Production of 6H30 single tube preamplifier

The origin of the 6H30 tube is the Soviet Union. The 6H30 belongs to the frame grid type electron tube. It adopts a super-reinforced structure with three layers of mica root bracket as auxiliary support, highly enhanced shock resistance, and a life of up to 10,000 hours. At that time, it was used in the fighter "Su-27". The mechanical structure was highly reinforced. At the same time, its electrical characteristics are similar to two parallel 6DJ8/ECC88s, with extremely high transconductance, large output current, low output impedance, ultra-low silence, and almost no microphone effect. It is a bit like the excellent Telefunken E182CC electron tube in those days. It can exert great efficiency under very simple conditions. Just because it is a Soviet military product, it was not lifted until the end of 2000, and it was known by tube amplifier manufacturers and tube amplifier enthusiasts.
ARC, a well-known American tube amplifier, uses four 6H30s in its newly launched Audio Research 3. This is quite indicative. As we all know, this Hi-End brand has always specialized in 6922/6DJ8 vacuum tubes. From pre-stage to power stage, it has been planning and designing around 6922/6DJ8 for decades. Now it has switched to 6H30 in its top reference series. If it were not for its excellent performance and compliance with its technical indicators, it would be difficult to be selected. And another well-known domestic brand, OBO, also uses a buffer circuit constructed by 6H30 as the output of the RCA output signal part in its reference-level Reference CD2.3 advanced CD turntable system. From the above two examples, everyone can have a rough idea of ​​the excellent performance of 6H30. Of course, the price of this tube is relatively high, but compared with antique tubes that cost thousands of yuan, the price of 200 yuan is not very expensive. At present, there are three types of Sovtek ordinary models, gold-foot models and EH gold-foot models available in China. The supply is relatively sufficient, and the quality is also proven. It is a tube worth exploring its potential.
I searched the Internet for circuits related to 6H30. I found two circuits with self-bias and constant current source as load for single-tube amplifier circuits; an amplifier circuit of an Italian enthusiast uses a fixed bias and inductive load screen output circuit; DIYZONE also has a test circuit with a constant current diode as load, and also found a 6H30 single-tube amplifier simulation software that can display the characteristic curves of various working points, which is very convenient when designing circuits.
Design concept:
The initial idea is a single-tube screen output, self-bias, and screen constant current source load.
The screen output is mainly due to the fact that the cathode follower output has no gain, and it may not be able to explode, and the power of 6H30 cannot be fully exerted. Although fixed bias voltage can flexibly select the working point, it is necessary to add input coupling capacitors, which increases the variables of circuit adjustment, and a pair of good coupling capacitors is also very valuable. The use of constant current source, first, helps to increase the gain of the user, and second, I have recently read Japanese tube amplifier books and found that constant current sources are widely used in the circuits of Japanese audiophiles. The overall evaluation is that there are more good sound opportunities.
There are many circuit structures of constant current sources. The common ones are the following: constant current diodes, field effect tubes, LM317, light-emitting LEDs (diodes, TL431) and crystal triodes, operational amplifiers and crystal triodes, electron tubes, transistors and electron tubes, field effect tubes and electron tubes, integrated circuits and electron tubes, etc. Due to space limitations, I will not explain it in detail here, and I will prepare another article for comprehensive analysis later. Considering that the voltage that the plate load of 6H30 needs to bear is relatively high, and at the same time, I hope to add some unique sound coloration of the pentode. Because I have seen a design circuit of a pre-stage abroad before, it is 6080 as a single-tube line amplifier and EL34 as a constant current source. The original author believes that: the frequency response of the triode is relatively straight, and the extension of the high and low ends is better. The sound of the pentode is condensed in the mid-tone and has strong resolution. The combination of the two characteristics should be a perfect sound. Influenced by this article, I plan to use the constant current source circuit composed of 6BQ5 (6P14, EL84).
I happened to meet Mr. Li from Taipei when I was searching for information online. Mr. Li was a maintenance worker at ARC's Taiwan agent. He has 30 to 40 years of experience in repairing top-level audio equipment and is very familiar with ARC's common pre- and post-stage amplifiers. When I talked to him about the 6H30 tube and my own ideas, he quickly drew a line amplifier circuit diagram of a 6H30 and 6BQ5 combination, as shown in Figure 1. Of course, strictly speaking, this circuit cannot be considered a constant current source, but a deformed SRPP structure, because the gate and cathode of 6BQ5 are connected to the 6H30 screen. When 6H30 inputs AC, its screen current will change, and the working current of 6BQ5 will also change. It is an SRPP circuit with asymmetrical impedance. Since 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 the upper and lower together, so it can only work like SRPP.

The preliminary setting of several resistance values ​​in the figure:
first set the working voltage of 6H30. According to the original factory data in Figure 2, the following working points are decided: screen voltage: 150V, grid voltage -9 V, screen current: 20MA. Through the simulation software, it is known that at this time, the amplification factor of 6H30 μ＝15, and the transconductance Gm＝9.58.
Calculate the cathode resistance: 9V/0.02A=450Ω, take 470Ω1W.
At present, the output level of CD players is generally around 2V, so the general grid bias voltage is 1.5V. Here we want to try 6H30 under high current working conditions. The characteristics of the sound, so the grid voltage is -9V. Although there is a certain gap in matching with the output swing of the CD, the amplification factor is lost, but the dynamic range of the input signal is improved, which can make the distortion of 6H30 in AC work smaller, and the advantages outweigh the disadvantages in improving the signal waveform. The screen voltage is also taken higher, so that the maximum undistorted signal amplitude of the output can reach more than 50V.
Let's look at the data of 6BQ5, class A amplification, screen voltage: 250V, screen grid voltage: 250V, cathode resistance: 135Ω, screen current: 48MA + 5.5MA of screen grid = 53.5MA, now it needs to be changed to 20MA, then the cathode resistance is: 53.5/20×135=361, and 320Ω1W is actually used.
The specific value needs to be adjusted in actual production.

Production and debugging
The chassis frame is made of aluminum profiles for decoration, and the upper and lower covers are made of 2.5 mm aluminum plates. The tube socket holes are easy to process with a hole opener, but for beginners who use it for the first time, they need to pay attention to two points. The first is to open a positioning hole of about 3 mm first. Don't use the hole opener directly to save trouble, otherwise it is easy to scratch 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. The nine-pin tube socket should use a hole opener with a diameter of 22 mm. After
the chassis is processed, for the sake of beauty, it is painted with black self-spray paint. When the paint film is dry, you can fix the switch, indicator light, power socket, and RCA socket, as shown in Figure 3, and then fix the power cow, tube socket, electrolytic capacitor clip and the wiring rack inside.
First weld the 220V AC power cord and switch. Then weld the filament connection part of the rectifier tube, 6H30, and 6P14.
The filament of 6H30 is at pins 4 and 5. You can directly ground one of the pins 4 or 5. If there is AC noise, you can also try to use two 10-20 ohm resistors, solder them to pins 4 and 5 respectively, and then solder the other ends of the two resistors together to the common ground.
The filaments of 6BQ5 are connected in parallel and floating, that is, only one group of AC 6.3V is connected, not grounded. After installation, when the power is turned on for testing, measure the cathode voltage of 6BQ5, and divide the voltage with two resistors, which is slightly higher than the cathode voltage.
After welding, first power on and test the voltage (at this time, you should pay attention to the joints of the high-voltage terminals of the power transformer, and you can wrap them with insulating tape first to avoid the danger of electric shock). After normal, you can plug in the rectifier tube, 6P14 and 6H30 tubes, power on and test, and test the filament voltage again. If the voltage is accurate and the tubes are lit normally, it means that the filament power supply part can work normally. It should be noted that the power consumption of the power grid is different during the day and at night, so the city power will fluctuate around 220V. Because the filaments are all AC powered, the fluctuation of the city power should be considered when checking the filament voltage.
Fix the high-voltage electrolytic and choke, weld the wiring, and check if there is no problem. You can first check it with power on. Note that the high-voltage part must be welded with the bleeder resistor before checking. Otherwise, because the chassis is relatively small, it is easy to be hit by the accumulated charge of the high-voltage electrolytic when adjusting the components inside.
If the power supply is correct, you can then weld the circuit amplification part. The amplification part is very simple. Be careful not to have cold soldering on the component pins. After welding, plug in the tube and you can do a preliminary test. After
turning on the power, the tube filament slowly lights up. Then you can test the circuit. During the test, it was found that because the current of the 6H30 tube is large, the voltage stabilization effect simply through CLC cannot meet the requirements. It is manifested as short-circuiting the input end and the AC noise voltage at the output end is high. The DA16 millivoltmeter test reached 20mV. Helplessly, the original plan had to be abandoned.
Because the opening of the chassis cannot be changed, it is planned to change the original tube rectifier to transistor rectifier, so that the output voltage can be increased, and add 6N5 as the power supply adjustment tube to the original rectifier tube position. This gourd tube looks very beautiful. The power supply part is changed to be processed and made on the epoxy board with rivet welding pieces and wires. First draw a sketch on paper and arrange the position of the components. In order to find the best wiring plan, it took more than half a day to determine the final plan. I opened the holes overnight and installed the rivets the next day. I was ready to weld the components.
When I was preparing the components by comparing the circuit diagram, I happened to see a message from Mr. Li in Taipei and found that a key problem had been overlooked, that is, the 6N5 adjustment tube had to meet the voltage drop of 60 to 90V. Although the output voltage of the transistor rectification increased, the final output voltage could only be around 280 to 290V, which could not meet the 350V voltage requirement of the line amplification. Helplessly, the newly made board had to be abandoned again, and now I had to choose transistors, field effect tubes or LM317 as the voltage regulator. In this way, the tube socket where the 6N5 was originally installed was vacant, and two treatment methods came to mind. One is to find a heat sink to cover the hole and serve as a heat sink for the transistor at the same time. The second is to install a large eight-pin voltage amplifier tube as the voltage amplifier tube of the voltage regulator. I remember that there was an article about the imitation of 300B in "Hi-Fi Audio" magazine, which introduced the soft start function. However, considering the interference of AC filament, I think the first method is better, so I started wiring and arranging the parts again. Alas, this 6H30 is really hard to serve!

The third power board uses the simplified board of Madis power supply. The circuit diagram is shown in Figure 4. The performance of this circuit is very good. I have made more than a dozen of them for my friends. The AC ripple voltage is only 0.1mV. No wonder Hong Kong and Taiwan enthusiasts often use it. Here, scaffolding welding is used. The wiring of the main filter part is shown in Figure 5, and the main voltage regulator part is shown in Figure 6. First, fix the MJE13007 and LM317 to the heat sink. Pay attention to installing the insulating gasket and insulating sheet, and install it on the case. Because there are few components, welding is completed quickly.
Turn on the power, the output voltage is 350V. Look at the waveform of the power supply voltage, a horizontal straight line, indicating that the voltage stabilizer works well!
The installation of the internal power board can be seen in Figure 7. It was originally planned to use a double choke balanced filter, but because the voltage drop of a choke is 20V when it is fully loaded, it is limited by the output voltage. 390V-20V-20V=350V cannot meet the input voltage requirements of the voltage stabilizer, so it has to be changed to a single choke.
Measurement of working point: The voltage of each point is as shown in the figure, B+ 348V, 6H30 screen voltage to ground 170V, cathode voltage: 9.2V, calculated screen current Ip = 19.6mA. It basically matches the designed working point.
Ground the input end, and then connect an oscilloscope to the output end to observe, almost a horizontal line, tested with a DA16 millivoltmeter, it is 5mV, there is still some effect, considering that the capacitor at the output end is open circuit, if the load is connected, the noise voltage is estimated to be lower, so it can be considered that the requirements are basically met. Having an oscilloscope will provide a lot of convenience in making tube amplifiers. Sometimes the circuit is self-excited at high frequency, and it is difficult to detect without an oscilloscope. Now a second-hand 20M dual-trace oscilloscope is only 300 or 400 yuan, so you should prepare one if you have the conditions.
There is a small episode. I started using a second-hand EH gold-pin 6H30. I found that the voltage difference between the left and right channels of the 6H30 was 20V. I replaced it with a pair of new Sugon 6BQ5. It was still the same. I changed to a new Sovtek 6H30. I tested and found that the voltage difference was less than 2V. It seems that if the requirements are high, the 6H30 tubes also need to be selected in pairs.
The working point of the circuit is basically normal. The next step is to make some detailed adjustments to the tone of the front stage. This circuit is relatively simple and can be done by slightly adjusting the working point and replacing the output coupling capacitor. For friends who are new to tube amplifiers, the screen load of the 6H30 can also be tested in several forms, such as resistance, inductance, constant current source, etc., to practice and increase some perceptual understanding.