Using 300B tube to make a single-ended class A integrated amplifier

I am a tube amplifier enthusiast. I have assembled many tube amplifiers, such as 6P14 single-ended small tube amplifier, 6L6 and FU-5 (805) single-ended amplifier, KT88 push-pull amplifier, etc. It has always been my dream to make a 30OB, so I ordered a kit by mail two years ago. After two years of hard work, I finally succeeded in making it. Now I share my fun with you. The final circuit is shown in Figure 1 (slightly modified from the circuit provided by the original manufacturer).

1. About 300B bile duct

In 1930, the American Wester Electric Company (Western Electric Company, referred to as Western Electric Company) produced the world-famous classical direct-heated triode electron tube 300 and 300A. At that time, the filament current was 1.0A, 1.2A, 1.4A, etc., and the power of the electron tube was also divided into 8W, 10W, 12W, etc. After several years of optimization and improvement, it was finalized as 300B in 1934 and has been used for about 80 years. Due to the reasonable internal structure design, moderate power, low internal resistance and excellent linearity of the electron tube, it has almost reached the perfect theoretical design limit. The single-ended Class A power amplifier assembled with it drives the high-efficiency horn speaker at that time, which can play smooth sound, and has attracted generations of audiophiles and music lovers. No wonder some audiophiles today describe it as the highest realm of audiophilia, a jewel inlaid in the crown of audio culture, and assert that "if you haven't heard the sound of 300B, you are not a tube amplifier enthusiast." Although this statement is a bit biased, it also explains the charm of the clear, transparent, sweet and authentic sound quality and tone created by 300B.

The correct name of 30OB should be WE300B3, because it is a patented product of Xidian. Other 300Bs such as those from Europe, Russia or my country are all imitations. Although there are no less than 30 varieties, no matter from the technical indicators or the sense of hearing, there is no one who can compare with WE300B. The development history of WE300B has also been full of twists and turns. In 1988, when transistors were popular and replaced electron tubes nationwide, the production line of WE300B, which had been glorious for nearly 60 years in the history of electron tube development in the world, was stopped. The remaining more than 30,000 electron tubes were quickly snapped up by the Japanese and French with a keen eye. As the goods gradually decreased, it became a sourceless water, causing its price to soar rapidly. In the United States, WE300B was sold at 750 US dollars per piece, and in Asia it was as high as 1,250 US dollars per piece, but it was still out of stock. Even some American tube amplifier enthusiasts who were obsessed with it had to turn to the Japanese or French for help, buying them back at prices several times or even more than ten times higher than before. It was not until 1992 that AT&T Group (WE's parent company) saw the broad prospects of the market under the historical background of the comeback of electron tubes and the requests of audio fans from all sides. They decided to spend 5 million US dollars to rebuild Western Electric, recall the chief engineer of the WE300B production line and several skilled technicians who were able to take charge of their own work, and regroup. They used new materials and new processes, and it is said that they used 7 new technologies, combined with extremely strict and even harsh management methods, to take the technical indicators of the new WE3OOB to a higher level. For example, the vacuum degree is improved 100 times than before. After the vacuum degree is greatly improved, in order to resist the atmospheric pressure, artificial crystal is added to the glass raw materials used to produce the tube shell. Not only does it make the tube look more transparent and beautiful, it also makes the sound subtly changed, and its service life miraculously reaches an average of 40,000 hours. Calculated at 3 hours of listening per day, it can be used for 40 years.

What is exciting is that during the gap between the discontinuation and resumption of production of WE300B of Xidian Company, Changsha Shuguang Factory, the largest electron tube manufacturer in my country, lost no time in cooperating with British PM Company to develop 300B and quickly launched it to the market under the brand of "Shuguang Golden Dragon" in early 1991. The new product has high quality and is close to the technical indicators of WE300B. It has attracted the attention of the global Hi-Fi community and was once called the best 300B product on the market after the original WE300B. Coincidentally, Liuzhou Guiguang Electron Tube Factory in my country also launched Guiguang 4300B (another name for 300B), which has a European style of sound and also attracted the attention of many audiophiles. What is even more surprising is that Quanzhen Electronic Tube Factory was also established in Tianjin, my country at the same time. It is said that the factory was a private enterprise established in 1998 by several retired engineers from the former Beijing Electronic Tube Factory. After Guiguang stopped production, some experienced and skilled workers also participated in it. Therefore, among many 30OBs in the world, the Full Music300B tube it produced is considered to be the only tube that can compete with WE300B. Whether in terms of theoretical parameters or listening experience, it is enough to challenge WE300B, and the price is much lower than that of Xidian products. Therefore, it is highly praised not only in China, but also among senior audiophiles in Europe, the United States and Japan.

The 300B tubes manufactured by several domestic tube manufacturers not only pushed my country to the throne of a major tube producer in the world, but also provided an excellent choice for the majority of audiophiles. The domestic 300B is of good quality and low price, making it affordable and playable for ordinary audiophiles. This is a great blessing for the domestic audiophile circle.

2. Circuit Principle

This circuit is both classic and familiar, and it has the following features:

(1) In order to give full play to the excellent potential of the 300B electron tube, the machine adopts the method of connecting two triodes in parallel in both the pre-stage voltage amplification and the driving stage. This method was once not favored by some audiophiles, who believed that the sound of the two tubes in parallel was not as clean and sharp as that of a single tube, and the resolution was reduced. However, the author does not think so. This is because: the double triode is to encapsulate two triodes with exactly the same characteristics in a glass shell. The two tubes have the same geometric shape and size, and the same materials are used. The vacuum degree inside the tube is shared equally, so the consistency of the two tubes can be fully guaranteed. Even if there are any changes in use, such as gradual aging, it is the two tubes that are synchronized. Therefore, the two tubes can be regarded as a very ideal single triode after being connected in parallel, and its technical parameters have been greatly improved, such as the internal resistance is reduced by half, and the output current is increased by 1 times, which makes the electrical performance of the entire circuit further optimized without increasing the number of amplification stages, such as widening the frequency band, greatly improving the damping characteristics, increasing the driving force, and enhancing the load capacity.

Figure 1 300B single-ended class A combined power amplifier circuit diagram

(2) The large-loop negative feedback has been eliminated. The negative feedback technology was once widely used in analog amplifier circuits, especially in transistor circuits. Due to the inherent deficiencies of transistors (such as sensitivity to temperature changes), the working stability is relatively poor. The addition of negative feedback will greatly improve the characteristics of the circuit. However, in the analog amplifier circuits of vacuum tubes, people increasingly feel that the addition of this technology is superfluous. Practice has also proved that after the large-loop negative feedback is eliminated, the sound becomes particularly vivid, passionate, and jumping, especially for the 300B tube with excellent linearity. Its sound quality and timbre are even more vivid and there is no sense of restraint.

(3) The power supply circuit has been carefully designed. First, the two channels have two independent power supplies, each performing its own function without interfering with each other, which greatly improves the separation of the two channels of the whole machine and makes the sound field and positioning more accurate; second, the rectifier uses an electronic tube, which not only makes the sound more beautiful and the tube flavor stronger, but also realizes soft start of the power supply, which plays a vital role in extending the life of the amplifier tube, especially the expensive 300B. Secondly, the filtering part of the two power supplies uses two 10H/150mA low-frequency chokes regardless of cost, that is, the high-quality L0 filter circuit is used, because the Class A single-ended amplifier has extremely strict requirements on the ripple coefficient of the power supply, and it is difficult to use the RC filter circuit. Unlike the push-pull machine, the upper and lower arms of the secondary side of the output transformer are symmetrical, and even if there are some AC salts in the power supply, they will cancel each other out. Finally, the power supply filter part, it is worth mentioning that two Kaidou brand 2OuF/400V high-quality film capacitors are added to the first stage of the filter circuit. Since the film capacitor charges and discharges very quickly, the inherent drawback of the tube amplifier, which is slow, is improved, making the sound clean and neat without any sense of drag, and the resolution reaches the level of high-end tube amplifiers.

3. Component Selection

All the tubes are made in China, and the power amplifier tubes are the export type of Shuguang 300B-98. By the way, I would like to say a few more words. In the years between the discontinuation of the old version of 300B of Xidian and the re-production of the new version, the 300B of Shuguang Factory in my country was taken back by a British company and branded as "Golden Dragon Brand". Then it was sold at a high price as a substitute for WE300B in the world. The price of the assembled whole machine was even more amazing. What's interesting is that some of our audiophiles are blindly pursuing imported tubes. The author believes that my country has become a major producer of electronic tubes in the world. At present, there are at least three manufacturers producing 300B. After the Guiguang Electronic Tube Factory in Liuzhou was closed in 2000, it was purchased by a knowledgeable person in 2007 and moved to Foshan City, Guangdong Province, the frontier province of my country's reform and opening up, and the new Guiguang NGG300B was produced at the same time. The author has not seen this tube yet. But I heard it is good. The 30OB produced in my country is of good quality and low price, and has been recognized by the world. Audiophiles can choose it boldly. The pre-stage tube is 12AX7 (two tubes are universal) from Beijing* and Shuguang; the driver stage is 6H8 produced in Nanjing in the 1950s, which has a particularly thick and beautiful sound. Later new models are 6N8P, 6SN7, etc. The influence of the inter-stage coupling capacitor on the sound should not be underestimated. The kit provides a red Kaili brand MKP capacitor. An audiophile has written an article specifically introducing this capacitor in a certain publication. Practice has also proved that this capacitor does have a good performance. Like the Xindak capacitor, it is the best among domestic capacitors. However, the author has several capacitors on hand. Audiophiles who play with machines play with the same model over and over again. The bile ducts produced by different manufacturers, different brands and different periods; the resistors and capacitors produced by different brands, different materials and different periods of the same specifications, especially the coupling capacitors, are fun to play with. Carefully experience the different timbre trends of different components and devices, train your ears, accumulate experience, increase knowledge, and get endless fun from it. Only people in the circle have this deep experience. The author has German WMA (Weima) capacitors on hand. The red ones with the words MKPIO printed on them are the best. The price is very low. The disassembled ones are only more than 1 yuan each. The sound is quite satisfactory. They were hyped for a while many years ago. The cost performance is very high. You may wish to buy a few more to try; the American VQ0.1 uF/6OOV Sibi oil-immersed capacitors are 30 to 40 yuan/piece for disassembled ones. They are heavy in the hand, the sound is steady and heavy, and the extension of high and low frequencies is better than other capacitors, but the low frequency is slightly fatter, and there will be obvious transmission after burning for a period of time. The American-made SPRAGUE has a more comprehensive performance, with a more balanced high and low frequency, especially the mid-high frequency is intriguing, but the price is a bit higher. The French SSOUEN SCR, also known as the Big S capacitor, has a good reputation among audiophiles and a high cost-effectiveness, so it is worth using. In addition, the author also has other antique capacitors such as the American-made Black Widow. Since there are too many types and different brands have different sound trends, I will not go into details here.

Here I would like to recommend two domestic capacitors to audiophiles. One is CZM-C capacitor, which is the white ceramic tube we often see, with wide pins sealed with iron hoops on both sides. The sound performance of this capacitor is extremely comprehensive, especially the sweet mid-frequency and bright treble, which is particularly suitable for listening to human voices and string music. The author uses a disassembled product, which needs to be burned in, otherwise it has a little bit of sand sound, which cannot be heard without careful taste, but this cannot be said to be a small flaw. The second is CZM-J, there are also oil-immersed capacitors marked JCZM-J, the color is mostly dark gray or dark green, and the appearance of the two capacitors is shown in Figure 2. The sound base of this capacitor is thick, and the extension of high and low frequencies is good, especially the bass performance is thick but not fat, which just makes up for the congenital defect of the lack of bass in the tube amplifier. It can cope with all kinds of music with ease, and the cost performance is also very high. If you have to find a disadvantage, it is that its size is slightly larger. In short, the timbre of the coupling capacitor is a matter of personal opinion. If you feel satisfied, then it is good.

Figure 2 Two capacitors are particularly recommended

Assembly and commissioning

When I got the kit and opened the back cover of the chassis, I saw that the power supply part had been installed and debugged by the manufacturer. The filament of the 300B power amplifier tube was powered by DC, and the 5V filament voltage and the rectifier and filter resistors and capacitors of the high-voltage power supply were neatly arranged on a printed circuit board. Two low-frequency filter chokes, 300B 920Ω/20W cathode resistors, 200uF/100V electrolytic capacitors and 1uF/400V film capacitors were placed on both sides, symmetrically designed for the left and right channels; the entire amplifier part was connected by scaffolding, because the scaffolding connection has no dielectric loss, the wiring is easier to arrange, and the resistors and capacitors can be directly welded on the pins, making the pupil line simple and short, so the distributed capacitance is also the smallest, and the sound is more beautiful. The internal wiring of the machine is shown in Figure 3. In order to further improve the signal-to-noise ratio, the input end connection uses high-quality Teflon signal shielding wire. The shielding net of the shielding wire can be grounded at one end, and must not be grounded at both ends, which may cause AC noise. The grounding of the resistor and capacitor components of each stage of the amplifier must be given enough attention. One of them must be welded on the same grounding point, which is the star grounding often mentioned by audiophiles. In particular, the grounding of the plate resistor, cathode resistor and cathode bypass capacitor is particularly important. No wire is allowed between the two. They should be directly welded between the tube pin and the grounding point. Because if the input plate resistor and the cathode resistor are far apart, there is a potential difference between the two for the AC signal, that is, an AC signal source is added between the plate and the cathode of the tube. After the subsequent stages are placed, the crying sound in the speaker can be imagined. In order to minimize the distortion of the amplifier and make the two channels more balanced, no bypass capacitor is added to the cathode of the input stage and the driving stage amplifier tube, that is, current negative feedback is applied to both stages. At the same time, in order to ensure the transient response characteristics of the whole machine, in addition to canceling the large loop negative feedback network, only a shallow inter-stage negative feedback network is added between the cathode of the input tube and the screen of the driving tube. As can be seen from the figure, the negative feedback resistor is as large as 30OkΩ. The author has also tried to simply cancel this network, but the distortion increased significantly. When the resistance was increased to 270kΩ, the sound was still a little dark and tight, and a little restrained. Only when it was increased to 3OOkΩ, the sound had no obvious distortion and sounded fresh, grand, and very pleasant to listen to.

Figure 3 Internal structure of the power amplifier

Specific debugging As mentioned earlier, since this circuit is classic and mature, a reasonable design is used to ensure the various technical indicators of the entire circuit. As long as the selected components are of high quality and are welded strictly according to the diagram, it can work almost without adjustment, but the author still carefully adjusted the machine. The method of debugging the machine is somewhat special. Of course, adjust the power supply first, and then adjust it step by step from back to front. The advantage of this is that because the power consumption of the power amplifier stage accounts for more than 80% of the power of the whole machine, only after the power amplifier stage is roughly adjusted, the driving stage and the input stage are easy to adjust. Some enthusiasts adjust from front to back, and some adjust the front stage without plugging in the power amplifier tube first. It should be noted that at this time, the load is too light and the voltage remains high. Once the power amplifier tube is plugged in, the working point adjusted under such a voltage will drop sharply and need to be readjusted, which is more troublesome. Regarding the adjustment of the power amplifier level, considering that most audiophiles (especially those who are just getting started) do not have special test instruments, the author's approach is as follows: first adjust the machine so that it can generally work, connect the ammeter in series with the 300B screen electrode, and measure the static current. After the static current stabilizes, send an audio signal to the input end. If the screen current becomes larger at this time, it means that the absolute value of the grid negative voltage is too small, otherwise it is the opposite. Since 300B adopts a self-supplied grid negative voltage method, the voltage value across the cathode resistor is the grid negative voltage value, and the size of the grid negative voltage depends on the size of the cathode resistor. The resistance value of the 300B cathode resistor is generally selected between 880 and 1000 Ω. After fine-tuning, when the cathode resistor is adjusted to 920 Ω, the signal is sent to the input end, and the 300B screen current changes the least, indicating that its working point is roughly at the midpoint of the straight line part of the screen current. Practice has proved that the distortion is also the smallest. At this time, the voltage between the plate and the ground of 30OB is 400V, the plate current is 70mA, and the cathode voltage is 64V. The tube voltage (the voltage between the plate and the cathode) of 300B is 400-64=336(V); power calculation: 336×0.07=23.52(W), the efficiency of the single-ended Class A tube amplifier is calculated as 1/3, that is, 23.52÷3≈8(W). The debugging data of the drive stage and the input stage are marked on the figure. These data are measured by the author using a 500-type multimeter. If audiophiles use other meters to measure, there may be discrepancies. As long as the difference is not big, it can be regarded as normal.

Actual listening

I use this amplifier with Shanling SI00CD player (after careful polishing and upgrading), three-way floor-standing speakers (10 inches + 65 inches + 1 inch), sensitivity 90dB, frequency response 30Hz ~ 27kHz. My listening habits are also somewhat different. I like to make a cup of Biluochun or West Lake Longjing in the evening, turn on the system, put on my favorite CD, sit on the sofa, turn off all the lights, close my eyes, and taste it carefully. Music needs to be understood with the heart, so whether listening to Teresa Teng's lyrics or Tsai Chin's confession, the velvety smoothness makes people forget the existence of the speakers, just right to integrate the singer and the listener. Teresa Teng's famous album "Light and Subtle Feelings" depicts the last emperor of the Southern Tang Dynasty, Li Yu, who was a ruler of the highest power, being reduced to a prisoner. The psychological impact caused by this huge gap makes people sigh from the heart, "How much sorrow can a ruler have, just like a river of spring water flowing eastward." This emotional masterpiece can only be presented to the listeners through Teresa Teng's outstanding mid-high pitch and lyrical singing. Her magnetic, gentle and melodious singing is refreshing and tear-jerking, and at the same time makes people fascinated and intoxicated. Unfortunately, we have no fate and it is impossible for us to go to the scene to listen to the original singing of this singer in person, but the 30OB tube amplifier performs this ancient masterpiece vividly and deeply. At this time, people can't help but connect it to the untimely death of this superstar in the music world. While making people feel deeply, they sigh secretly: time is ruthless and life is short. After the record was over, people still sat on the sofa stupidly and continued to immerse themselves in the charm of music, and they couldn't come back to their senses for a long time.

Let's get back to the point. Listening to string music such as guzheng or violin can show the performer's inner world to you without reservation. When listening to string solo, even the position of other instruments in the accompaniment can be clearly heard. The author is not good at describing the sound with words, especially the heavenly sound of 300B, which is known as the king of gallbladders. I can only say that listening to 300B is about charm. It is this charm that has attracted generations of people and is the dream "lover" that many audiophiles and music lovers have been pursuing tirelessly for 80 years. 20 years ago, those who heard the sound of 300B were proud of it. Owning a 30OB tube amplifier is a symbol of identity, status, qualifications and economic strength in the audiophile circle. But today, 300B tubes are not rare in the domestic audio market. No wonder some senior audiophiles sighed: Today's audiophiles are both lucky and happy. Why don't we take this opportunity to DIY and satisfy our 300B addiction!