Single ended high quality tube power amplifier. Scheme, description

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Single ended high quality tube power amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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This article continues our discussion of single ended power amplifiers. As you can see, the amplifier circuit is almost no different from the amplifier circuit published in my article in the Radio Amateur magazine No. 9 for 2003.

The author of the scheme, A.I. Manakov, detector@surguuttel.ru built an amplifier on two finger lamps 6N2P and 6P43P. Many radio amateurs who repeated this amplifier were pleasantly surprised by its soft natural sound with the relative simplicity of circuitry and the low cost of components. However, the questions that have come in regularly since the publication are mainly about two things: output power and the applicability of lamps with an octal base.

Going towards the wishes of radio amateurs, and after consultations with A.I. Manakov, I propose the following version of the amplifier.

The diagram of one channel of the amplifier, as well as the power supply for both channels, is shown in the figure.

Rice. 1. Schematic diagram of one channel of the amplifier, as well as a power supply for both channels (click to enlarge)

There are two main differences, these are increased output power, about 4 W per channel, and kenotron power, which has a beneficial effect on the sound.

The input signal is fed to a dual variable resistor, which is the volume control. I used ALPS, but due to its high cost, you can use any, preferably a wire resistor, group "B" (logarithmic dependence). Two separate volume controls can be applied, one for each channel.

One of the best (from my point of view) domestic small-signal triodes, 6H9C, was chosen as the preliminary stage lamp. Both halves of the lamp are connected in parallel. This achieves a reduction in the internal resistance of the lamp, which entails an improvement in load capacity and signal-to-noise ratio. Setting up the cascade consists in setting the voltage on the cathode of the 6H9C lamp in the range of 1,3-1,5 volts by selecting the resistor R3. Resistor R4 is chosen for the best sound quality. If you want to use another triode, for example 6H8C, then the resistance of the resistor R4 will be 20-25 kΩ, in which case you will again have to select the resistor R3. The 6H8S tube sounds more analytical, it has a lower gain (21 versus 70 for 6H9S), but maybe someone will like this sound more. The choice is yours.

The output stage is made on a 6P13S beam tetrode connected by a triode. It is the triode inclusion that is most optimal in terms of sound quality. The output stage has no features. The only thing that will have to be done is to select, using the resistor R8, the current through the lamp within 60-65 ma. This resistor can be made up of two resistors connected in parallel, for example, 1 kΩ 2 watts each. If you wish, you can use the common lamp 6P3S or 6P7S. The quiescent current of the output stage in this case should be in the range of 70-75 ma. However, I want to note that in this case the power will decrease to 2 watts (when using 6P3S) and the total harmonic coefficient of the amplifier will increase. I tried the 6P7S lamp and I want to note that it sounds good. When using it, the resistor in the automatic bias circuit is selected within 220-230 ohm 2W, and the resistor between the second grid and the anode is within 150-230 ohm 2W. The quiescent current in this case will be about 70 ma. The output power of the amplifier in this case will be about 3W per channel.

Now for the details. The sound of the amplifier as a whole depends on the quality of the coupling capacitor C3. I used Jensen, and from domestic ones, you can use K71, K78, K73, K40U-9, K40U-2, K42U-2 for the corresponding voltage from 250V.

Capacitors of constant capacity, shunting electrolytic, in circuits of automatic displacement of lamps - film. Shunting electrolytes with fixed capacitors improves sound transmission in the high frequency region.

The capacitance of these capacitors can be one or two orders of magnitude less than the capacitance of the electrolytic one. Electrolytic shunt capacitors in power circuits can be used K73; K77, and the electrolytes themselves in the filters of the power supply - Teapo, Samsung, etc. In automatic lamp bias circuits, try to use the best quality electrolytic capacitors, such as Black Gate. When using them, it is possible to abandon shunt capacitances altogether.

Output transformer TW6SE of the Moscow firm "Audioinstrument". By visiting the company's website at audioinstr.h1.ru, you can view and order the lamps, transformers, chokes, lamp panels, etc. you are interested in.

Fixed resistors P1-71 with a tolerance of 1-2%. You can use the aircraft, as well as the more common type C2-33N or MLT, corresponding to the power.

There are a number of questions regarding the cap put on the anode of the 6P13S lamp. There are many suggestions in the amateur radio literature about this. I have been successfully using spark plug wire tips from any car in my designs for a long time and with success. Due to the design of the tip, the contact is tight and reliable, and, importantly, you can change its inner diameter, since it is different for different lamps. If the tip does not solder well, use flux for soldering steel or non-ferrous metals.

The power supply is made on the kenotron 5Ts3S (5Ts4S, 5U4G). The use of kenotron power, in comparison with diodes, makes the sound of the amplifier warmer and more coherent.

Try it and see for yourself. Many articles have been written about kenotron power, so I will not go into details. The power transformer has four secondary windings. Two of them feed the incandescence of the lamps of the first and second amplifier channels, one feeds the kenotron, and the anode one, with a midpoint output, is designed for 300v x 2 at a current of 200 ma. As a first approximation, how many volts are on the transformer winding, so many are at the output, after the chokes and power capacitors.

Chokes can be used DR-2LM, DR-2,3-0,2 from black and white TVs, unified D 21, D 31, the data of both are on the website igdrassil.tk.

The inductors I use in this circuit are also from Audio Instrument. Their inductance is 5H, they are designed for a current of 300 ma.

The amplifier was mounted in a hinged way, the conclusions of the parts themselves and the contacts of the lamp panels were used to the maximum. The ground bus is made of a single-core copper wire with a diameter of 0,8 mm and has contact with the chassis at one point, next to the input. The wires going to the filament terminals of all lamps must be twisted together. This is necessary to reduce the background of the alternating current. Resistors R9-R12 also serve the same purpose. You also need to twist the wires from the input jack to the volume control. As these wires, I also use single-core wires, with a diameter of 0,4-0,7 mm, each of which (to protect against short circuits) is insulated with silk insulation (a thin shoelace is used).

In conclusion, I want to say that this amplifier is not just a circuit, but a really manufactured and well-proven device. I have been using it for about three months now and am very pleased with its sound. For those who think that 4 watts per channel is not enough, I will say that in a room of 16 square meters when using KEF Q1 acoustics (sensitivity 91 dB), the amplifier develops sound pressure commensurate with the sound pressure developed by a transistor amplifier with a power of 40 watts per channel (these are the results of a subjective assessment of my friends - musicians). But the sound is different. The amplifier perfectly feels the slightest nuances of the sound of instruments or voices and, as it were, "breathes" (forgive me if the comparison is not very correct). The sound does not tire him, he wants to listen and listen.

Author: Puzanov V.S., caravan@online.bryansk.ru, Bryansk, Russia; Publication: radioradar.net

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