6N2 tube preamplifier circuit diagram

The tube pre-stage circuit is shown in the attached figure. It is a commonly used two-stage resistor-capacitor coupled voltage amplifier. All common components are used in the circuit. The power transformer is a dismantled product of a tube radio, model DB-21-169, with reliable performance. Brand new 6N22 and 6241, about 6 yuan, the choke is replaced by an obsolete 8W fluorescent lamp ballast, the resistor is a domestic Dahongpao, the capacitor is a domestic CZ type, and the electrolytic capacitor is a domestic Tianhe brand.

Since there are not many components, the base is very flexible to make. You can decide according to your own specific situation. The shape can be round or square, which fully demonstrates the fun of DIY. The author assembled it with two 100mm×200mm×55mm switching power supply shells. Pay special attention to the installation of the power transformer. The farther the distance from the electron tube is from the base, the better. If conditions permit, try to shield the transformer.

Audition and debugging

The debugging of the tube amplifier is different from that of the transistor circuit. Since the tube amplifier has a higher voltage, beginners should pay special attention to safety. After the entire circuit is soldered, do not rush to turn it on. You should carefully check the circuit diagram to see if there are any wrong connections or missing connections. After checking, plug in the power supply and turn it on. After turning on the switch, do not let your hand leave the switch button and pay close attention. The filaments of each tube should light up immediately. Once there is an abnormal sound or ignition and smoke, turn it off immediately. Do not turn it on again until the problem is found and solved.

After the circuit is normal, you can try listening to the music. Use CD, MP3, or radio as the sound source, and connect 600Ω~1000Ω headphones to the output end. Music should be played in the headphones, and you can proceed to the next step of fine-tuning.

Disconnect the audio source, and there should be a slight rustling sound in the earphones, which is the thermal noise of the circuit and is normal. If there is a loud AC sound, there is a problem with the power supply. Check the filter circuit, ground wire, and electrostatic shielding of the power transformer one by one to see if they are well grounded, and whether the transformer is installed in a suitable position. Find the root cause of the fault and eliminate it.

Adjusting the static operating point

The sound quality is best when the tube works in the best condition, which requires adjusting the working point of the tube.

The preamplifier generally works in Class A state, and the working point of the electron tube is taken at the middle point of the gate voltage-screen current characteristic curve. The electron tube used in this machine is the audio-specific tube 6N2. According to the basic data and screen characteristic curve given in the "Handbook of Vacuum Devices for Radio Communications", the gate negative voltage is adjusted to about -1.5V, the screen current of the first stage is about 1.5mA, and the second stage is about 2mA.

During adjustment, changing the resistance of the amplifier tube's screen load resistor or cathode resistor can change the gate negative voltage value and the screen current.

When adjusting the static operating point, short the signal input terminal to the ground, remove the cathode resistor first, and use a 1.5kΩ2W wirewound potentiometer to weld it to the cathode resistor. Put the sliding contact of the potentiometer in the middle position, and connect a 20V voltmeter at the same time. Slowly turn the potentiometer left and right. When the voltmeter shows a suitable voltage value, remove the potentiometer, measure the resistance value with a multimeter, and weld it with a resistor of the same resistance value. You can also adjust it while listening to the sound until the sound quality is the best, and then replace it with a fixed resistor of the same resistance value.

Then connect the DIY pre-amplifier to the eight-power amplifier and you can listen to it.