Fabrication of "6H30 single tube bladder front stage, 6H30 amplifier
Key words: fabrication of 6H30 single tube bile duct front stage
The 6H30 pipe originated in the Soviet Union. 6H30 belongs to frame grid electronic tube. It adopts a super reinforced structure with three-layer mica root support as auxiliary support, which highly enhances seismic resistance and has a service life of up to 10000 hours. At that time, it was applied to the fighter Su-27, and its mechanical structure was highly strengthened. At the same time, its electrical characteristics were about two 6dj8 / ecc88 connected in parallel. It had extremely high transconductance, large output current, low output impedance, ultra-low silence, and almost no microphone effect. It was a bit like the deluxe Fenggen e182cc electronic tube with excellent appearance in that year, As long as very simple conditions can play a great role. It was only because it was a Soviet military product that the ban was lifted by the end of 2000, which was known to gall machine manufacturers and gall machine lovers. Arc, the famous American venture machine, has adopted four 6H30 in the newly launched audio research 3, which is of great significance. As we all know, this Hi end brand has always focused on 6922 / 6dj8 vacuum tubes. From the front level to the rear level, it has been planned and designed around 6922 / 6dj8 for decades. Now it is transferred to its top reference series and adopts 6H30. If it is not for excellent performance, I'm afraid it's difficult to be selected if it meets its technical indicators. Another well-known brand in China, Obo, also uses the buffer circuit constructed by 6H30 as the output signal in its reference cd2.3 advanced CD turntable system. From the above two examples, we can know the excellent performance of 6H30. Of course, the price of this tube is also higher, but compared with the antique electronic tube that is prone to thousands of yuan, the price of 200 yuan is still not very expensive. At present, there are three types of sovtek ordinary model, Golden Foot Model and eh golden foot model that can be purchased in China. The supply is sufficient and the quality is also proved. It is a tube worth tapping its potential. I searched the 6H30 circuit on the Internet, and found two self-sufficient bias circuits with constant current source as load for the single tube amplification circuit; An Italian enthusiast's amplifier circuit adopts a fixed bias and inductive load plate output circuit; DIYZONE also uses constant current diode as load test circuit, and has found a 6H30 single tube amplification simulation software, which can display the characteristic curves of various working points, so it is very convenient to design the circuit
Design idea: the preliminary idea is single tube plate output, self generated bias, plate constant current source load. The plate output is mainly due to the fact that the output of the cathode follower has no gain, so the explosion may not be affordable, and the power of 6H30 can not be fully played. Although the fixed bias can flexibly select the operating point, it needs to add the input coupling capacitance, which increases the variable of circuit adjustment, and a pair of good coupling capacitors are also valuable. The use of constant current source, first, helps to exhaust the gain of the tube, and second, recently read the Japanese courage books and found that constant current source is widely used in the circuits of Japanese enthusiasts. The general evaluation is that there are more opportunities for good sound.
There are many types of circuit structures of constant current source. The common ones are as follows: constant current diode, field effect tube, LM317, light emitting LED (diode, TL431) and crystal triode, operational amplifier and crystal triode, electron tube, transistor and electron tube, field effect tube and electron tube, integrated circuit and electron tube, etc. they are limited to space and will not be described in detail here, Prepare another comprehensive analysis later. Considering that the 6H30 plate load needs to bear a relatively high voltage, and we hope to add some unique sound dyes of the pentode, because we have seen a foreign front-end design circuit, which uses 6080 as a single tube line amplification and el34 as a constant current source. The original author believes that the frequency response of the triode is relatively flat and the high and low ends extend better. The sound of the pentode is condensed and has strong analytical power, Mixing the two characteristics should be a perfect sound. Influenced by this article, the constant current source circuit composed of 6bq5 (6p14, EL84) is prepared.
Just when I was searching for information on the Internet, I met Mr. Li from Taipei. Mr. Li was once a repairman of arc's Taiwan agent. He had 30 or 40 years of experience in repairing top audio equipment. He was very familiar with the common front-end and rear-end power amplifiers of arc. He talked to him about the 6H30 tube and his own ideas. He soon drew a circuit diagram of the combination of 6H30 and 6bq5, See Figure 1. Of course, strictly speaking, this circuit is not a constant current source, but a similar deformed SRPP structure, because the grid and cathode of 6bq5 are connected to the 6H30 plate. When 6H30 inputs AC, its screen current will change, and the working current of 6bq5 will also change. It is an SRPP circuit with asymmetric impedance. Because the upper and lower impedances are different, The midpoint output is not half of the B + voltage, and the maximum output clipping is not clipping up and down together, so it can only work like SRPP.
Preliminary setting of several resistance values in the figure: first set the working voltage of 6H30. According to the original data in Figure 2, it is decided to adopt the following working points: screen voltage: 150V, grid voltage - 9V, screen current: 20mA. The amplification factor of 6H30 is known through the simulation software μ＝ 15, transconductance GM = 9.58. Calculated cathode resistance: 9V / 0.02A = 450 Ω, take 470 Ω 1W. At present, the output level of CD player is generally about 2V, so the gate bias is generally 1.5V. Here, I want to try 6H30 under high current operation. Because of the characteristics of sound, the gate voltage is taken as - 9V. In this way, although there is a certain drop in matching with the output swing of CD and the magnification of loss point, it improves the dynamic range of input signal, reduces the distortion of 6H30 in AC work, and has more advantages than disadvantages in improving signal waveform. The screen voltage is also higher, so that the maximum undistorted signal amplitude can reach more than 50V. Looking at the data of 6bq5, class a amplification, screen voltage: 250V, curtain grid voltage: 250V, cathode resistance: 135 Ω, screen current: 48mA + curtain grid 5.5ma = 53.5ma, now to be changed to 20mA, the cathode resistance is 53.5/20 × 135 = 361, actually used 320 Ω 1W. The specific value should be adjusted in the actual production
Production and commissioning
The chassis frame is bent with decorated aluminum profile. The upper and lower cover plates are made of 2.5mm aluminum plate. The pipe seat hole is easy to process with a hole opener. However, for novices who use it for the first time, two points need to be paid attention to. First, a positioning hole of about 3mm should be opened first. It is not easy to save trouble. The hole should be opened directly with a hole opener, so it is easy to flower the aluminum plate. The second is to drip some engine oil or sewing machine lubricant when opening the hole, so that the edge of the hole is very smooth and flat. A 22 mm diameter hole opener shall be used for the nine foot socket.
After the chassis is processed, in order to be beautiful, it is painted with black self painting. When the paint film is dry, the switch, indicator light, power socket and RCA base can be fixed, as shown in Figure 3, and then the power cow, tube base, electrolytic capacitor clip and the wiring frame inside can be fixed.
Weld 220V AC power line and switch first. Then weld the filament wiring of rectifier tube, 6H30 and 6p14.
The filament of 6H30 is pin 4 and pin 5. For grounding, one of pin 4 or pin 5 can be directly grounded. If there is AC noise, you can also try to weld two 10 ~ 20 ohm resistors on pin 4 and pin 5 respectively, and then weld the other ends of the two resistors together to connect to the public ground.
The two filaments of 6bq5 are connected in parallel and floating, that is, only a group of AC 6.3V is connected without grounding. After installation, during power on test, measure the cathode voltage of 6bq5, and divide the voltage with two resistors, which is slightly higher than the cathode voltage.
After welding, power on and test the lower voltage first (at this time, pay attention to deal with the connector of the high-voltage terminal of the power transformer, which can be wrapped with insulating tape to avoid the risk of electric shock). After it is normal, plug in the rectifier tube, 6p14 and 6H30 tubes, power on and test, and test the filament voltage again. If the voltages are accurate and the tubes are lit normally, It shows that the filament power supply can work normally. It should be noted that the mains power will float up and down at 220V during the day and at night because the power consumption of the power grid is different. Because the filaments are AC power supply, the fluctuation factor of mains power should be taken into account when checking the filament voltage.
Fix the high-voltage electrolysis and choke, weld the wiring, and check with the power on test if there is no error. Note that the high-voltage part must be welded with the discharge resistance before checking, otherwise, because the chassis is relatively narrow, it is easy to be hit by the electric charge accumulated by the high-voltage electrolysis when adjusting the components inside.
If the power part is correct, you can then weld the amplification part of the circuit. The amplification part is very simple. Note that the component pins should not be soldered. After welding, plug in the tube and you can conduct a preliminary test.
When the power is turned on, the tube filament slowly lights up, and then the circuit can be tested. During the test, it is found that because the current of 6H30 tube is large, the effect of voltage stabilization by CLC alone can not meet the requirements. It is shown that the input terminal is short circuited and the AC noise voltage at the output terminal is high. Test with da16 millivoltmeter to reach 20mV. Helpless, the original plan had to give up.
Because the opening of the chassis cannot be changed, it is planned to change the original electronic tube rectifier to transistor rectifier, so that the output voltage can be increased. Add 6n5 to the original rectifier position as the power regulator. This kind of gourd tube also looks very beautiful. The power supply part is processed on the epoxy plate with rivet welding sheet and conductor. First draw a sketch on the paper and arrange the position of the components. In order to find the best wiring scheme, it took more than half a day to determine the final scheme. Open the hole overnight and install the rivet the next day. Prepare welding components.
When preparing the components according to the circuit diagram, I just saw Mr. Li's message in Taipei. I found that a key problem was ignored, that is, 6n5 is used as an adjusting tube to meet the voltage drop of 60 ~ 90V. In this way, although the rectifier output voltage of the transistor increases, the final output voltage can only be about 280 ~ 290v, which can not meet the 350V voltage requirement of line amplification. However, the newly made board will be abandoned again. Now we have to choose transistor, FET or LM317 as regulated power supply. In this way, the original 6n5 tube seat is empty. I thought of two treatment methods. One is to find a heat sink and cover the hole. At the same time, it is used as the heat sink of the transistor. The second is to install a large eight pin voltage amplifier as the voltage amplifier of the regulated power supply. I remember that there was an article on imitating 300B in the magazine of high fidelity sound, which also has the function of soft start. However, considering the interference of AC filament, I think the first method is better, so I started wiring again and arranged the position of parts. Alas, this 6H30 is really hard to serve!
The third power board adopts the simplified board of MADIS power supply. The circuit diagram is shown in Figure 4. This circuit has very good performance. It has been made for more than a dozen friends, and the AC ripple voltage is only 0.1mv. No wonder Hong Kong and Taiwan enthusiasts often use it. Shed welding is adopted here. The wiring of the main filter part is shown in Figure 5, and the main voltage stabilizing part is shown in Figure 6. First fix MJE13007 and LM317 to the heat sink, Pay attention to the installation of insulating gasket and insulating sheet on the casing. Because there are few components, the welding will be completed soon.
Power on, output voltage 350V, look at the waveform of the power supply voltage, a horizontal straight line, indicating that the regulated power supply works well!
See Figure 7 for the installation of the internal power board. It was originally intended to use double choke balanced filtering. However, due to the voltage drop of 20V when one choke works at full load, 390v-20v-20v = 350V due to the limitation of output voltage, which can not meet the input voltage requirements of regulated power supply, so it had to be changed to single choke.
Measurement of working points: the voltage of each point is shown in the figure. The plate voltage of B + 348v, 6H30 to ground is 170V, and the cathode voltage is 9.2v. The calculated plate current IP = 19.6ma. Basically consistent with the design working point.
Connect the input terminal to the ground, and then connect an oscilloscope to the output terminal to observe. Almost a horizontal line is tested with a da16 millivoltmeter, which is 5mv. There are still some effects. Considering that the capacitance at the output terminal is open circuit, if the load is connected, it is estimated that the noise voltage will be reduced, so it can be considered to basically meet the requirements. Making a gall machine with an oscilloscope will provide a lot of convenience. Sometimes the circuit is high-frequency self-excited, which is difficult to detect without an oscilloscope. Now the second-hand 20m double trace is only 300 or 400 yuan. If possible, you should prepare one.
There is also a small episode. The second-hand eh gold pin 6H30 was used at the beginning. It was found that the voltage difference between the left and right channels of 6H30 was 20V. A pair of Dawning brand-new 6bq5 was replaced. Still, it was found that the voltage difference was less than 2V after replacing the brand-new sovtek brand 6H30. It seems that if the requirements are high, the 6H30 tubes also need to be matched.
The working point of the circuit is basically normal. In the next step, you can make some detailed adjustments to the timbre of the previous stage. This circuit is relatively simple and can be carried out by slightly adjusting the working point and replacing the output coupling capacitor. For those who have just come into contact with courage, the 6H30 plate load can also be tested in several forms, such as resistance, inductance and constant current source. First, they can practice their hands and second, they can increase their perceptual knowledge.
Because the tubes and resistance capacitor parts are brand-new, they should be powered on for aging for a period of time. If there is a cooker disc, you can also try it.
During the actual test, the following pipes were auditioned:
6H30: sovtek ordinary type, eh gold foot
Active load: Beijing 6p14, Shuguang EL84 / 6pq5.
The sound source is an old DVD player with pcm1716 decoding board. It has been rubbed for many times, and the sound is quite satisfactory. The speaker is a small box made of Foster's fe126 full frequency horn, and the sensitivity of the horn is 93dB. The power amplifier uses the lm1875. The wiring form of the power amplifier board PCB refers to an improved scheme in the April 2007 issue of EPE (everyday practical electronics) magazine. The signal-to-noise ratio is as high as 105dB, which is very suitable for promoting the high-sensitivity speaker fe126.
As for the specific sound performance, because it has a great relationship with equipment and sound testing environment, I don't do much narration. I initially feel that the sound is more atmospheric, which is worthy of further detailed adjustment. After listening to this front stage for a period of time, I think it is characterized by fast speed and sufficient low-frequency sense, but it lacks the charm of courage and is more straightforward. After consulting Mr. Li in Taipei again, according to Mr. Li's suggestion, the current of 6H30 is reduced, 470 Ω is changed to 560 Ω, 320 Ω is changed to 420 Ω, and the 0.33 micro method capacitor at the output end is connected in parallel with a 0.022 micro method capacitor, Slightly increase the medium and high frequencies to achieve the balance of listening feeling. After such adjustment, the timbre has been significantly improved. At present, use this configuration first, and report to you later if there is a better scheme., Read the full text“
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