FU-7 Tube Amplifier Homemade

The sound of the 300B Class A single-ended output tube amplifier has fascinated countless audiophiles, and some even describe it as the highest level of audiophilia. However, the power of less than 10 watts limits it from being paired with some inefficient high-quality speakers, which is a great pity. If there is an excellent tube amplifier that has both the sound quality of the 300B single-ended output and the power of the 300B push-pull output, and the dynamics, damping, and resolution are all better than it, I believe it is the dream of many audiophiles. Unfortunately, the price of the tube amplifier that can meet this requirement is no longer affordable for ordinary people. However, for "homegrown" audiophiles, it is possible to use their expertise to weld a tube amplifier with excellent sound quality by themselves. Moreover, there is no secret in the technology of tube amplifiers at present. The debugging points and manufacturing process are very mature. As long as you are willing to do it, you will succeed. The only difference is the grade of the sound.

This article introduces a "homemade" tube amplifier to audiophiles who like to get their hands dirty. Its mid- and high-frequency feel is as good as that of a single-ended Class-A amplifier made with 300B, and it even surpasses it in terms of sound density, texture, and resolution.

Principle introduction :

The voltage amplifier stage uses a 6DJ8 (6N11) connected to form an SRPP circuit. This is a circuit that has become popular in recent years. Compared with the traditional classic common cathode amplifier circuit, it has high resolution, good high-frequency response, low output impedance, and easy debugging. The tone is clearer and smoother than the common cathode amplifier circuit, and the speed is faster. It can be understood as focusing on the characteristics of "sound", but it is not as strong as the "tube flavor" of the common cathode amplifier circuit. This stage plays a key role in the tone of the whole machine. When 12AX7 (6N4) is used to connect to the common cathode amplifier and the working current is selected at about 3.5mA, the host can obtain a more "musical" performance. The second stage uses 6SN7H (6N8P) plus a constant current source to form a long-tail inverter circuit with a gain of about 15dB. Transistor BG1 and D1, R11, and R12 form a constant current source of the inverter circuit, which is beneficial to the balance of the two arms of the circuit and reduce distortion. The working current of the inverter stage has a great influence on the sound quality of the whole machine. Please adjust R12 to 4-8mA. The author takes 6mA. Too large or too small a value will have a crucial impact on the treble and bass. Increasing the current will make the sound warmer and the mid-frequency richer, and the treble will be relatively dim. Reducing the current will make the high frequency better, and the mid-frequency will lose some thickness. The specific value depends on your personal taste.

The third power output stage uses four FU-7s in parallel push-pull output. The reason for using FU-7 is not just for the sake of cheapness, but more importantly, its vocal performance is comparable to the famous 300B. A Taiwanese DIY expert once praised FU-7 for being more "humane" than 300B in expressing the musical essence of vocals. Another point is that this machine uses two tubes in parallel for auxiliary output. Generally speaking, after the tubes are connected in parallel, the mid-low frequency becomes thicker and the sound density increases, while the high frequency becomes hazy, and the resolution decreases to varying degrees. After experiments, different tubes have different characteristics. I have tried the overall performance of EL34 and KT88 in parallel. Except for the increase in output power, the overall performance is not as good as that of a single tube. When the FU-7 is used with a load of about 6kΩ, the high frequency difference is very small, the mid-frequency texture increases, and the vocals are richer; the improvement of dynamics, control, and output power is even more expected. Each FU-7 operates at 480V, 50ma, and consumes about 2W, which is close to the maximum anode dissipation power of FU-7, 25W. However, as long as you choose genuine tubes, there will be no problem with stability. The author's sample has been used for two years without any aging phenomenon. Moreover, its price is only one-fourth of KT88, and the market supply is sufficient. FU-7 can be said to be one of the most cost-effective tubes.

The sound quality of FU-7 is determined by the triode connection. Although it lacks a bit of masculinity, it can be compensated by using double-tube well connection. Since the open-loop characteristics of the whole machine are very good, the large-loop negative feedback value of this machine is relatively low, which effectively reduces the negative effects such as transient distortion and high-frequency phase shift caused by the large-loop negative feedback, making this machine have excellent transient response.

The whole machine has only one level of capacitor coupling, and the capacity is very small, so the impact on the sound quality is not great, so you can choose a tonic capacitor. I have tried several famous capacitors, such as WIMA, ERO, and Sulun, and finally I used WIMA capacitors because of their more neutral sound, superior resolution and speed. If you like a more "musical" performance, you can also choose Sulun capacitors.

Production :

The production of the chassis can be seen in Figure 2. The selection of materials has a certain impact on the sound. I have experimented with iron plates, aluminum plates, stainless steel plates, etc. Different materials have different effects on the timbre. Aluminum plates are the most ideal in terms of mechanical, electrical, and magnetic aspects, but they are not very ideal in terms of sound effects. Its sound is a bit divergent and the sound image is a bit fuzzy. Iron plates are the least desirable. The reason is simple. The internal magnetic and electrical characteristics of the rack are not ideal. The leakage magnetic field of various transformers will produce complex electromagnetic interference in the iron plate, and produce magnetic coupling to the components close to the iron plate, causing sound quality pollution. In the author's experiment, using the same components and circuits, when using iron plates as racks, it can be clearly felt that the sound quality has deteriorated. Stainless steel plates are a controversial material. Some people accuse it of having a dull and astringent sound, but the author's comparative experiment still thinks it is better than aluminum plates and iron plates. It is mainly manifested in the solemnity of the mid- and low-frequency bands, which is incomparable to the other two materials, and it is also more beautiful. The disadvantage is that due to the high hardness of stainless steel, it is more difficult to process and manufacture in amateur situations.

The output transformer is the key to the success of a tube amplifier. This machine is self-wound with high-quality iron core and oxygen-free copper wire. The winding method is relatively complicated. The primary and secondary are separated from each other. The primary is divided into three layers and six groups, one side is wound forward and the other side is wound reversely. The secondary is wound in the same way and sandwiched between the primary layers. There is a gap between the turns, and it cannot be wound closely, so that a good high-frequency response can be obtained. Be careful not to mix up the taps of each winding. After winding, insert the iron core, connect the secondary with an AC power of about 6V, and then connect the taps of each winding of the primary. Note that the voltage measured for each group connected should increase. If it decreases, it means that the connection is reversed, and it should be switched until the superimposed voltage is the highest (connected in phase). As long as you are careful and operate according to the diagram, the success rate is extremely high; it is much better than some finished output transformers that are in name only. The iron core size is larger, the medium and low frequencies are good, but the leakage magnetic flux, leakage inductance, and distributed capacitance are large, and the high-frequency response decreases. The iron core is smaller, the mid-high frequency is good, but the mid-low frequency is worse, and the sound is thin. There is also debate in the industry about the material of the iron core. The finished output transformers on the market now basically use high-value oriented chips, which are small in size and low in copper loss. They can reduce distributed capacitance and inductance and get good high-frequency response. The classic medium-value silicon steel sheet, under the same parameters, has a warmer and richer sound and a stronger tube flavor. After countless experiments, the author took 4×6 medium-value silicon steel sheet core and carefully wound it according to the diagram, achieving a good frequency response and a very beautiful sound (in the actual production of this machine, the iron core of the output transformer of two 275W electronic radios was used for winding).

The power supply is very important. This machine is a dual-mono design, and two power transformers are required. The author uses a 450W iron core with conventional winding, so I won't go into details. The choke is also a controversial component, with mixed reviews, but one thing is certain. That is, the use of the choke is not good for the speed and transient response of the power supply. The author believes that as long as the DC ripple of the power supply is small enough, it is better not to use it.

After rectification, except for the first-stage filter, which uses electrolytic capacitors, all other filter capacitors use CBB60 polypropylene capacitors plus #0.68uFCBB small capacitors. Multiple motor start capacitors of 30uF/500V can be used in parallel (Note: The multi-motor start capacitors reported on the market are all packaged after two polarity electrolytic capacitors are connected in series with the same polarity. The quality of this capacitor is very poor and it must not be used!). If the bottom plate space is large enough, it is better to replace the first-stage filter capacitor with CBB, but pay attention to the capacity. You will find that this is a "trick" for beautiful sound. The author often uses this method when tuning the tube amplifier, and it works every time. It is obviously better than "high-speed electrolytic" and "audio-specific electrolytic" in terms of mid-frequency texture and resolution. It is necessary to explain again that the capacity of the filter capacitor must be large enough and the withstand voltage must have a margin, otherwise the power supply with unclean filtering will seriously affect the sound quality.

Since the circuit is relatively simple, scaffolding welding is more conducive to sound quality. The components and wire ends to be welded should be cleaned first, and the oxide layer should be removed before twisting and welding. The flux can be a mixture of rosin and alcohol. Do not use flux paste.

The wiring and grounding of the tube amplifier are very important, and audiophiles with little experience should not take it lightly. This machine uses independent ground wires at various levels to converge the power ground, and then ground it near the input socket on the rack, which is the so-called single-point grounding method. The ground wire used must be as thick as possible, and it is best to use a single thick copper wire.

The quality of early domestically produced electronic instruments (before the 1980s) is trustworthy. 6N8P, 6N11, and FU-7 are all well-known models in the audio industry and can be selected with complete confidence (see Figure 5). As for the products after the 1980s, they are a mixed bag, and it is difficult to guarantee the stability and consistency of the products, so it is best not to use them.

In short, the success of making a good sounding homemade tube amplifier is closely related to the careful production and debugging bit by bit. Except for the output transformer, although the selection of any component or a debugging step may not necessarily have a great impact on the sound quality, but when accumulated bit by bit, it will add up to a lot, and the impact on the sound quality cannot be ignored. Each component has different brands, different production periods, different production processes, and different working conditions, and has different timbre characteristics. Especially for electronic tubes and capacitors, it is essential to master the timbre characteristics and characteristics of these components to successfully make a high-level homemade tube amplifier.

Debugging :
1) Adjust the four grid negative voltage adjustment potentiometers to make the current of the corresponding four FU-7s 50mA, and adjust it again after working for 1 hour (new tubes need to be aged, that is, do not connect high voltage and let the filaments ignite for 15 minutes). Then connect high voltage, adjust the grid negative voltage, age for 10 minutes at 10mA and 20mA respectively, and then increase to 50mA.
2) Adjust the constant current source resistor R12 to make the inverter current 5mA.
3) The current of the SRPP amplifier must not be lower than 6mA, otherwise the sound quality will be discounted. The resistor R3 can be increased or decreased to meet the requirements.
4) The filaments of 6DJ8 and 6SN7H are not grounded, but 100V+ voltage is taken out through voltage division and added to both ends of the filaments through two balancing resistors. The circuit diagram is not drawn, and readers can design it by themselves. This is because the cathode potentials of these two stages are both above 100V, which exceeds the maximum cathode filament voltage.
5) Check the negative feedback phase. Since the negative feedback of this machine is very small, it may not howl when the phase is reversed. You can do an experiment to disconnect the negative feedback circuit. If the music volume becomes louder after disconnection, it means that the feedback phase is correct. If the volume becomes smaller, it means that the phase is reversed. You can swap the wiring of the two input terminals of the output transformer. Pay attention to observe that when there is no signal, if the FU-7 screen turns red and the tweeter has irregular noise, it means that there is high-frequency self-excitation. You can wrap the top screen lead of FU-7 five times to form a small inductor, which can generally solve the
problem. 6) Do not open the output terminal during power-on to avoid self-excitation and breakdown of the output transformer.
7) The inverter power supply is 380V, which can be increased to 430V, and the control is better at this time.
Since this machine has a high gain, it can be directly connected to the CD player and then connected to the front pole, which seems a bit superfluous.
The performance of this machine is unique, different from the general low-end tube amplifier, the sound line is clear, and the dual-unit structure has an extremely excellent sound field and positioning. Since the open-loop index of the whole machine is very good, there is only shallow negative feedback, which makes the sound lively, dynamic and fast. The low frequency is thick but not fat, and the bounce is very good. There is no soft and fat phenomenon that is common in general tube amplifiers. The performance of the intermediate frequency is particularly satisfactory (the parallel use of FU-7 is indispensable). The human voice is particularly tasteful, especially the voices of several famous female singers are fully exerted, full and magnetic. Generally, the sound of the solid-state machine has more or less a sense of toughness, but this machine does not have this feeling at all. The interpretation of the high frequency band is also very distinctive, with a sweet and moist feeling, very close to the feeling of 300B single-ended machine. At first glance, it seems a little sharp, but it does not lose resolution and transparency, which makes people never get tired of listening. It is the sound that many high-end enthusiasts are striving for. The cost of making such a dual-monopoly machine is about more than 1,000 yuan, less than one-tenth of the 300B brand machine, and the cost performance is very good. In fact, the practice is valuable. When a tube amplifier is born in your hands, the sense of accomplishment and satisfaction is something that no famous brand machine can give.

Main performance of this machine:
Power: 40W×2, 8Ω
Distortion: <0.2 1kHz/5w
Frequency response: 18～30000Hz
Listening equipment:
Speakers: 1) AE1 1I
2) Gentleman's treasure unit Homemade earth cannon box
Turntable: DENOR 2800AL
Decoder: Li Te 38S
Signal velvet: NBS Little Snake King
Speaker cable: AQ MIDNIGHT 3