Making a Biliary Preamplifier Using High-μ Tubes

Nowadays, some discs are made in HDCD format, which has greater dynamics, richer details, clearer layers, wider sound field, and a vivid sense of presence than ordinary discs. How can I make my existing playback system reproduce music with high fidelity? The tube preamplifier made of high μ triode can achieve this goal.

The circuit is shown in the figure below. The input stage voltage amplification is a common cathode amplifier circuit using a high μ dual triode 6N9P (foreign model is 6SL7). Due to the high voltage gain, the cathode resistor does not have a bypass capacitor, so Ve1 has negative feedback of the current at this stage, which reduces distortion. The μ of 6N9P is 70. It is characterized by clear, gorgeous, and delicate sound, wide sound field, and sharp dynamics. It performs better when the screen power supply voltage is higher. Foreign famous machines often use this tube as input stage small signal amplification. V is also a cathode output device, using a medium μ dual triode 6N8P. Although this stage has no voltage gain, the input stage gain has a large margin, and this machine still has sufficient energy to excite the rear stage power amplifier. V also acts as a buffer stage to isolate the front and rear stages, while reducing the output impedance, so that the front and rear stage power amplifiers match better.

This machine is equipped with a large loop negative feedback. The feedback voltage is drawn from the output end of Ve2 and sent to the cathode of Ve1 through resistor R8. The feedback amount is about 4dB to 6dB. There are two forms of feedback voltage drawing for large loop negative feedback. In the figure below, there is only one output capacitor, and the feedback voltage is drawn through C2. In this connection method, only C1 affects the timbre and frequency response of the amplifier, and it is easier to calibrate the sound. Another connection method is shown in the figure above. The feedback voltage is drawn between C1 and C2. C1 and C2 have a greater impact on the timbre and frequency response of the amplifier. When calibrating the sound, C1 and C2 must be selected in combination, and the capacity of C1 must be smaller than that of C2.

After welding, debugging and sound calibration are required. The cathode voltage of Ve1 is 1.5V~1.8V, and the grid voltage of Ve2 (also the screen of Ve2) is about 80V (not more than 100V). The cathode voltage is 3V~4V higher than the grid voltage, which can be used as a bias voltage. If the cathode voltage is low, the voltage drop will increase if the cathode resistor is appropriately increased. After listening, adjust the resistance of the negative feedback resistor R8 according to your needs to make the tone color meet the requirements.

This machine is suitable for the playback system of transistor amplifier or 300B tube amplifier. If it is used in a tube amplifier system with good tube flavor, the tube flavor will be too strong.

The improvement method is to replace VE2 with 6N6, because the timbre of 6N6 is more neutral, and the bass performance will be better. Therefore, before installing the machine, consider the situation of the playback system of this machine, and then decide whether VE2 should use 6N8P or 6N6.