates the inherent "soft, leathery, and watery" drawbacks of tube amplifiers in terms of strength, and the sound sounds particularly fresh and elastic.

2. Component Selection

The bile duct must be selected from J grade or above, and should be matched as much as possible. There are many 6P1 tubes produced in China, and the technology is quite mature. There is no problem with the quality. As long as you buy tubes of the same brand and batch, there is almost no need to match them. The tube has a particularly strong overload resistance. The author has done experiments and found that even if it is used to the extreme parameters, there will be no sparking phenomenon, and at most the screen will be slightly red. However, it is not recommended to use extreme parameters.

5670 is an American product. It is smaller than the domestic 6N3 tube, but its performance is much better than that of domestic tubes. Its sound quality and tone are truly outstanding among similar tubes.

The selection of resistors is divided into two parts. The amplification part uses domestic Dahongpao metal film resistors. Except for the power indicated on the schematic diagram, all 1/2 w resistors are used. The power supply part uses the US military RcLAL resistor with aluminum alloy heat sink shell. The resistor works stably. Due to the heat sink shell, the temperature rise is very low, which is very beneficial to reduce thermal noise and improve the signal-to-noise ratio of the whole machine. The 330Ω/6w resistor of the power rectifier output stage is composed of three 1K/2W American AB carbon resistors in parallel. This resistor has two functions. First, it has been determined that the sound of AB carbon resistors is warm and thick, so it is used here to polish. Second, the filter electrolytic capacitor has a large charging current at the moment of rectifier output, and this resistor plays the role of current limiting and protecting the rectifier tube.

The choice of capacitors has a great impact on the sound quality of the whole machine. Here, the author has selected German wIMA film capacitors. This capacitor was very popular in China a few years ago. Due to its comprehensive sound performance, wide sound range, and extremely low price, it has a very high cost performance. If you don't care about the cost, you can also use French SSOLEN SCR (Solen) and American SPRAGUE (Sprague). In fact, domestic capacitors also have good performance. For example, Xindak, Kaili and other MKP capacitors have a good reputation among audiophiles.

The role of power supply in the circuit has long been determined. Among audiophiles, there is a saying that "laymen focus on circuits, experts focus on power supply" or "three parts circuit, seven parts power supply". When it comes to power supply, it is nothing more than rectification and filtering. It seems simple, but it is quite particular to do. Take filtering as an example (including decoupling). The principle recognized by audiophiles is "big head, thin tail and small middle". If the tail is big, it is easy to produce sharp and sharp sounds, the sound speed lags and lacks dynamics, the bass is foggy, and it is extremely unbearable to listen. The author used two Dutch Philips Blue Hexagonal 100uF/385V electrolytic capacitors at both ends of the filter inductor. This capacitor is small in size, has little leakage, and is fast. It is one of the rare supplementary capacitors. If the domestic Tianhe brand or Heping brand electrolytic capacitors are used, the effect is no worse than that of foreign products, but the size is slightly larger. The third-level filter capacitor uses Kaili 20uF/400V MKP film capacitors, which have excellent performance in the filter circuit. The decoupling capacitor uses the American ARCO 10uF/300V film capacitor, which is very small in size and is second-hand, so the price is very low. After instrument testing, all indicators are excellent and very stable. Since the MKP film capacitor is added to the filtering and decoupling circuit, it is very beneficial to reduce the internal resistance of the entire power supply and improve the response capability.

The output transformer is the throat of the tube amplifier and also the bottleneck that restricts the further improvement of the level of the tube amplifier. The author also selected a famous brand of power transformer.

3. Installation and debugging

This tube amplifier uses the scaffolding welding process recognized by audiophiles. It uses the tube socket welding feet as the basis and several bakelite connecting pillars removed from old instruments to weld all the resistors and capacitors into the chassis.

The machine is very easy to debug. Because the final stage works in the Class A amplification state, the working point must be selected at the midpoint of the screen current straight line. In the absence of special instruments, the multimeter can be set to 100mA and then connected in series to the screen circuit. When the audio signal is input, the screen current becomes larger, indicating that the grid negative voltage is too small. On the contrary, if the screen current becomes smaller, it means that the grid negative voltage is too large. When there is an audio signal input, the screen current basically remains stable or does not change much. At this time, the working point is basically adjusted. The cathode resistor of 6P1 uses 300Ω, and the screen current is about 45mA. In addition, 6P1 works according to the standard connection method, so it is not connected according to the triode method. There are two main considerations: first, the machine itself is a small tube machine with low power (2×4w), so if it is connected according to the triode method, the power will be smaller. Although the triode connection method has a more mellow and soft sound, the standard connection method can maximize the potential of the power amplifier tube and release a passionate and unrestrained sound. In order to improve the signal-to-noise ratio of the whole machine. The center tap of the filament winding is not grounded. Instead, it is connected to the cathode of 6P1. In this way, you can hardly hear any noise when your ear is 20cm away from the speaker.

4. Scope of use

In the listening environment of a room below 20 square meters, with bookshelf speakers above 90db, this machine has excellent performance. I once introduced the audio of the bedroom TV to this machine, paired with full-band speakers, and played the large-scale symphony of the Central Music Station. The sound effect is simply incomparable to the original TV. I also used this machine to replace the multimedia active speakers that came with my computer. I didn’t know it until I changed it, and I couldn’t forget it once I changed it. Although the original active speakers have a subwoofer, the sound of the drumsticks is a fuzzy "buzz, buzz", while the sound of this machine is a clear and shocking "boom, boom". Recently, I connected this machine to a high-definition player, especially playing some music in APE or FLAC lossless compression format. It is no exaggeration to say that the sound effect is completely up to or close to the level of CD. Moreover, all these music are downloaded from the Internet, which is inexhaustible. In this way, the frequency of use of the CD player in my living room has been greatly reduced.

Audiophiles who have been using transistor amplifiers for a long time may wish to install a cheap but high-quality 6P1 tube amplifier, which will definitely refresh your ears and experience another world of music.