Several technical issues in tube amplifiers

In the final stage of the tube amplifier, there are push-pull working mode and single-ended working mode. Since it is easier to obtain high power by using the push-pull mode, it is a very common circuit form. However, since the push-pull working mode is a superposition mode, there are some distortions objectively, and there are additions and subtractions in the push-pull superposition. In this addition and subtraction, some small things that were not originally there may be added, and some small things that were originally there may be subtracted. If a single tube is used in the final stage circuit to work in the single-ended Class A state, there will be no problems that cannot be avoided by the push-pull working mode. Therefore, the single-ended is much better than the push-pull in terms of listening, especially in some tiny details. However, it is difficult to make the single-ended power very large. For example, using the same type of tube, it can only achieve 10w in single-ended mode, but it is easy to achieve 30w in push-pull mode. It costs some price to increase the power. At the same time, in terms of technology, the single-ended machine is more difficult to handle than the push-pull machine. Therefore, the single-ended circuit is often used in high-end machines, and the push-pull circuit is used in popular machines.

2) The difference between different connection methods of electron tubes in the final push-pull circuit

The electron tubes used in the final push-pull circuit are often tetrodes and pentodes, so when using these tubes, there are three-stage tube connection, super-linear connection and standard connection. The difference between them is that the three-stage tube connection has the smallest distortion and the smallest output power; the standard connection has relatively larger distortion and the largest power; the super-linear connection is between the two. In terms of listening, each has its own advantages, and the three-stage tube connection is relatively better. However, the three-stage tube connection has a loss in the life of the electron tube, so it is rarely used in industrial production.

3) The impact of the driving circuit on the sound quality and tone

Generally speaking, the driving circuit and structure have a great effect on the orientation of the sound quality and tone. Different driving circuits can achieve different effects on the low-frequency strength of the sound, the speed of the mid-high frequency and the density of the mid-frequency. There are many kinds of driving circuits, and it is difficult to judge the quality of the product from the difference in driving circuits. Choosing what kind of circuit is entirely a choice of the designer's tone orientation.

4) The impact of different types of tubes on sound

The common electronic tubes used in the pre-stage driving circuit are elf82, 6f1, ef86, 12ax7, 12au7, 12at7, 12bh7, 6dj8, 6sn7, 6sl7, 6sj7, 6n1, 6n2, 6n3, 6n6, etc. In principle, these tubes can make good sounds when used in tube amplifiers, but each model has its own characteristics, and designers will decide which model to use based on many factors. Generally speaking, elf82, 6f1, ef86, 12ax7, 12au7, 12at7, 12bh7, 6dj8, 6sn7, 6sl7, and 6sj7 are the most commonly used electronic tubes in foreign audio, and many manufacturers produce them, so they are interchangeable, so they are commonly used in exported or foreign machines. The commonly used electron tubes in the final stage circuit are: kt88, 6550, el34, 6l6gc, 2a3, 300b, 211, 845. The first four electron tubes are side-heated tetrodes or pentodes, which are often used in push-pull circuits with higher power. The last four electron tubes are directly heated triodes, which are more commonly used in single-ended Class A (2a3 and 300b are also often used in push-pull circuits). Relatively speaking, the timbre of directly heated triodes is slightly better than that of side-heated quadrupoles and pentodes. However, although they are all triodes or tetrodes, the sound quality and timbre of each model have some differences and characteristics. Since tube amplifiers are plug-in devices, it is convenient to replace them directly, so changing tubes and playing with the machine has become a great pleasure in the use of tube amplifiers.

5) The influence of output transformer on tone

The output transformer has a great influence on the indicators and listening experience of the whole machine. The bandwidth of the excellent push-pull output transformer is 10hz-100khz, and there is no problem at all if the distortion is below 1%. It can be said that the transformer is no longer a key component that affects the indicators of the tube amplifier. However, the structure, process, and material of the transformer still have a great influence on the sound of the whole machine. In fact, after the indicators of the transformer exceed a certain range, the higher the indicators are, the better they are. If the tube amplifier does not have an output transformer, such as OTL, its listening experience will be different from that of the traditional tube amplifier. Therefore, the timbre of the tube amplifier is greatly related to the output transformer.

6) About integrated amplifiers and pre- and post-amplifiers

The integrated amplifier has the following characteristics: 1. When the signal source is at a certain input level, the output of the amplifier can reach full power; 2. The amplifier has multiple groups of signal source input options; 3. The amplifier has a level control function; 4. The left and right channels are combined into one, and a high and low tone control device can also be provided. In the early days, the output level of the signal source was relatively low, generally around 0.2V, so the input level of the integrated amplifier had to be below 0.2V, but modern signal sources have changed a lot. For example, CD players have been widely used, and the output level of modern signal sources is between 0.5-1V, so the input sensitivity requirements of modern amplifiers have also changed accordingly. Of course, no matter how it changes, as long as the first three requirements of the integrated amplifier are met, it is an integrated amplifier. The pre- and post-amplifiers integrate 1 source selection 2 level control 3 voltage amplification into one (those with the 3rd item are active pre-amplifiers and those without the 3rd item are passive pre-amplifiers). The pure post-stage integrates the voltage amplification and power amplification into one (or two) body with one left and one right input, no level control and source selection (input level is between 1-2v). This approach is more reasonable in structure, distribution and materials. Therefore, the grade of the pre- and post-stage split amplifiers is higher than that of the combined ones, and the price may also be much higher.