Using Shuguang 350C tube to make tube amplifier

WE350B is very famous, and 350C is a replica of the famous Western Electric WE350B antique beam quadrupole power amplifier tube, which can satisfy audiophiles' pursuit of high-quality electron tubes.

The screen power dissipation of 350c is 30w, the filament voltage and current are 6.3V/1.6A, and the maximum screen voltage is 400V. It is similar to WE350B, 6L6, 6L6G, and 6P3P, and can be directly used interchangeably. The glass shell of 350C is much larger than that of 6P3P and 5881A, which is conducive to heat dissipation and makes it work more stable. When the screen voltage is 250V, an output power of 15w can be obtained. The commonly used 6L6, 5881, KT66, etc. only have an output power of a few watts in this state. If the load resistance is reduced to 2kΩ, there is still an output power of about 10w when the gate negative voltage is 14v, and the distortion is smaller at this time.

If the audiophile already has a tube amplifier with 6L6 or 6P3P output, it can be replaced with 350c with a slight modification. However, it is necessary to make appropriate adjustments according to the power supply of the original machine. The main thing is to adjust the screen voltage and grid negative voltage. Because the screen current of 350C is large, if the power supply capacity of the original machine is not enough, there will be insufficient power supply, making the sound quality unstable and the transformer overheating. Therefore, it is necessary to make appropriate adjustments according to the specific situation of the power supply. By adjusting the cathode resistance value, the 350c can establish a suitable working point.

When using 350c as the output tube of a tube amplifier, you should pay attention to the matching of the amplifier tube and the rectifier tube. Although the output power of 350 is relatively large, its tone is very delicate and smooth, with high resolution and crystal clear music. It should be taken seriously when combined with voltage amplifier tubes to achieve a more perfect performance.

The circuit of this machine is shown in Figure 1. The whole machine adopts a first-stage voltage amplification, a first-stage push, and a first-stage single-ended A1 power amplifier circuit. In order to improve the performance of the whole machine, the amplifier circuit is equipped with a large loop negative feedback.

The input stage V1 is a voltage amplifier stage of triode class A amplification, using high μ dual triode Shuguang Collector's Edition NO:01 12AX7B, with a voltage gain of about 30dB. The driver stage uses medium μ and high screen current dual triode 12AU7 to form an SRPP circuit, with a voltage gain of about 20dB. V1 and V2 are both voltage amplifiers with a strong musical flavor, which complement the 350c. V2 can be replaced with Beijing Kaiping 6N1, which has the same strong flavor. If V1 uses 6922, the music will be thicker, but the treble will be slightly inferior.

There is a switch K2 at the screen grid of 350c, which can put the tube into two working states: triode and standard circuit. When K2 is at point ①, the screen grid is connected in parallel with the screen electrode, forming a triode working state. At this time, due to the deeper negative feedback of the screen grid, the output power is reduced, but the tone becomes more delicate, smooth, and has higher fidelity. If K2 is set to point ②, it is the standard circuit working state, the output power is restored to the rated value, the dynamics will be enhanced, and the tone will be more bold. The feedback amount from the output end to the input stage V1 can be adjusted through W2.

The power supply uses Nanjing 5U4G for full-wave rectification. If the power amplifier V4 circuit is properly modified, other types of beam quadrupole power amplifier tubes can be directly replaced, such as 6L6, KT66, 6P3P, 5881, 6V6GT, etc., as shown in Figure 2. That is, add a switch K1 and a resistor to the cathode circuit of V4. When K1 is at point ①, it is suitable for 350c playback. After K1 is set to point ②, it can be used for 6L6, KT66, etc.

This machine uses the EI-type power transformer of the 150W amplifier from Shanghai No. 27 Factory. The internal connecting wires also have a certain influence on the tone.

The assembly process has a great impact on the sound quality and signal-to-noise ratio, so it should be taken seriously. This machine adopts the method of shed welding, and the layout of components is rigorous and reasonable. The components are placed from front to back according to the signal flow, and the arrangement is based on the principle of the shortest routing.

After the assembly and welding is completed, before powering on, measure whether the B+ high-voltage circuit is short-circuited to the ground. The method is to use the DC resistance of the multimeter to measure the DC resistance of point A to the ground. Generally, it is normal if it is more than tens of kΩ. If the DC resistance is very small, there is a short circuit to the ground. The cause should be found and eliminated. Then connect the speaker and measure it after powering on. The B+ output voltage (point A) should be about 280V~300V and should be very stable. The voltage of the third foot of 350c is 250v~270v, and the cathode voltage (the eighth foot) is 14v~18v, indicating that the power supply and power amplifier circuit are basically working normally. The voltage of the eighth foot of V2 is 3V~4V, the voltage of the sixth foot is about half of the voltage of the first foot, and the cathode voltage of V1 (the third foot) is 1.5v~2v. At this time, if the volume potentiometer is appropriately turned up, the metal part of the small screwdriver in the hand touches the "+" end of the input RCA jack, and the speaker will make a "beep" sound, and you can connect the tube preamplifier and the sound source to listen.