Application and improvement of 6P15 in power amplification

6P15 was originally used for video output of tube TVs, but was replaced by transistor TVs before tube TVs became popular. It can be said that it was born at the wrong time, and few people have been interested in it since then.

In fact, 6P15 is a broadband voltage and power amplifier tube with outstanding performance. It can amplify video signals with a bandwidth of several Hz to 6.5MHz with high fidelity and good linearity. It should be easy to use it to amplify audio signals with a bandwidth of 20 KHz.

The reason why 6P15 is neglected is that it is not compatible with commonly used power amplifier tubes. For example, the standard operating voltage and optimal load impedance of 6P1, 6P6P, 6P14 and other tubes are the same or close, and they are very compatible with each other. However, the parameters of 6P15 standard board voltage 300V, Ug2 150V, and load impedance 10 KΩ are far away from this range. Because most audiophiles regard winding transformers as a daunting task, especially output transformers, which are time-consuming and laborious but may not achieve the expected results, this has largely limited the application of 6P15 among audiophiles.

The high transconductance of 6P15 makes the screen grid voltage have a great influence on the stability of the working state. Some beginners think that 6P15 is unstable and easy to age. The main reason is that they ignore the high Ug2 when comparing it with 6P14 and other circuits.

6P15 is an electronic tube with excellent linearity. After consulting a lot of information, the author designed and produced two 6P15 single-ended Class A small amplifiers, the sound quality of which is far better than 6P1, 6P6P.

1. Application and improvement of 6P15 in power amplification

Over the years, some senior audiophiles have been exploring how to make good use of 6P15. The several circuits introduced in the available materials have their own advantages, but they are still a bit complicated for beginners. Only by keeping its good linearity and stable state, and designing the parameters within the range of general tubes such as 6P14 and 6P1, can this famous tube reproduce its glory. The following introduces two tube amplifiers made with 6P15.

Figure 1 is the common power supply circuit of the two machines. When Ua is 230V, Ug is 2200V, and Uk is 4V, the load impedance is 5kΩ, which is common with the 6P1 series. The sum of Ia and Ig2 of 6P15 is about 40mA. Generally, the power transformer of a five- or six-lamp radio outputs about 60mA, so it is somewhat inadequate to use it in dual-channel in this circuit. It will feel hot when listened to indoors for one hour in midsummer, but it can work for two or three hours in spring and autumn. If it is used for low-volume playback in the bedroom or long-term appreciation, a power transformer of no less than 80mA is required.

Figure 2 is a pentode connection power amplifier circuit: Figure 3 is a triode connection power amplifier circuit. In order to make 6P15 close to the general ranks, the shield grid, curtain grid and plate connection are changed to triode. When Ua200V and UK6V, la does not exceed 30mA, and the above power transformer is just suitable. Although the best load RL2.5kΩ, the internal resistance of the triode is low, and the requirements for load RL are relatively loose. The best RL=R ixB. In the formula, Rj is the internal resistance of the triode, and B is the load factor. As can be seen from Figure 4, B is relatively flat from 1.5 to 3.5, and the 6P15 R i is about 1.4 KΩ. The load RL can be from 2 KΩ to 5 KΩ. Except for the slightly lower power sensitivity and output, there is no difference between the two when listening to 5 KΩ and 7 KΩ output transformers.

2. Selection of voltage amplifier tube

The output amplitude of modern sound sources is usually not less than 1V, and the driving voltage of Class A amplifiers cannot be higher than the cathode voltage UK. 6P15 is a high-transverse tube, so it does not need a high amplification factor. The low-resistance medium μ tube 6N1 is the first choice, but it is abandoned because of its 600mA filament current. The 6N3 with the same μ but much smaller filament current is selected, and the sound quality is comparable to 6N1.

　　
After the 6P15 transistor is connected, μ is only 25, and the thrust of 6N3 is obviously insufficient, so we have to use high μ tube 6N2 instead. To prevent "top-up" distortion, a 4.7 KΩ attenuation resistor is connected in series at the input end, and the loop negative feedback is increased. The 300P capacitor can eliminate high-frequency noise and possible ultra-high frequency oscillation.

3. Selection, installation and debugging of original parts

1. A good quality power supply is the basis for good sound. L1 and L2 are filter inductors removed from a scrap switching power supply, and C1 to C5 are used to filter out high-frequency noise. These details cannot be ignored. The filter resistor should be kept away from the electrolytic capacitor due to its high heat generation. The effect of L and R being separated into two power supply paths from the filter resistor is better than one combined supply.

2. Both amplifiers use common components. The sheet leads of the early old-fashioned carbon film resistors are copper-tinned. Although they are not beautiful, they are very suitable for scaffolding welding and have good sound quality. Some later round lead resistors, whether metal film or carbon film, are mixed with copper and iron. When using them, try to use copper leads.

Nowadays, electrolytic capacitors of both domestic and imported brands rarely have the year of production marked, not to mention the disassembled ones. Before using the machine, be sure to charge and check the capacitor with 80% of its withstand voltage, disconnect the positive and negative poles after charging for 30 minutes, and measure the voltage values ​​at both ends one by one after 24 hours, and choose the one with the highest UC.

The grid-leakage resistors of the voltage and power amplifier tubes must be welded reliably, otherwise the lack of negative grid bias will cause excessive current and burn the tubes.

Both amplifier circuits have been rigorously tested in practice and can be assembled according to the marked values. This newspaper has previously introduced the arrangement of components, routing, grounding and other precautions, so I will not repeat them here.

3. The 6P15 on the market now are all inventory goods, which were produced a long time ago. At that time, some parts were processed manually, so the parameters of the tubes were relatively discrete. In order to ensure the consistency of the parameters of the L and R channels, the error of la and 1g2 of the two tubes should not exceed 10%, and the Ug2 of the pentode amplifier is slightly lower than 200V, which can further stabilize the working state and not affect the sound quality.

The filament current of 6P15 is as high as 760mA, and the heat generated by the tube body is much greater than that of 6P1, etc. Be careful not to get burned when testing and plugging.

4. The output transformer is the key to the sound quality. I once used a 5kΩ output transformer with a 16mmx16mm core from an accessory factory, and the sound quality was dry and dull.

The core quality is poor and the cross-sectional area is too small, resulting in insufficient bass. The leakage inductance and distributed capacitance caused by too many turns weaken the treble, so try to choose a core with a larger cross-sectional area. You can also use the following method to select: connect a 1W, 1W)n resistor in series with the primary to a 220V AC power supply, and use a digital meter to measure the voltage across the resistor. The smaller the value, the better. If it is wound by yourself, the lower limit fD is 60Hz~80HZ, which is a high standard. It is used to drive bookshelf boxes. A low fD will not produce a shocking effect. Too many turns and too large a core will weaken the mid-high frequency.

Applying a large loop of deep negative feedback to a tube amplifier will produce a dull feeling, and a small amount of appropriate negative feedback will improve the sound quality of a pentode amplifier better than a triode amplifier. This may be because the nonlinear distortion of the pentode is greater than that of the triode.

4. Comparison of sound quality

The sound quality of the power amplifier made by connecting 6P15 to pentode is far better than that of 6P1, and is comparable to the 6P14, which is known as "high-quality and elegant". It has its own unique charm in the beautiful details of the treble.

The sound quality of the amplifier connected with 6P15 transistors is unexpectedly good, which is significantly improved compared with the pentode. The tone becomes warm and sweet, with distinct layers, natural and transparent, which is suitable for listening to music with a small-power bookshelf box.

The price of electricity and the limitation of buildings have restricted the pursuit of large tube amplifiers with power consumption of 200-300W and output power of tens of watts by salary-earning audiophiles, making Class A small single-ended amplifiers popular. The 6P15 amplifier with five-to-three conversions is the best choice. The price of the 6P15 amplifier is very low, but it can produce elegant sound quality. This is the charm of the 6P15 tube amplifier.