The history of tube amplifier

It has been nearly a century since the invention of the electronic tube. Since the late 1950s, due to the appearance of transistors, the territory of the electronic tube has gradually shrunk, and it has been almost forgotten. Due to energy policies, electronic tubes have even been restricted. There is also an exaggerated publicity for transistors, so the tube is almost extinct. If it had not been used in high-end audio these years, many young people may not know what a tube is now.

Vacuum tube

The invention of the electronic tube dates back to the 1980s. At that time, the great inventor Edison in the process of improving the incandescent lamp unexpectedly discovered that an additional electrode was added to the bulb and heated by electricity. The filament will discharge to the charged electrode, that is, a current is generated in the electrode and the filament circuit. This phenomenon is called the "Edison effect" in the physics book in our middle school. The important role of this discovery was not noticed by Edison at the time. It just registered a patent. By the beginning of this century, the British scientist J.A.Fleming developed the "Edison effect" into a diode for rectification and detection, and named it Bulb or Valve. Since then Fleming Bulb has been laid The basis of the electronic tube. After two years, Lee Do. Forest of the United States put an additional electrode (Grid and grid) into the diode. They found that this kind of tube can detect and amplify. Since then, the electronic tube has developed a new era.

In 1913, the electronic tube was first used in regenerative receivers. But when the electronic tube was used in the audio began, I can’t say exactly. The earliest information that can be found is about 30 in this century. At the beginning of the decade, the Western Electric company in the United States invented the WE300A (the predecessor of the famous 300B). The RCA Company in the United States invented the 2A3. At that time, the two tubes still shining now began to play A glorious page was written in history.

WE300A is used in the company's WE86 audio film amplifier, and 2A3 is installed in RCA's luxury record player-Electroller D22. Because WE300A is used in professional systems, it is rarely noticed. 2A3 is used In the home system, it was watched. At that time, enthusiasts began to use 2A3 push-pull to make 22-watt "high-power" amplifiers. After this, about 300, WE300A was improved to WE300B, this tube is in Japan Known as "Dream Ball". Many audiophiles are fascinated by it. I think its timbre cannot be surpassed. The WE300B was only discontinued in the late 1970s. This tube is produced in the UK and Japan with similar specifications. The model produced in the UK is called 4300B.

From the 1920s to the 60s of this century, it can be said to be the heyday of electronic tubes. Since the 1920s, in order to improve the high-frequency characteristics of transistors, quadrupole and pentode have been gradually developed, as well as many special tubes and ultrahigh frequency power tube. Such as magnetron, microwave tube, metal ceramic tube, this kind of tube still dominates this field. Many of the tubes used in our audio have basically been born around the 1940s and 1950s. Of course, at that time, there was no intention to produce special tubes for audio. The large number of electronic tubes used in audio was after the end of the Second World War. At that time, a lot of spare materials (including electronic tubes) were idle into the market after the war, and many audiophiles have the conditions to make their own amplifiers and publish a large number of application articles.

At that time, the more used and better reflected tubes were: 6V6, 6L6G, 6F6, 807, 805, 811, 813, 826, 829, 845, 211 and so on. The first Hi-Fi amplifier was marked by the "Williamson" amplifier that was born in 1947. In 1948, the compact disc was born, providing a high-fidelity signal source, which further stimulated the development of Hi-Fi amplifiers. Around this time, the British G.E.C company appeared the KT66 improved from 6L6 and 807.

6L6 tube amplifier

PHILIPS produced EL34. Then G.E.C. launched KT77 and KT88. The GE Company in the United States also launched 6550 accordingly. These tubes were not specifically developed specifically for audio and are mostly used for instruments. In the 1970s, Japan’s NEC company was commissioned by Japan’s LUXMAN to develop several audio special The tube. Such as 8045G, 50CA10, etc. 8045G is a peculiar tube, it sounds very good, but the life is very short. The filament voltage of 50CA10 is 50V (Japanese power supply voltage is 100V , two transformers can be omitted in series). The tube is actually a beam-emitting quadrupole , but it is internally connected as a triode. In the 1970s, in addition to this counter-current work in Japan, the entire world of tube production was at a low ebb. At this time, many tube manufacturers have cut production, stopped production or discontinued.

With the resurrection of tube audio in the early 1980s, the market and manufacturers' demand for electronic tubes has increased again. At this time, there are almost no factories producing electronic tubes in Europe and the United States. Even if there are still some in production, the price remains high. So some smart tube distributors began to come to China to find sources of supply. China's Changsha Shuguang Electronic Tube Factory started taking orders to produce audio tubes after 83 years. Prior to this, the factory mass-produced models were 6V6 (6P6P), 6L6G (6P3P), 807 (FU-7), 805 (FU-5), etc. There are some thumb tubes such as 6N2, 6K4 and TV tubes such as 6P13P.

 

6K4 tube

Therefore, the factory has more production experience in the production of power tubes. The first audio tube produced by the factory is EL34. Since then, KT88, 6550, 6L6GC, 50CA10, 2A3, 211, 845, 50CA10, 6336A, etc. have been developed and produced one after another, and a large number of audio power tubes such as KT100, 300B, etc. 5U4G, 5AR4 audio rectifier tube. The second branch of China's Beijing Electronic Tube Factory has also modified and adjusted the original design according to the needs of foreign markets, and developed and produced audio-specific thumb tube. Such as: 12AX7A, 12AT7, 12AU7, 6DJ8, 6GH8, EF86, EL84, etc. Since then, China has become a major country in the production of dedicated audio tubes.

The sound quality and sound quality of the electronic tubes produced in China are better than similar tubes produced in other countries (especially power tubes), and even more than the original ones. The reasons are not only the differences in materials, but also the differences in structure. For example, the KT88 produced in China is almost the same as the original one (the maximum screen pressure is slightly lower), but the external dimensions and internal structure are different from the original design. The original KT88 is larger than the Chinese one, and the external dimensions of the 6550 in China are the same as the original ones, and the internal structure is far from the same.

The parameters of the original 6550 are basically the same as KT88, and the highest screen voltage is 150V lower. The sound quality and timbre are also inferior to KT88. China's 6550 is different. China's 6550 is actually the same die as the Chinese-made KT88, but the glass bulb has a different shape. Therefore, China's 6550 is an overall winner over the original 6550. According to personal use experience and the introduction of Shuguang factory experts, in fact, the performance of China’s 6550 is better than that of China’s KT88. Although the cores are the same, due to the different shape of the glass bulb, the sealing temperature of the glass bulb is different.

6550 tube

The shape of 6550 is good for sealing at lower temperature, and the sealing temperature is about 100 ℃ lower than KT88. Therefore, the internal oxidation of the 6550 is much lighter, which brings the 6550 better than KT88 in stability, consistency and yield. Of course, the strict screening of KT88 is also good.

In addition to the production of electronic tubes in China, several countries in Eastern Europe also began to produce audio tubes in the late 1980s. Including the Czech Republic, the former Yugoslavia and the former Soviet Union, their joining has a great impact on the production of electronic tubes in China. Although the sound of tubes in China mentioned earlier is better, there are more quality problems. Besides, their geographical location is also more favorable than that of China. At present, these countries have developed and produced many audio tubes, such as: 5881 (6L6GC premium product), EL34L, KT77, KT90 (KT99 and KT90 are actually the same tube, but the tube base is slightly different), 5AR4, etc.

KT100 is produced under the background of KT90 (KT99). It is based on KT88 and has made many improvements in the materials used. For example, the anode material has been changed from iron-based lead-nickel-coated to copper-based. The sound quality and timbre have been greatly improved. For example, the maximum anode power has been increased from 42W to 48W, and recently developed and produced 6L6GCWB (similar to 5881), EL34C, etc.

KT100 tube

We talked about the past and present situation of many electronic tubes in the front. Let's talk about all the tubes in the tube. The amplifier is similar to the crystal machine, which has a pre-amplifier, a post-amplifier and a combined amplifier. In the pre-amplifier, the main function of the tube is voltage amplification, so the tube is required to have a larger amplification factor and less noise. This is especially true of the pre-amplifier with phono.

In the early years, pentodes (such as EF86, 6AU6 and high-µ low-noise tubes such as 12AX7A (ECC83) were generally used instead of pentodes, but 12AX7A, 12AT7, 6DJ8 (ECC88), etc. were also used. However, 6SL7 and 12AU7 are also useful. However, 12AU7 is generally only used in the front output stage. The pure rear stage and the merger without the front stage are basically the same in circuit structure, generally there are buffer stage, voltage amplification stage, and inverting stage (for push-pull), the promotion level and the final level (strong release level).

At present, it is rare for general machines to have a buffer level, and most of them are not used. In the famous machine, Marantz9 has a first-level buffer level, and the tube used is 6DJ8. The voltage amplification stage generally requires a larger voltage amplification factor. At the same time, it is necessary to consider what kind of tube to choose according to the size of the negative feedback of the whole machine. Here, the noise requirements of the tube are not as high as those of the previous stage. , Medium μ tube, commonly used is 12AX7A, 12AT7, 6DJ8, 6SL7. It is also useful to use a pentode as a triode connection method.

12AX7A tubes

Commonly used EF86 (6267) is also used in low and medium µ tubes, such as 12AU7 and 6SN7, at this time mostly for non-negative feedback and low feedback machine selection. For example, Jadis’s JA-30 voltage amplifier stage uses 12AU7, for the push-pull machine, an inverting stage is required after the voltage amplification stage. This stage is mostly used for medium and low µ tubes, such as 12AU7, 6DJ8, 6SN7, 6FQ7 / 6CG7, 12BH7, 5687 and so on. There are also a few that use medium to high µ tubes, such as 12AT7 and 12AX7 (such as Jadis's JA-30 is 12AX7A).

For some machines with large output power (usually more than 70W for push-pull machines) or the input impedance of the final stage is low, it is often necessary to use a negative-level output for the first-level push-level. , Commonly used are 12BH7, 5687, 6FQ7, 6CG7, 6SN7 and so on. 12AU7 and 6DJ8 are also used, but the effect is not as good as the previous tubes. This level also has requirements on the power of the tubes used, I hope it is better.

The levels discussed above are a relatively standard circuit, but there are many idioms. Such as using a tube to do all the pre-level work; Such as using a WE310A or 6SJ7 or 6BA6 pentode to push 300B for single-tube Class A work. Use a composite tube or twin tubes for pre-level work, for example, use one 12BH7 to push one 300B, one 6BM8 to push 211, and one 5687 to push 845 for single-tube Class A work.

There is also such a typical example in the class AB push-pull. For example, the historical Dynaco MK II uses a 6AN8 to push a pair of 6550, and the Dynaco70 uses a 7199 to push a pair of EL34. That is, the pentode part in the composite tube is used for voltage amplification, and the triode part is connected as a P-K split inverting and driving. The output power of the whole machine with this connection will generally not exceed 50W. When the driving power requirement is large, it is also useful to connect EL84, EL34, 6L6GC or 6V6 into a triode for driving.

There are many tubes used in the final stage, currently the most commonly used are: KT88, KT100, 6550, KT90 (KT99), EL34, 2A3, 300B, 211, EL84 and 807. Generally, KT88, 6550, EL34, EL84, 2A3, 807 are basically used for push-pull operation (also in 300B), and there are also multiple tubes in parallel with push-pull, while 300B and 211 are mostly used in single-tube class A circuits. There are also many tubes that are very good in audio, and are not used much for various reasons, such as 6L6GC, 845, 6GB8, KT66, KT77, 50CA10, etc.

6L6GC tubes

There are also some used by audiophiles in the early years but not used in commercial machines, such as 811, 813, 826, 829, DA30, DA100, PX25, EL156, 2E22, etc., the final selection is generally based on the power The size, requirements for sound quality and timbre and the tube you can get. If you play by yourself, in principle, any tube can be used. If it is a commercial machine, you should pay attention to choose the more popular model. The reason is very simple. First, the tube replacement is easy, the second is to give enthusiasts a chance to "play the machine."

One of the characteristics of the electronic tube is that the same type of electronic tube produced by different manufacturers in different countries has some slight differences in sound quality and tone. Enthusiasts can change the tube that suits their taste according to their own preferences. This is convenient for audiophiles and also provides audiophiles with an opportunity to play on their own. Some models of electronic tubes in China also have good performance on audio, such as 6N1, 6N3, 6N6, 6C3, 6F1, 6F2, 6C19, 6C16, etc., which are rarely used by commercial machines for the above reasons.

The above mentioned is a tube amplifier with an output transformer. The requirements for the final tube in OCL and OTL tube amplifiers without an output transformer are mainly low internal resistance and good consistency. This kind of pipe has 6336A, 6080, 6AS7G (equivalent to 6N5P or 6N13P), and China 6C19 these pipes have low internal resistance, but the consistency must still be selected to meet the requirements.

6N5P tube

The output transformer

Throughout the history of tube amplifiers, there are few historical devices using this circuit structure, while modern tube amplifiers have few. Why this kind of circuit can save the throat part of the amplifier-the output transformer, but why not get a lot of applications? I think there are several reasons.

First, the amplifier is not only subject to the output transformer, but also benefits from the transformer. The amplifier without the output transformer and the amplifier with the output transformer are very different in sound quality and timbre. The "bile taste" of a tube amplifier without an output transformer is very special, and there are few people who like it. Besides, the frequency response of modern output transformers is no longer a big problem. For example, the frequency response of the Japanese TANGO brand and TAMRADIO brand output transformers has reached 5Hz-100KHz ± 3dB.

Second, it is not easy to match amplifiers without speakers and speakers. OCL and OTL circuits are suitable for matching high-impedance speakers, preferably above 16Ω, but 4Ω and 8Ω are not suitable.

Third, stable work and debugging are not easy. Stable work is very important. Since a pair of slightly better speakers now cost several hundred dollars, I don’t think anyone wants to let their beloved speakers work with an unstable machine. I think these reasons may be the reason why the amplifier without output transformer is not easy to promote.

Let's talk about the use of some electronic tubes. Some beginner fever friends do not understand the electronic tube and suffer from "cholephobia". One is that the electronic tube will explode, the other is the high voltage of the electronic tube, and the third is the short life of the electronic tube. Some people think that electronic tubes are not as good as transistors. Actually these friends don't know enough about the tube. The electronic tube itself will not explode, at most it will crack when it is not used properly, such as being strongly impacted during use, or the liquid falling on the electronic tube being used. Attention should be paid to high pressure in the process of installing the bile machine, but users of bile machine high pressure generally do not have to worry, and designers of bile machines generally pay more attention to the treatment of high pressure.

Vacuum tube

Except for a few models, the high pressure of the electronic tube is introduced at the top of the tube (such as 807, 805), the rest are at the bottom, and the safety measures are introduced at the top, so the user can be completely assured. Regarding the service life of the electronic tube, it was established from the military standard in the early years. At that time, the military standard required extremely high. The manual gives 500 hours or 1000 hours, which means that all the parameters of all tubes in this range are not guaranteed below the given index.

Judging from the actual use, the life of the thumb tube is generally 10,000 to 20,000 hours, and the large tube is more than 2,000 hours. It is not a problem to use a tube like 300B for 5 or 8 years. On the other hand, the reliability of the electronic tube is extremely high, and the overload capacity and impact resistance are incomparable to the transistor. Transistors may be damaged within 10,000ths of a second, and electronic tubes cannot be damaged in 3 to 5 minutes in a harsh environment.

Because of this, the crystal machine needs to protect a lot of circuits, but the amplifier is almost not used. This is what many audiophiles don't understand. Friends who think that the electronic tube is behind the times are not as good as the transistor. This is after all a minority. In this day and age, the vacuum tube machine is still popular in the world, and the fact that it is equally divided with the crystal machine in the Hi-end market does not need to explain this problem. Because the tube is stable and reliable, the circuit of the amplifier is much simpler than that of the crystal machine. A 50W two-channel machine generally has 8 tubes, and the same crystal machine does not have 30 transistors.

Vacuum tube

To really judge the quality of a tube, it still needs to be actually used or tested with an instrument.

As long as the tube is in use, it is necessary to pay attention to the voltage of each pole matching the design value. The main point is to pay attention to the negative voltage of the gate. For the thumb tube, the self-sufficient gate bias is generally used. Most of the tubes are fixed (external) grid bias voltage. Generally, the grid bias voltage is most negative (maximum) before starting, and then the negative grid voltage is gradually reduced based on the monitoring screen flow after starting.

Also note that for new tubes that have not been aging, do not adjust the screen flow in place when adjusting the bias voltage. It can be adjusted to one-third of the normal value at the beginning. This is because the cathode oxide of the new tube is activated slowly, and the electron emission gradually becomes normal. It should also be noted that the filament voltage should not exceed 5% of the rated voltage. If it exceeds 10%, it is easy to cause the screen to become red.

With the advancement of modern technology, the life of the electronic tube has been extended several times, and the older generation of audiophiles who are tired of the hard and dry digital have nostalgia for the long-lost sweetness of the electronic tube. Coupled with the urgency and help of many manufacturers, this old man who has been living for more than half a century has finally revived the prestige of the fifties!

AIYIMA