A Brief Discussion on Tube Amplifier

                                          A Brief Discussion on Tube Amplifier

I have been in contact with tube amplifiers for more than ten years. The first one I bought was the Japanese Lux EL34 tube amplifier. Similar to the original audio, it was also out of interest in technology. From then on, I began to explore tube amplifiers.

In recent years, I have tried to make tube amplifiers by myself. I have DIYed single-ended, push-pull, preamplifiers, phono amplifiers and other models. Because there are still some basics, I always design, choose and debug according to my own ideas. Now the results are not good. Yes, I have some superficial experience. At present, I often listen to a 2A3 single-ended machine and a pair of mono 6L6 push-pull machines. That tens of thousands of Lux tube amplifiers have become decorations.

I think the biggest advantage of tube amplifiers is that the sound is good, the sound is harmonious and natural, especially the treble is sweet or melodious. If the DIY process is not deliberately raised, such a sound is relatively easy to reproduce. On the contrary, some of its weaknesses are overemphasized. Such as output power, low frequency control, etc. may be counterproductive, and those are the strengths of stone machines.

Another advantage of tube amplifiers is that they have a good tolerance and a wide range of parameters, but this does not mean that DIY can mess around. We must respect science, at least ensure that the circuit is correct and the components are free of defects. For the same reason, it is not recommended to challenge technology that is difficult to support. Models, this is not a matter of chance.

The third advantage of tube amplifiers is simplicity and directness, which is very different from the defense of stone machines everywhere. It is natural to have less noise interference and freshness preservation. This also reminds us that DIY tube amplifiers should be simple rather than complicated, with less fancy functions and self-seeking. Worrying design is good for everyone.

The predecessors of the amplifier circuit framework have designed it, and there are not many commonly used amplifiers. Find ready-made drawings or draw by yourself. , It can also avoid being misled by unreliable so-called ready-made drawings.

The circuit design can learn from mature circuits such as famous machines, especially the details, take the essence, there should be no obvious bottlenecks such as distortion and bandwidth on the circuit, and there is no need for excessive idealization. Priority and second, stability is the first priority of listening experience. The listening experience will vary from person to person, from system to system, and from music type to music type. The most important thing is to feel good yourself.

The design itself is a process of compromise, with a high starting point and a final landing. For example, sense of hearing and efficiency are a pair of contradictions. Convenience, gain, conversion, etc. are all efficiency, and power can also be regarded as an extension of efficiency. If you want to have a good sense of hearing, you have to endure low efficiency. It does not mean surrender, and the cost can be reduced by means of technology, cost, and complementarity.

Tube amplifiers pay attention to the optional materials, because the status of simple circuit components is elevated, especially some nonlinear components can even affect the final sense of hearing, such as output. In fact, all audio products are of this virtue, the frequency band is narrow, the distortion is high, and the cost is high. It is only because the power output has no choice but to use it, which makes it the last gate.

And capacitors. Different materials, capacities, and even pressure resistance will affect the sense of hearing. I have compared various coupling capacitors such as film, paper, and oil. I have the best impression of polystyrene metal foil capacitors. I like the transparent and fullness. Bypass, decoupling, and filter capacitors have also undergone multiple tests, and the effects are clear, such as clarity, fineness, freshness, etc.

Electronic tubes are now called tubes, because they can be replaced at any time and I treat them as independent accessories. Different from the two types of passive components listed above, it is an active nonlinear component. Not only its own brand, origin, age, etc. are related to the sense of hearing, but also work under different voltages and currents.

I would recommend choosing a new power tube, because it is easy to match, which is especially important for push-pull machines, and it can help reduce odd harmonic distortion. The voltage gallbladder affects the tone. If you like the sound of the law, I recommend the new domestic gallbladder. If you want to pay attention to the charm, you can choose the antique famous gallbladder. The price is acceptable. The first choice is European, followed by the United States before the 1950s. This kind of old antique should be retested on the machine after returning. If there is a deviation in the G/GT type gallbladder parameters or occasional noise, it may be better to re-weld the gallbladder feet.

About the bottom noise of the amplifier. This problem depends on how it came about. It must not be caused by DIY mistakes, such as unreasonable structure, improper grounding, poor power filter, etc., and the noise of components and AC filaments, etc., if the treatment is too clean, it may affect the sense of hearing. , as long as the speaker is 20 centimeters away in front of the speaker and it is quiet enough, I can accept it. In fact, the environment itself has background noise, and the cleaner the music, the better it sounds.

As mentioned above, stability comes first, which mainly means that when the tube amplifier is working, the tube will not deviate from the working area due to signal changes, the power supply voltage will not fluctuate greatly due to the increase or decrease of output power, and the whole machine will not be self-excited due to poor design. wait. The self-excitation phenomenon is sometimes very tricky, and it cannot be found without power transmission, especially in circuits with loop negative feedback above the third level. To deal with high-frequency phase-shift self-excitation, it is best to reduce the amount of negative feedback instead of compensation, otherwise it will affect the high-frequency hearing. To deal with "beep" sound and low-frequency self-excitation, you must first check the decoupling at all levels, and the working voltage should be from the rear stage to the front. With a gradient, it can also moderately reduce the RC coupling time constant and increase the ratio of screen resistance to biliary internal resistance, etc.

Listening and debugging is a headache. It is almost impossible to have no problems. The most common problems are excessive noise or slight self-excitation, but you will always find a solution. The current status is numbered. To understand the current state, some test tools are inseparable. In addition to multimeters, such as oscilloscopes, function signal generators, AC millivoltmeters, autovoltage regulators, etc., only in the correct state can you not go wrong when combined with auditory debugging.

The sense of hearing is not without rules. For example, the low-power single-ended machine is fresh and sweet, and the high-power push-pull machine is melodious and full. Different circuits, parameters, and materials have corresponding senses of hearing, such as the sound of raising the voltage of the screen electrode. The strength is strong but may feel rough or thin, increasing the current sound density seems to be strong but will lower the voltage, etc. When debugging and tuning, I think that in addition to following certain rules and DIY experience, my own understanding of music and good sound is also very important.

Finally, I would like to talk about the two tube amplifiers I am using. I mentioned at the beginning that they are 2A3 single-ended amplifiers and 6L6G push-pull amplifiers. Why are these two tube amplifiers?

The 2A3 single-ended machine was my first DIY show. Although I had made enough theoretical preparations at that time, there were still many uncertainties. I hoped to be more secure when delineating a low-power single-ended machine, and it happened to be a different style from the Lux amplifier. 300B has also been considered, because it has too high requirements on the driving voltage, and it is difficult to meet the standard if you don’t want to use a three-pole tube for pentapole tubes. In addition, it was rejected due to cost factors.

I believe that the correct sound must have low distortion, which is the direction rather than the indicator. This concept has always been in all my DIY processes. The linearity of the 2A3 tube itself is good, and the driving voltage of 50V is suitable for many voltage tubes, including the 6SN7 that I admire the most. Not to mention the smallest distortion, the more important thing is that the harmonic distortion of the odd order is much lower than that of the even order, which will lay a good foundation for the later tuning.

The output power of this machine is small, but it is more than enough to match my pair of 93dB Tannoy cobalt magnetic speakers. The sound is very ear-catching. I like the vocals and violin music it plays the most.

The low-power single-ended machine has a beautiful sound but lacks momentum, while the push-pull machine has high efficiency, and there is no DC component restriction in the output, so the performance can be fully utilized, which should make up for the above problems, but this machine has odd harmonic distortion. It is too large, and the power gallbladder has a great contribution.

Check the tube manual to know that under the standard connection method of 6L6 series power beam tubes, when the load impedance is lower than 5KΩp-p, the harmonic distortion of the odd order is much smaller than that of the even order, and the power is also moderate. There is no reason not to choose it, and let it work at the same time. Class A, set the static balance adjustment potentiometer, etc., to further reduce the odd harmonic distortion, and the pre-stage part still uses the voltage tube 6SN7 with the smallest distortion.

The listening experience of this machine is different from that of the single-ended machine, and it exceeds expectations as expected. The most obvious is that it has a greater sense of scale, large lung capacity, quick movements, good texture, and a little stickiness. Playing strings and symphonies is not enough to listen to. The feeling that the so-called poisoning may be nothing more than that.

Written by : Davecl Audio.
The Audio Equipments Manufacturer
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