The single-end gallbladder is the best partner for full frequency. The author already has three separate single-end post-stages, namely 805, 300B and 2A3. These three post-stages are all well-known and well-known, and their output powers are 40W, 8W and 3.5W. Although their sound performances are different, they also have their own characteristics, and the smaller the output power, the thinner the sound, and the more realistic the emotion of expressing the human voice. So there was an idea: to build a single-end post with lower power.
1. Found bold
When it comes to making low-power single-ended, many people think of using some beam tubes such as 6P1, 6P14, 6V6, 6L6, 6P3, 6P6 and pentode to use triode to make power tube, but I don't like this This method, because the beam tube and the pentode use the triode connection method, its linearity can not always be compared with the real triode. By chance, the author found a brave that has been forgotten, it is the protagonist of this article-6C19.
6C19 is a side-heating high-power single transistor, the main purpose is to regulate power supply regulator and OTL power amplifier. The characteristic curve and pin arrangement are shown in Figure 1, and the main parameters are shown in the attached table. The 6C19's internal resistance is as low as 300Ω and the anode dissipation power is 11W. Its performance is no better than the thumb. Because it has not been heated by the market, the price is very cheap, and it is too cheap to make people dare. I believe that such a low-cost bile will have a good voice performance. In fact, how can the voice of a gallbladder be directly linked to its price? What is more important is the quality of the gallbladder itself and whether it can be used well.
Figure 1 6C19 characteristic curve and pin arrangement
Main parameters of Schedule 6C19
Parameter Symbol Index
Filament voltage Uf 6.3V
Filament current If 1A
Internal resistance Ri 300Ω
Transconductance S 7.5mA / V
Maximum anode current Imax 140mA
Maximum anode power dissipation Pmax 11W
2. Circuit principle
According to the characteristics of 6C19, the author designed and produced a low-power single-end post-stage. The whole electromechanical schematic diagram is shown in Figure 2.
Figure 2 Schematic diagram of the whole machine
To work well, the selection of its working point and load impedance is very important. The internal resistance of 6C19 is 300Ω, and the impedance of the output transformer is preferably 1 ～ 1.5kQ. I just saw someone selling a single-ended output transformer with 5W / 2.5kΩ on the Internet. The power is appropriate, and the impedance is slightly larger. However, a larger impedance helps the performance of low frequencies, so I bought a pair. After actual use, the effect is good. Combining the characteristics of 6C19 and the recommended working state, the final working point is 200V screen voltage and 45mA anode static current. In order to improve the dynamic performance and low-frequency control of 6C19, instead of using a common self-bias, a fixed bias is adopted, which is rectified by D1 and D2. After filtering by C7, it outputs a negative pressure of about 250V, which is stabilized by R10 and DW1. After the voltage is divided by W2 and R11, the gate voltage required by 6C19 is provided. In addition, the 6C19 uses a fixed offset to improve power utilization. Since the negative pressure of 6C19 is as high as about -60V, that is to say, if the input sensitivity is set to 1V, the driving stage must have a gain of not less than 60 times to achieve 6C19 full power output, so the requirements for the driving stage are higher . There are several common practices:
(1) Adopt medium μ tube or low μ tube for two-stage amplification;
(2) Adopt high μ for first-level amplification, usually using common cathode and SRPP connection method, commonly used tubes are 6N9 and 12AX7;
(3) Use a high-magnification pentode for first-level amplification;
(4) Use the middle μ tube and push the cow (using its boosting and amplifying effect) to make a direct push.
In my opinion, if one level of amplification can meet the needs, then two levels will not be used, because in this way, it will bring many problems such as increased phase shift, deteriorated signal-to-noise ratio, and more complex circuit. Finally, his eyes locked on the 6F2 composite tube. 6F2 contains a high magnification pentode and a triode, the pin arrangement is shown in Figure 3.
Figure 3 6F2 pin arrangement
The signal is amplified by its internal pentode, buffered by the triode, and then pushed through the coupling capacitor C to push 6C19. Since the output internal resistance of the pentode is high, the direct push 6C19 distortion will be greater, the high-frequency characteristics are poor, and the dynamic and low-frequency control forces are also affected, so a buffer stage composed of 6F2 internal triodes can be added.
The power supply part adopts direct heating rectifier 5Z4 for rectification, and CLC forms a π-shaped filter circuit. The two-channel 6F2 filaments share a set of power supplies and are supplied by the three-terminal regulator LM7806.
3. Component selection
6C19 can use the common bile ducts produced in Beijing. Of course, it is better to find those made in the former Soviet Union or Europe. Due to the low price of 6C19, prepare a few more when preparing materials. Try to find two matching ones, which will help improve the sound. The 6F2 is easier to find from the Shanghai brand. Its European model is ECF82 and the American model is 6U8. If you can find it, you can compare the sound characteristics of each and choose according to your personal preferences.
150W is enough for power transformer B2. Due to the large number of voltage windings, if the core window cannot be accommodated, an independent filament transformer can be added.
The coupling capacitor C4 has a great influence on the sound of the whole machine. Using some expensive capacitors, such as Jensen M-cap capacitors, of course, the effect is good, but the price is too expensive, and does not correspond to the price of this body. When I made it, I chose a 0.15uF / 400V CJ10 metallized paper dielectric capacitor in parallel with a 0.1 uF / 250V Wima capacitor. The thick and warm mid-frequency of the metallized paper dielectric capacitor and the clear high-frequency of the Wima capacitor blend the sounds of the two to complement each other. At the same time, these two capacitors are inexpensive. Oil-immersed capacitors can increase the charm and musicality of the amplifier, which has become a consensus among enthusiasts, but it will affect the dynamic performance of the amplifier, and the oil-immersed capacitor is larger and difficult to fix, so the oil-immersed capacitor is in the gallbladder It should not be used more in the machine, let's just treat it as MSG and use it for seasoning, so the power part uses 2 oil-immersed capacitors with a capacity of 7 uF.
The volume control potentiometer uses the common ALPS potentiometer. The Noble rotary type power switch is originally characterized by good feel, high reliability, and helpless price. Later, I accidentally discovered a 500-ohm Nissan potentiometer with a switch, which can be used as a rotary power switch. After actual use, it is found that its feel and reliability are comparable to Noble, and the price is only 1 yuan / piece. Cost-effective.
4. Production and debugging
We make a tube amplifier, not only requires its sound quality, but also its appearance can not be ignored, because an audio equipment, its function is not only for us to enjoy music, but also should be a piece of art of home decoration, visually requires beautiful , Generous, giving people a pleasing feeling. For this reason, when making this amplifier, the author adopted a symmetrical design, and its bottom plate processing is shown in Figure 4.
Figure 4 Floor processing diagram
The casing is made of pressed stainless steel with a thickness of 0.5mm. The size is 240mm in width, 320mm in depth, and 50mm in height, with solid wood panels on both sides. The size is 340mm × 60mm, and the thickness is 15mm. The appearance photos of the complete machine are shown in Figure 5, for reference during production.
The whole machine only has one signal input, so the panel knobs are relatively simple, and there are only two knobs in total. One controls the volume and the other controls the power switch.
After all the components of the whole machine are welded, and the inspection is correct, you can start debugging. The debugging of this machine is very simple. Just adjust W2 and W3 so that the voltage across the two cathode resistors R8 of 6C19 is 0.37V to start the sound test.
5. Listening with the phone
Although this machine has only a few hundred yuan investment, but its sound performance is unexpected. When expressing vocals, it is particularly clear, true and natural, and the emotions expressed to the singer are most vividly expressed, and it is three points. When replaying string music, such as erhu and guzheng, the sweet and pliable timbre, full of long reverb and air, make people fascinated and unable to stop. Of course, the output power of this machine is only 2W, and vocal and string music are its special tricks, such as letting it replay large dynamic symphonies and drums, it seems to be difficult.
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