Preamplifier made of 2P29 tube

The low-power direct-heated pentode 2P29 is a small seven-pin tube, mainly used in high-frequency oscillation transmission circuits and low-power amplification. This tube is connected into a triode for use in audio pre-stage voltage amplifier circuits. It has good linearity, beautiful tone, and high transparency. The author uses this tube to make a pre-stage and achieves satisfactory sound effects.

1. Circuit Principle
The circuit of this machine is shown in the figure, which adopts the classic common cathode amplifier, RC and DC isolation coupling output. Its main features are:

1. In order to avoid transmission distortion of ordinary volume potentiometers, this machine uses an audio-type extremely low-noise V-MOS field-effect tube for finger-touch volume control. Compared with the key-controlled volume circuit, some components are reduced and shielded, so that the noise coefficient of the volume control part is less than 1dB (the noise coefficient of the V-MOS field-effect tube is about 0.5dB), which can compete with high-end vacuum stepping potentiometers.

The V-MOS field effect tube has a high internal resistance and is a voltage-controlled device. A charging capacitor is connected between the gate and the source. Since the gate leakage current is extremely small, the capacitor voltage can remain basically unchanged for a long time when the tube works in the adjustable resistance area.

Its drain-source resistance will be controlled by the gate-source voltage, that is, the voltage of the capacitor. At this time, the tube is equivalent to a voltage-controlled variable resistor. When the finger-touch (conducting by finger resistance) switch S1 is closed, the capacitor is charged, and when the finger-touch switch S2 is closed, the capacitor is discharged. Thus, the purpose of controlling the drain-source resistance by voltage is achieved. Connect it to the audio equipment to adjust the volume. S1 and S2 can be made of thin silver or thin copper sheets, with a spacing of about 1mm. After debugging, the volume increase and decrease is set at about +2dB. The author installed the thin sheet on the top of the knob.

2.2P29 adopts triode connection to reduce internal resistance and improve linearity.

3.2P29 uses self-supplied grid bias, which is softer than fixed grid bias.

4.2P29 The selection, size and matching of the plate resistor directly affect the noise, gain, frequency response and distortion of the previous stage. The noise of the resistor is divided into two categories. One is generated by the thermal motion of free electrons in the resistor, which is called thermal noise. The other is the excess noise of the resistor.

It is related to the voltage across the resistor, and its value is much larger than the thermal noise. There is usually a high DC voltage across the plate load resistor of the electron tube. The larger the plate resistance value, the greater the voltage drop. The excess noise generated is large. The best value of the plate load resistor is usually 3 to 5 times the internal resistance. After 2P29 is connected as a triode, the internal resistance is about 3kΩ.

In order to ensure good low-frequency response, the resistance is calculated as 5 times, that is, about 15kΩ is used to obtain a better signal-to-noise ratio, and the resistance is divided into two parts at a ratio of about 1:10 with reference to the classical phono frequency equalization circuit (McIntosh-C22), that is, two resistors with a nominal value of 1.2kΩ (focusing on high frequency) + 12kΩ (focusing on medium and low frequencies) are connected in series as the 2P29 screen load resistor.

In this way, the plate load voltage is borne by two resistors respectively, which reduces the excess noise. At the same time, the frequency characteristics of this pre-stage are further improved. Low-noise resistors can be selected for the two resistors.

5. The power supply uses 6Z4 rectification. The voltage regulator WY4P stabilizes the voltage. The use of bile tube rectification and bile tube voltage regulation eliminates the switching distortion and tunnel avalanche distortion of transistors. Since the working current of the 2P29 front stage is not large, the cold start non-thermal power loss voltage regulator is directly used for voltage regulation. Compared with the bile tube voltage regulator circuit with thermal power loss, it has less thermal noise and also avoids increasing the internal resistance of the power supply. In addition, the voltage regulator is connected in parallel to the power output end. It is equivalent to a sound calibration capacitor, which has a beautifying effect on the sound quality.

6.2P29 is a direct heating tube. The change of filament voltage has a more significant impact on the electron emission than the side-heating tube. Therefore, the 2P29 filament of this machine is powered by a 2.2V DC regulated power supply to improve the signal-to-noise ratio. The power module LM-317 is used for regulated output, the maximum output current is 2.2A, and the AC ripple is less than 0.4mV.

2. Component
configuration: effect tube should be audio type, VMOS or NMOS type low power tube, such as IRFDll3 (60V/Vds, 0.8A/Ids, 1W/Pd), BSl70 (60V/Vds, 0.3Mid, 0.63W/Pd), etc. The two channels share a set of touch switches.

The output coupling capacitor is preferably a Teflon capacitor, or an oil-immersed silver film capacitor, such as Sprague VQ oil-immersed silver film capacitor, West oil-immersed capacitor, etc. This type of capacitor has good frequency difference conduction characteristics, low high-frequency loss, and mellow and pleasant sound.

2P29 cathode capacitor and filter electrolytic capacitor are made of domestic Tianhe, Changfeng, Jianghai, Wuxi and other large aluminum shell electrolytic capacitors. Fast-response camera flash electrolytic capacitors such as CDl7H can also be used. The power transformer has a power of 100VA, and the choke is 15H/20mA, or an R-type power transformer with small magnetic leakage radiation can be used. The filament transformer is 220W6V, with a power of 3W.

The 2P29 filament voltage-stabilized power supply part of this machine is not suitable for scaffolding welding because of its large AC radiation induction and poor shielding effect. It can be modified with the circuit board in the energy-saving bulb, which is convenient for production and helps to improve the AC signal-to-noise ratio.

3. Assembly and debugging
Choose a suitable base plate for reasonable layout. In particular, the power supply should be far away from the front stage and isolated and shielded with metal plates. Considering the skin effect, use a 1.5mm diameter silver-plated single-core copper wire for welding according to the circuit diagram. The signal input and output lines in the machine should still use TV coaxial lines, and take anti-vibration and shielding measures. All related connectors should also use silver-plated products. Adjust the cathode voltage of 2P29 to 2V, and the corresponding current is 2mA.

4. Listening effect
: Connect this machine to the system, the sound source is the computer sound card, virtual Dolby AC-3 surround software decoding, homemade 2x50W V-MOS field effect tube amplifier, the signal line is also made of TV coaxial cable, the speaker is 8-inch 91dB full-range speaker, turn on the power, there is no noise, click a song "Passionate Dongjiang Water" from the MP3; then use the DVD to try to play "Hugo Fever Disc 8" and other full-range discs and pop music, I was stunned by the sound, the high, medium and low frequency performance is comprehensive, the vocal music emotion is beautiful, and the taste of the tube is strong, especially the transparency and realism of the tone, which is clear and visible, and it is hard to describe.