Experiment of 2-in-1 single-ended Class A tube amplifier

Add a set of tubes and a small amount of resistors and capacitors to one amplifier to form two amplifiers of different styles. The two amplifiers share a power supply and a pair of expensive output transformers, and the two tube amplifiers only take up the space of one amplifier. The two pre-stages can choose triode pre-stage or pentode pre-stage at will. Two post-stages, one uses the side-heated pentode FU-50, and the other uses the direct-heated pentode FD-422. You only need to flip the switch to select the required pre-stage and post-stage working mode. The figure below is the circuit principle.

　　
This power amplifier is a single-ended Class A amplifier with a fully symmetrical mirror design. Volume potentiometers are set on the left and right sides of the front of the base, the power switch is on the outside on the right, and the DC high-voltage switch is on the inside. On the first row of the base, the left and right positions are 6N8P first set of preamplifier tubes, and on both sides are FU-50 power amplifier tubes. In the middle of the second row is a 523P rectifier tube, and on both sides are 6J4P second set of preamplifier tubes. In the middle of the third row is a C-type choke, on both sides are FD-422, on the outside are two oil-immersed capacitors, and in the middle of the last row is a power transformer, and on both sides are 50w output transformers.

1. Preamplifier

In order to obtain good stability, reduce the noise and distortion of the whole machine, this pre-stage does not pursue high sensitivity, but uses low μ tube 6N8P, half of which is used for signal amplification and the other half for voltage driving. This combination of pre-stage is very classic, with strong voltage amplification ability, good frequency response and phase characteristics, very suitable for CD, DVD and various digital sound sources, enhancing the charm of music.

2. Pre-amplifier

This stage is only a 6J4P-stage amplification. When the pentode is used as a voltage amplifier, the noise and distortion are slightly larger than those of the triode. However, the pentode also has its advantages, and the amplification factor is much larger than that of the triode. The screen load resistance RL of 6J4P is relatively small, because the pentode Ri is high, and if the load resistance RL is too large, the high frequency attenuation will be large. The RL of 614P is controlled at 56Kn, and the high frequency response can be extended to more than 30kHz, and the higher the UP, the smaller the distortion and the better the dynamic response. In order to ensure the fidelity of this stage, a voltage stabilizing circuit is set for the 614P screen grid, because the stability of the pentode screen grid voltage is extremely important and closely related to the sound quality.

3. Power amplifier stage

FU-50 is a medium-power pentode with a plate voltage of 800V and a plate power consumption of 40W. It was a military product in the early years and was used for high-power mobile wireless transmission in vehicles. The upper frequency limit of this tube is very high, so it is no problem to use it in audio amplifiers. The triode connection has a wide linear area, high fidelity, and a wide frequency response. The biggest feature of this tube is that the tone is very transparent, crystal clear, and has a unique musical charm.

When connected as a triode, FU-50 can reach an output power of 10W. It is a cheap power amplifier tube that is deeply loved by audiophiles.

The other power amplifier uses FD-422. FD-422 is a direct-heated pentode power tube. In order to prevent external interference, a graphite layer is sprayed on the glass shell wall during the manufacturing process, which plays a good shielding function. The single-ended Class A amplifier made of this tube has a wide sound field, good transient response, and realistic restoration of string music and vocals, with a strong tube flavor. When connected to a triode, the sound quality is very close to that of the King of Tubes 300B.

The author installed the machine to pursue the enjoyment of high sound quality, so he chose the single-ended Class A circuit. Although the efficiency is lower, the sound is very pleasant. Since the two sets of power amplifier stages are classic circuits, that is, the pentode screen grid is connected to the screen electrode with a small resistance resistor, the circuit is simple and easy to make, so the author does not have much space to describe. What remains is how to make good use of the two tubes, FU-50 and FD422, to maximize their potential. The following is the author's experience for reference:

First, you need to invest in the power supply system; second, you need to choose good components; third, you need to work hard on debugging. Only by doing the above three things can you achieve beautiful sound.

4. Power supply

Power supply plays a very important role in audio, especially for Class A equipment, which has higher requirements for power supply. According to a rough estimate, two FU-50s with higher power consumption, plus 5Z3P rectifier power supply, consume about 150W of power. To ensure the quality of the whole machine, a 300W power transformer is selected, with a reserve of 150W. This is a more conservative configuration. Although it is less economical, it has a small internal resistance and can work safely for a long time in a high temperature environment.

It is better to use oil-immersed paper dielectric capacitors for high-voltage filter capacitors, which have excellent frequency characteristics, reflected in the high-frequency transparency and the low-frequency elasticity. The sound quality is much better than that of electrolytic capacitors. The choke coil uses a C-type iron core, 15H 250mA.

5. Selection of components

The output transformer is the most valuable component of this machine. Choosing a good output transformer is crucial to the sound quality of the amplifier. If a bad output transformer is used, the installation will lose its meaning. This machine uses a primary side impedance of 3.5KΩ and a power of 20W to 30W. However, the author used a 50W one, which is much more expensive, but is good for low-frequency diving, and there is no need to worry about magnetic saturation during large dynamics. Another point is to choose a product with a good reputation. The author uses a Kaili brand transformer, which feels good.

The coupling capacitor uses the early domestically produced CYM-2, a 0.1μF oil-immersed capacitor in parallel with a 0.047μF oil-immersed capacitor. Compared with imported oil-immersed capacitors such as Spi VQ, Western Electric, BlacKMat, etc., the tone is not inferior to foreign brands at all.

The bypass capacitors are all oil-immersed and thin-film non-polar capacitors, which have a certain improvement on the sound quality and timbre. The sound is obviously clearer and brighter, the low-frequency force is greatly increased, and it is full of elasticity.

The value of the decoupling capacitor must not be too large, otherwise a lot of details will be lost. Oil-immersed capacitors or polypropylene capacitors are preferred.

When designing the chassis, make it as large as possible, and use non-polar capacitors for all the capacitors in the entire machine. This will greatly help improve the sound quality and tone.

6. Debugging

After the machine is assembled, the important task is debugging. Usually the power amplifier tube is adjusted to 80% of the standard screen power consumption, because the screen power consumption at this time is just in the linear area, and it is also beneficial to extend the working life of the power amplifier tube.

For example, the FU-50 amplifier group of this machine has a total power supply of 500V, and after the output transformer voltage drop, the measured screen voltage is 482V. The screen current I=UR-60700-0.085A, and the power P=UI=(482V-60V)x0.085=35.87W.

The total power supply of the DF422 amplifier stage is shared with the output transformer, the screen current I=UR=29450=0.0644A, and the power P=U1=(482V-29V)X 0.0644=29.1W.

Debugging is a very complicated task and is also the most technically demanding job in playing audio. Only by making proper adjustments can you get a good sound and extend the life of the power amplifier tube.

VII. Precautions for use

Before switching the front stage, turn off the high-voltage switch for a few seconds, and switch after the energy in the filter capacitor is consumed, so as to avoid the explosion sound caused by high-voltage ignition.

8. Two groups of pre-stage audition results

The first group uses a two-stage amplifier circuit with a triode 6N8P. When playing music, the sound is sweet and clear, especially the female voices such as Tong Li and Gong Er, which are so sincere that the author is intoxicated. Listening to the violin, the sound is delicate and smooth, the overtones are just right, and the high frequency extends upward naturally. The trumpet is bright and cool, very clean and refreshing.

The first group is pentode 6J4P, which has a mellow sound. For example, when playing Zhao Peng's record, the bass is excellent and penetrating. When playing dynamic music, the background is quiet. When combined with the FD-422, the treble is delicate and round, the midrange is plump and stretched, and the bass drum beats are vigorous and powerful, which goes straight to the heart.

9. Two groups of post-stage audition results

When FD422 is connected to a triode circuit, the sound is very close to that of 300B tube, and the sound has a classical charm. Although the charm and details of 300B are better than FD-422, the price difference between the two tubes is more than ten times. I think FD-422 is more suitable for elderly audiophiles and audiophiles of the working class.

As for FU-50, the sound is cleaner and sweeter, giving the listener a feeling of vigor and vitality. Although FU-50 cannot be compared with 300B, the low-frequency strength of FU-50 is much better than that of 300B.

The above two tubes can be said to be good tubes with good sound quality and low price and high cost performance when used in audio equipment. Their sound quality and timbre are better than those of tetrodes.