Author: Zhou Jun
The following figure is a single-ended schematic of 6J8P + 6P6P. Each 6J8P pushes a 6P6PO signal from the source. First, after the adjustment of W1, enter the gate of 6J8P to enlarge the gate. R1 is 6, J8P cathode resistance, the size of the resistor determines the high low of the negative gate pressure, the value of the unit is 1 kΩ, ensuring that the negative gate pressure is around -1.5V: R4 is 6J8P anode electrical resistance value of 100KΩ, The resistance resistance value selection will have an impact on gain and high frequency characteristics. Of course, the difference from the five-poular-curtain gate voltage will also affect the gain of this level, and can be made according to the actual needs in accordance with the parameters provided on the manual. This mechanism is slightly changed, to obtain a stable curtain voltage, improve nonlinear distortion, and can also appropriately change the voltage gain of the stage by adjusting the curtain gate pressure. The signal after the primary amplification is coupled to the 6P6P gate of the power amplification stage. Since the negative gate pressure of 6P6P is shallow, the push voltage is low, so the pre-amplification and promotion amplification of the unit is combined into one level. Each channel is completed by one 6j8p, and the 6P6P of the power amplification stage uses a simple and practical "self-bias" circuit. The self-bias circuit has the function of automatically adjusting the work point, and it is possible to prevent the current overload due to the changes in the anode voltage, the work is relatively stable, the exhaustion, and no need to adjust when changing the tube, but the disadvantage is to reduce the output of the power amplifier tube. power. R6 is 6P6P cathode resistance, adjusting the resistance to change the working current and negative gate pressure of the power vessel, it is recommended that you will be adjusted in accordance with the working current given in the manual, and the excessive or too small current is not good. The working screen designed by this unit is about 6P6P's maximum screen, which is full of 6P6P characteristics and guarantees the life of the tube. The measurement of the gate negative pressure is: On the 8 feet of the red table pen on the 8 feet of the power amplifier, the black table pen is directly on the 5th feet of the power amplifier tube. At this time, the reading of the multimeter is the gate negative pressure. The value of the gate negative pressure is the same as the voltage value of the cathode of the power amplifier tube, only the cathode is a positive voltage, and the voltage of the gate to the cathode is a negative voltage. R5 is 200kΩ, this resistance is a gate resistance, the supply circuit of the gate negative pressure is caused by the resistor, and the RC network composed of the resistor and C2 also determines the lowest operating frequency of the transmission between the front and rear stages.
The unit did not use the large-loop voltage negative feedback, but a single-stage current string negative feedback circuit, because the large loop negative feedback can improve the ultimate machine frequency, improve the whole machine distortion, reduce the whole machine noise, etc., but also It will reduce the vitality and musical notes of the sound. Everyone may notice that there is no cathode capacitance in the predecessor 6J8P and power amplification level. It is right! The purpose of this capacitor is to get some negative feedback. The amplified audio alternating signal current generates an alternating voltage on the preamble and the power amplifier electron tube cathode resistance R1 and R6 when flowing through the cathode resistance, and the signal voltage input from the gate is exactly the opposite. After superposition, the intensity of "weakening" input signal will be ", since the feedback is available, the input signal is increased when the same output signal is obtained. If the cathode voltage of 6p6p shown in the schematic shown is 13.6V, the effective value of the push voltage of the pre-stage should be around 13.6V, that is, the valid value VRMS = 13.6 V of the 6P6P control gate input push signal. However, if a 100μF capacitor is connected to the 6P6P cathode, then the "current string negative feedback" function of this stage is not, then the peak value of the predecessor push voltage should be around VP = 13.6V. That is, the input push signal of the 6P6P control gate is valid VRMS = 13.6V ÷ 1.414 = 9.6V, and the push voltage will appear "Top" distortion if it is higher than the above value. Therefore, the addition of negative feedback reduces the magnification of the amplifier, but can improve the stability of the amplifier, improve the frequency distortion of the amplifier, improve the nonlinear distortion of the amplifier, reduce the internal noise of the amplifier, etc., of course, the disadvantage is to reduce the output of the power amplifier tube power. Because if there is no cathode decoupling capacitor, some audio signal will be applied to R6; and the cathode decoupling capacitor will pass by bypass the audio signal on R6! "Self-supplied bias circuit" The impact of the cathode capacitance on the output power is depends on whether the audio AC signal is generated in R6; the "fixed bias circuit" and "self-biased bias circuit" and "self-biased circuit" affect the output power. Since different circuits are produced differently from the efficiency of the power supply. This is different, please pay attention to you! Many "bile friends" very much like the "fixed bias circuit" of the power amplifier tube. When obtaining higher output power, it can easily increase the small working current in the case of increasing the anode voltage, and the work and transport pipe work in the state of "high voltage, small current", to obtain personal preferences "Sound bottom tranquility, treble The sound effect of clear ". In fact, the increase in output power is improved. The "self-sufficiency circuit" can also obtain the sound effect of "Sound of Sound, Tall", mainly what everyone comes to school, how to match the predecessor, how to make the damping, frequent sound Wait for match your speaker. Reach the sound you need. Valid value VRMS peak VP, peak-peak VP_P, mean VAVG is some of you often hear, use parameters, there is a transformation relationship: VP_P = 2 × VP; VP = 1.414 × VRMS; VAVG = 0.9 × VRMS . These translational relationships must be familiar with the heart, often used when calculating, tested, and calibrating the machine. After the machine is installed, it is turned on and confirmed that the welding is correct. When the power is turned on without inserting the tube, first measuring whether the filament is supplied normally: the filament voltage is known as the A voltage, the 524P 2, 4 feet are 5V, 6P6P and 6J8P 2, 7 feet are 6.3V, when The voltage is high, it is normal; whether the high pressure winding of the transformer is normal, the set of voltage is referred to as B voltage. After everything is normal, you can turn off the power and plug all tubes. Before you turn on the power, be sure to put the fake load on the active speaker output terminal, use coupal presence, do not load the speaker terminal, otherwise it will excellent damage to the cost of expensive output transformers. First, measure the B power supply, the voltage of each point is basically the same as the label parameters on the drawings. If there is too much access to turn off the power, then check the circuit, pay attention to the boot, the B power supply is slowly elevated, and the highest will rise to the power supply The transformer is 1.414 times, and then the 6J8P and 6P6P filaments are constantly warming, each tube is slowly entering the working state, and the high pressure of the power supply will slowly fall, and it has been reduced to the design of the working voltage. So the reading should wait until the tube is fully preheated, so it will get the correct, real value. The cathode voltage of 6J8P and 6P6P is then tested, and too high or too low, it is necessary to check the circuit, components, and the like. Turn off the power after the test is complete. Replace the fake load into a speaker, pick up the audio input signal, slowly rotate the volume knob after the boot is fully preheated, taste this pure voice, and appreciate your own masterpiece. It is also necessary to tell you: debug, do not quickly, frequently, turn off and turn off the power switch when using the asterisk, so it is not good, you must turn it again after the charge of the power filter capacitor can be turned on again, remember! For quality and low price "civilians" devices: WiMA unpolatile capacitors, domestic metal film resistance, dawn tube, etc. When the first-time friend, a friend who is in the preparation and commissioning must be careful, voltage test, especially high pressure test, and must follow the principle of single-handed measurement, generally put a black table pen on the ground, ensuring that it will not fall off. To avoid short-circuit, unnecessary loss or damage, after the pipetting, power-on must have a phenomenon such as smoke, odor or electronic pipe screen, otherwise disconnect the power immediately. Good damage to the tube is calculated in minutes. It will not be able to invade like a semiconductor, so everyone does not have to worry too much. The screen of the tube is not damaged in a short time. The audio uses the philipscd850mkii laser player, homemade 12-inch large box, domestic manufacturers use Vifa unit imitation bookshelf boxes and friends' import 3 / 5A small bookshelf box. This is only a small power of less than 2 × 3W. It really can't be extravagant, but the tattoo, lubrication, lubrication, is obsessed with it, if you abandon the bass. You absolutely don't believe this is 6P6P sent. In order to expand the applicability of the unit, a group of 640 headphone output is widely wound, which is convenient for the headset to listen. The data in the table below is a test indicator measured by this unit. The test instrument is CAT-TEK's CA-Type 01 signal source, HP's 54600 digital oscilloscope, HP 3478A high-precision three-meter, GW GAD 1 201G type Automatic distortion tester. Scientific rational "burning" is the ultimate pursuit of every courage, and the indicator will only talk about the sound. The author believes that a sound excellent courage must have a good parameter indicator; and a kind of as a parameter indicator is very poorly unable to make a sound. Replace 6J8P 6J8P 6 × 8 or Nanjing 6J8P, replace 6P6P for Nanjing 6V6 comparison audition, feel 6 * 8 bass slightly, the test data is also in line with the change of hearing.
Test frequency
20Hz
40Hz
100 Hz
400Hz
1khz
2khz
5khz
10khz
15khz
20khz
30khz
Pre-level input level
500MV
500MV
500MV
500MV
500MV
500MV
500MV
500MV
500MV
500MV
500MV
Premium output level
14.4V
1417V
14.4V
14.5V
14.0V
14.4V
14.4V
14.4V
14.4V
13.9V
12.0V
8Ω end output level
1.9V
3.4V
4.0V
4.29V
4.3V
4.3V
4.31V
4.3V
4.29V
4.28V
3.89V
distortion
7%
3.47%
2.50%
2.20%
2.40%
2.50%
2.7%
4%
2.30%
4.90%
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