Production of 6C19 Low Power Single-Ended Tube Amplifier

Since I started playing with the pair of 12-inch full-range units called "poison speakers" on the Internet, I have fallen in love with its smooth and natural sound, and I have also deeply realized that single-ended tube power amplifiers are the best partner for full-range. I already have three split single-ended power amplifiers, namely 805, 300B and 2A3. These three power amplifiers all use well-known and well-reputed beautiful tubes, with output powers of 40W, 8W and 3.5W respectively. Although their sound performances are different, they also have their own characteristics, and the smaller the output power, the more delicate the sound, and the more realistic the expression of human voice emotions. So an idea emerged: to make another single-ended power amplifier with smaller power.

1. Discovering a good tube When it comes to making low-power single-ended tubes, many people will think of using some beam tubes and pentodes such as 6P1, 6P14, 6V6, 6L6, 6P3, 6P6, etc. to make power tubes using triode connection. However, I don't like this approach because even if beam tubes and pentodes are connected using triode connection, their linearity can never be compared with that of real triodes. By chance, the author discovered a good tube that has been forgotten. It is the protagonist introduced in this article - 6C19. 6C19 is a side-heated high-power single triode tube, and its main uses are voltage-stabilized power supply adjustment tubes and OTL power amplifiers. Its characteristic curve and pin arrangement are shown in Figure 1, and its main parameters are shown in the attached table. 6C19 has no better performance among thumb tubes due to its low internal resistance of 300Ω and anode dissipation power of 11W. Since it has not been hyped in the market, its price is very cheap, so cheap that people can't believe that such a low-priced tube can have any good sound performance. In fact, the sound of a tube cannot be directly linked to its price. What is more important is the quality of the tube itself and whether it can be used well. Figure 1 6C19 characteristic curve and pin arrangement

2. Circuit Principle The author designed and manufactured a low-power single-ended post-stage according to the characteristics of 6C19. The whole machine electrical schematic diagram is shown in Figure 2. Figure 2 Whole machine electrical schematic diagram For the tube 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 1~1.5kΩ. It happened that someone sold a 5W/2.5kΩ single-ended output transformer online. The power was suitable, but the impedance was slightly large. However, a larger impedance is helpful for low-frequency performance, so I bought a pair. After actual use, the effect is good. Combining the characteristics of 6C19 and the recommended working conditions, 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, the common self-sufficient bias is not used here, but a fixed bias is adopted. D1 and D2 rectify and C7 filter to output a negative voltage of about 250V, which is stabilized by R10 and DW1, and divided by W2 and R11 to provide the gate voltage required by 6C19. In addition, after 6C19 adopts a fixed bias, the utilization rate of the power supply can be improved. Since the negative voltage of 6C19 is as high as about -60V, that is to say, if the input sensitivity is set to 1V, the driver stage must have a gain of no less than 60 times to enable 6C19 to reach full power output, so the requirements for the driver stage are relatively high. The conventional methods are as follows: (1) Use medium μ tube or low μ tube for two-stage amplification; (2) Use high μ tube for one-stage amplification, usually using common cathode and SRPP connection, commonly used tubes are 6N9 and 12AX7; (3) Use high amplification factor pentode for one-stage amplification; (4) Use medium μ tube plus driving transformer (using its boost amplification function) for one-stage direct drive. In my opinion, if one-stage amplification can meet the needs, then two stages will not be used, because this will bring many problems such as increased phase shift, deterioration of signal-to-noise ratio, and complex circuit. Finally, the eyes were locked on the 6F2 composite tube. 6F2 contains a high amplification factor pentode and a triode, and the pin arrangement is shown in Figure 3. Figure 3 Pin arrangement of 6F2 The signal is amplified by the internal pentode and buffered by the triode, and then drives 6C19 through the coupling capacitor C. Since the output internal resistance of the pentode is relatively high, directly driving 6C19 will result in large distortion, poor high-frequency characteristics, and affected dynamic and low-frequency control, so a buffer stage composed of 6F2 internal triodes is added. The power supply part uses a direct-heated rectifier tube 5Z4 for rectification, and CLC forms a π-shaped filter circuit. The 6F2 filaments of the two channels share a set of power supplies, which are supplied by the three-terminal regulator LM7806.

3. Component selection 6C19 can be made of common Beijing tubes. Of course , it would be better if you can find those made in the former Soviet Union or Europe. Since 6C19 is cheap, prepare a few more when preparing materials, and try to find two pairs to help improve the sound. The easiest 6F2 is the Shanghai brand. Its European model is ECF82 and the American model is 6U8. If you can find it, you can compare their sound characteristics and choose according to your personal preference. The power transformer B2 has a power of 150W. Since there are many voltage windings, if the core window cannot accommodate them, you can add an independent filament transformer. The coupling capacitor C4 has a greater impact on the sound of the whole machine. Using some expensive capacitors, such as Jensen M-cap capacitors, the effect is of course good, but the price is too expensive and does not correspond to the price of the machine. When I made it, I chose a 0.15uF/400V CJ10 metallized paper dielectric capacitor and a 0.1 uF/250V Wima capacitor in parallel. The thick and warm mid-frequency of metallized paper dielectric capacitors and the clear and beautiful high frequency of Wima capacitors, the two tones blend with each other, complementing each other's strengths. At the same time, these two capacitors are cheap. It has become a consensus among audiophiles that oil-immersed capacitors can increase the charm and musicality of tube amplifiers, but it will affect the dynamic performance of tube amplifiers, and oil-immersed capacitors are large in size and difficult to fix. Therefore, oil-immersed capacitors should not be used too much in tube amplifiers. Just think of it as MSG and use it for seasoning. Therefore, two oil-immersed capacitors with a capacity of 7 uF are used in the power supply part. The potentiometer for controlling the volume uses the common ALPS potentiometer. The power switch was originally favored by Noble's rotary type, which is characterized by good feel and high reliability, but the price is too high. Later, I accidentally discovered a Japanese potentiometer with a resistance of 500Ω and a switch, which can be used to modify it into a rotary power switch. After actual use, it was found that its feel and reliability are comparable to Noble, and the price is only 1 yuan/piece, which is very cost-effective.

IV . Production and debugging When
we make a tube amplifier, we not only require its sound quality to be beautiful, but also its appearance cannot be ignored, because the function of an audio equipment is not only for us to enjoy music, but also should be a piece of art for home decoration. It should be beautiful and generous visually, giving people a pleasing feeling. For this reason, when making this tube amplifier, the author adopted a symmetrical design, and its bottom plate processing is shown in Figure 4.

Figure 4 Bottom plate processing diagram
The casing is pressed with a stainless steel plate with a thickness of 0.5mm. The dimensions are 240mm in width, 320mm in depth, and 50mm in height. Solid wood boards are clamped on both sides, with a size of 340mm×60mm and a thickness of 15mm. The appearance photo of the completed machine is shown in Figure 5 for reference during production.
The whole machine is only set with one signal input, so the panel knobs are relatively simple, with a total of only 2 knobs. One controls the volume and the other controls the power switch.
After all the components of the whole machine are welded and checked, you can start debugging. The debugging of this machine is very simple. You only need to adjust W2 and W3 so that the voltage across the cathode resistors R8 of the two 6C19 is 0.37V and then you can start testing the sound.

5. Listening with the machine Although the machine only costs a few hundred yuan, its sound performance is unexpected. When expressing the human voice, it is particularly clear, real and natural, and the singer's emotions are expressed vividly and deeply. When replaying string music, such as erhu and guzheng, the sweet and flexible timbre, the abundant and long reverberation and air feeling make people intoxicated and unable to stop. Of course, the output power of this machine is only 2W, and human voice and string music are its forte. If it is asked to replay large-dynamic symphony and drum music, it seems reluctant.