6N3 tube preamplifier

I bought a "Ma Ti Shi" tube preamplifier circuit board. The circuit diagram is shown in Figure 1. The first stage is played by 12AX7A, and the second stage is played by 12AT7. Since there are no 12AX7A and 12AT7, I decided to use the cheap and good 6N3 instead. The amplification factor of 12AX7 is higher, about 95, and the amplification factor of 6N3 is about 35. The circuit structure is not changed, only the circuit parameters are adjusted.

If the original preamplifier circuit is to use 6N3 to replace 12AX7 and 12AT3, the static operating point needs to be redesigned. The characteristic parameters of 6N3 are shown in Table 1. The first step in remodeling the circuit is to determine the plate working voltage.

The chassis of a scrapped CD player is used as the chassis of the preamplifier. The voltage of the power transformer winding originally matched with the board is shown in Figure 2. The secondary input voltage is switched between 200V and 230V. Since the working voltage of the screen of 12AX7 is 250V, and the working voltage of the screen of 6N3 is 150V, the usage of the transformer is changed. The 300v winding is used as the primary, and the input AC 220V. The 30v winding is used as the secondary output winding. The transformer has two filament voltages of 12.6v and 6.3v. Since the 300v winding is used as the primary, the 6.3v winding voltage will drop to about 5V. The 12.6V winding voltage drops to about 10v. The filament current of 6N3 is 0.35A, and the two filaments are about 0.7A. The voltage of 6.3V is obtained by reducing the voltage with a resistor, R=(10-6.3)/0.7=5.2Ω, and 6Ω is selected.

The secondary winding of the power transformer of Kenwood DP-880Gs is used as a filter inductor to form an L-c type filter. The electron tube preamplifier has very high requirements for the power supply. Many articles introduce the use of electron tubes for rectification and voltage stabilization, which is costly and troublesome. In fact, both electron tube rectification and semiconductor diode rectification convert AC into DC, which will not affect the sound quality too much. Similarly, the effect of using electron tubes as voltage stabilization adjustment tubes and using semiconductor triodes as adjustment tubes, the author believes that semiconductor triodes are better.

The rectifier bridge is the induction cooker disassembled part KBJ2508, 25A/800v, ​​which meets the requirements. The series type voltage regulator power supply adjustment tube uses the color TV power supply adjustment tube D1403, the main parameter is 1500V/12A, which meets the requirements. It is difficult to find a voltage regulator tube above 100V, so the 24v voltage regulator tube series method is used to obtain the required voltage regulation value.

The specific steps are as follows. Drill 4 φ4mm holes according to the size of the mounting hole of the electron tube power transformer to fix the power transformer. Drill holes to fix the rectifier bridge D01 and the power adjustment tube on the bottom plate. Note that D1403 needs to be insulated with mica sheets. Glue 3 100uP/400V filter capacitors on the bottom plate with AB glue feet facing up, and use 1.5mm. Connect the multi-strand copper core wires. After 6 24V voltage regulator tubes are welded together in series, they are insulated with hemp leather tubes and installed in the air through D1403. R01 and R02 are connected in series, and they are also installed in the air with hemp leather tubes. This voltage regulator has a very good effect. The output voltage can be changed by changing the voltage regulator value of the voltage regulator tube.

Calculation of the static working point of 6N3 The screen voltage of the first-stage amplifier circuit is selected as 143V, and the screen current is 8mA. Draw a load line on the characteristic curve of 6N3, the load resistance RL=143V/8mA=17.8kΩ, take 18kΩ. This straight line is the load line of Ua=150V, RL=18kΩ. The negative gate voltage is 1.25V, and the current corresponding to the intersection of the load line is 3mA. Then, the cathode resistance corresponding to the working point is Rg=1.25V/3mA=0.416kΩ, take 420Ω. The second-stage amplifier circuit is similar to the first-stage amplifier circuit. The amplification factor of 6N3 is about 35, and the feedback resistance is adjusted to 15kΩ.

Installation. Install the circuit board in the original CD player circuit position. Drill 4 φ4mm holes on the CD player bottom plate according to the size of the circuit board installation holes. Place them between the bottom plate and the circuit board to prevent the circuit board from touching the bottom plate. The original 2 pairs of output plugs of the CD player are used as the input and output sockets of the preamplifier.

The preamplifier does not have any noise when it is turned on. The reason is that the transformer is far away from the circuit board, the filter capacitance is large, there is an LC type filter circuit, and the most important thing is that a voltage stabilizer is used. The preamplifier is matched with a self-made: 250W+250w pure Class A power amplifier, and the effect is quite good. The control strength is improved, the dynamic range is larger, and the sound quality maintains the sweet tone of the original machine. Since the pin arrangement of 12AX7 and 6N3 is different, reconnect according to the 6N3 pin.