Amplifier on lamps 6H13S. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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The advantage of tube amplifiers is known - a narrow range of harmonic distortion due to the use of triodes with linear anode characteristics, when it is possible to refuse interstage deep feedback. The use of push-pull stages helps to reduce the overall level of harmonics and increase the maximum power obtained from specific types of lamps. This eliminates the magnetization of the magnetic circuit of the output transformer, which significantly reduces its size and greatly facilitates the design of two-channel amplifiers. For high quality amplifiers, it is preferable to use tubes with low internal resistance in order not to use output transformers with a large primary winding inductance. These are the general design principles that have been taken into account in the development of the amplifier.

The amplifier has the following characteristics: reproducible frequency range 20 Hz ... 20 kHz with unevenness of 1,5 dB; output power - 8 W at a load of 8 ohms.

Fig. 1

The amplifier brought to your attention is assembled according to the classical push-pull circuit (Fig. 1). The amplifier input has a volume control. On the left triode of the VL1 (6H8C) lamp, a voltage amplifier is assembled, on the right - a phase inverter. Voltage amplifiers are made on the VL2 (6H7C) lamp, creating a signal voltage of up to 30 V at the output. Such a rather high amplitude is necessary for the normal operation of the push-pull output stage of a power amplifier operating in class A. The first two VL1 and VL2 lamps are powered by an increased anode voltage, this has a positive effect on the dynamic response of the amplifier.

The amplifier uses lamps of the octal series - 6N7S, 6N8S and 6N13S. They have the largest zone of linear sections of load characteristics, where the lamps work without distortion. The 6H13C triode has a low internal resistance, which makes it possible to obtain a low output impedance of a powerful stage and, as a result, lower resistance and inductance of the output transformer windings. Both triodes of the 6H13C lamp are connected in parallel.

The unified TPP-303 series (for powering semiconductor equipment) were used as output transformers, with which a good frequency response of the UMZCH is achieved. They are ideal for amplifiers with a two-tube 6H5C or 6H13C output stage.

Each channel has its own power switch and works independently (two monoblocks in one housing - photo in Fig. 2).

As network transformers, TS-180-2 (or similar analogues from the TS-180 series), previously used in televisions, were used. They are preliminarily disassembled, and one winding is wound on top of each coil (in the winding diagram III and IV) of 200-250 turns with a wire with a diameter of 0,2 mm. One of these windings feeds a 100 V grid bias rectifier for 6H13C lamps, the other - an additional anode voltage rectifier for VL1 and VL2 lamps. One such transformer freely provides (in light thermal conditions) the operation of one channel of this amplifier.

Capacitors can be used in the design - oxide imported (Jamicon, Rubicon, etc.) or domestic K50-7, K50-26, K50-27, isolation capacitors - paper KBG-I, BMT-2, K40U-9, film K73-9 , K73-17 or similar; resistors - MLT, VS or other similar ones of appropriate power. Deviation from the nominal value is allowed no more than ±10%. In rectifiers, it is convenient to use diode bridges of the RS407 type (for a current of 4 A and a voltage of up to 1000 V). Instead of the RS407 bridge for positions VD2, VD4, you can use a domestic-made diode bridge, for example, KTs403G (double), halving the capacitance of the capacitors SYU, C19. Instead of an imported incandescent lamp (HL1), you can use any other 6,3 V lamp by connecting it to one of several filament windings of a network transformer.

You should not save on the power supply: filter chokes are no less useful than high-quality oxide capacitors. All this will positively affect the final result.

When using imported oxide capacitors, I recommend providing them with an operating voltage margin of up to 80 ... 100 V. For example, if the supply voltage is 300 V, then use 400 or 450 V capacitors. Instead of one large capacitor, you can connect several smaller capacitances in parallel. It can be confirmed the usefulness of shunting oxide capacitors (C2, C3, C9, C11, C18) with film (1 μF) or paper (up to 10 μF) shunts to the appropriate operating voltage.

The amplifier is assembled on a rectangular wooden frame measuring 450x320x80 mm from pine bars, previously dried in a natural way. Their ends are cut at an angle of 45 ° on the "angle cutter", glued with epoxy. A stiffening rib is installed in the supporting frame, on which the chassis is additionally supported (so as not to be made of thick metal). A selection was made along the perimeter of the frame, into which a bottom panel of plywood 6 mm thick is inserted, where 20-30 holes with a diameter of 10 ... 12 mm are drilled for ventilation. The assembled wooden frame is coated four times with the "Belinka-TOPLAZUR" tinting compound in the color "mahogany" (No. 23). A 2 mm thick steel chassis sheet is inserted into the sample in the upper part of the frame, where all functional units and lamps are located. The case, which covers the mains and output transformers, is soldered from foil fiberglass 2 mm thick. The body and chassis are painted with matte black acrylic enamel.

Separate metal housings house miniature incandescent illuminated input indicators (HL1) and volume controls. The lamps are separated from them by tinted glass 6 mm thick. Four holes are drilled in the glass, through which it is bolted to the rear walls of the indicator housings.

In addition to the aesthetic function, the output pointer devices serve as indicators of amplifier overload and channel gain difference. Between the blocks of indicators there is a panel cut out of tinted glass 5...6 mm thick. It performs a decorative function: through the tinted glass, the glow of the lamps is beautifully visible. The glass protects the hand from accidental burns on the lamp bulb when the amplifier is turned off, since the mains switches are located on the horizontal part of the chassis.

Behind the lamps in a row are network and output transformers. All of them are closed by one common metal screen of a box-shaped design. The design features of the amplifier are clearly visible in the photo: in fig. 3 shows the placement of the amplifier elements inside the chassis, and in fig. 4 - placement of connectors, terminals and fuse holders on the rear panel, mounted on a wooden frame.

I always try to make the maximum number of finished nodes so that at the last stages of assembly I work with only two tools: a screwdriver and a soldering iron.

I want to give some general recommendations for the manufacture of tube amplifiers

• Lamps should be selected according to the anode current the most identical, if possible.
• There is no need to close the lamps, let it be seen that they glow; natural convection is necessary to avoid overheating of nearby nodes.
• Use high-quality dried wood in constructions, preferably from valuable species.
• Aluminum and brass are noble metals, they can be polished and coated with a colorless alkyd car varnish. Steel parts can be painted black.
• Use parts made of natural glass, preferably tinted, they really enliven the design and create style.
• If possible, make the volume knobs of the original design by making them on a lathe or milling machine.
• Do not save on materials and do not rush, then you will make an apparatus that will delight your hearing and vision.
• Establishment should begin by checking the power supply and its nominal voltages. Naturally, without load, its voltage will be slightly higher than the nominal values.
• When establishing the channels of the amplifier, the current of each lamp of the output stage operating in class A mode is set to 80 mA, while the bias voltage on the grid is approximately -97 V. The anode voltage of VL3 and VL4 is 210 V. In the process of tuning the lamp amplifier VL3 and VL4 align with the anode current with grid bias regulators R14, R15.

Author: O. Platonov, Perm; Publication: radioradar.net

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