Single-cycle stereo amplifier on pentodes. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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When creating this amplifier, the task was to make the most simple design with good parameters and a decent design, suitable for repetition by novice radio amateurs. The amplifier uses only ready-made winding units (mains and output transformers, choke) from household lamp equipment of the last century.

A diagram of one of the amplifier channels is shown in fig. 1. The input signal goes to the volume control R1 and then through the capacitor C1 to the control grid of the lamp VL1. The 6Zh1P pentode was chosen for reasons of economy (lamp filament current - 180 mA), non-scarcity and decent parameters (this lamp was used in professional microphone amplifiers).

Rice. 1. Diagram of one of the amplifier channels (click to enlarge)

From the load resistor R3 through the capacitor C4, the amplified signal is fed to the control grid of the lamp of the output stage VL2. The VL2 lamp operates with automatic bias, but its cathode circuit includes a current source on the KR142EN5V microcircuit (voltage stabilizer + 5 V). The input of the stabilizer is connected to the output of the cathode of the lamp, and a current-setting resistor R9 is connected to its output. With a resistor of 100 ohms, the anode current of the VL2 lamp is 50 mA. The use of a current source made it possible to improve the linearity of the frequency response of the amplifier.

Switch SA1 in the power supply circuit of the screen grid VL2 allows you to select the mode of switching on the lamp - pentode or triode. In pentode mode, the maximum output power of the amplifier is about 2 W at a load of 8 ohms in the operating frequency band of 120 Hz ... 25 kHz at a level of -3 dB. In a triode connection, the amplifier power is at least 1 W at a load of 8 ohms and a band of 40 Hz ... 12,5 kHz at a level of -3 dB. In addition to differences in the electrical characteristics of the modes, the nature of the sound is also significantly different.

The amplifier is assembled on a chassis, which is a steel frame with dimensions of 240x150x54 mm, soldered to a steel sheet with dimensions of 240x150x0,7 mm, in which the necessary holes are pre-cut for installing radio tubes, network and output transformers, for ventilation. The soldering points of the sheet and frame were thoroughly washed, cleaned and puttied with polyester automotive putty, after which the entire chassis was primed and painted with automotive enamel (Fig. 2).

Rice. 2. Amplifier elements

The output transformers are placed in steel screens. This is more of an aesthetic than a technical necessity. Suitable metal boxes were found for this purpose, inside of which M3 screws with pre-sawn off heads were soldered at the corners. The screws should protrude 6...8 mm beyond the edge of the box; with their help, the boxes are attached to the chassis from the bottom side with four M3 nuts. Screens of transformers are also primed and painted with car enamel.

The front panel of the amplifier is cut out of an AMG-5 aluminum sheet 1,5 mm thick, polished with fine sandpaper under a stream of warm water and covered with a colorless automotive varnish. The panel has a backlit power switch and channel volume controls. The panel has dimensions of 240x65 mm, its upper edge protrudes 10 mm above the chassis.

On the rear wall of the chassis there is a network connector with a fuse, a WP4-18FB push-in terminal block for connecting acoustic systems and two RCA jacks RS-104 or RS-115 (tulip type) for inputs of the left and right channels.

The installation of the amplifier begins with laying the power supply circuits for the incandescent lamps with an intertwined pair of wires. All parts of the amplifier are mounted in the basement of the chassis in a hinged way on the terminals of the lamp panels and on the pads made of strips of textolite glass foiled on one side. Capacitors C6-C8 are fixed on Plexiglas plates (Fig. 3). Also, in the basement of the chassis along its front panel, a tinned copper wire with a diameter of 1,5 ... 2 mm is soldered, which serves as a common wire.

Rice. 3. Mounting the amplifier

All fixed resistors - VS, MLT or ULI, variables - series 16K1 or S16KN1 with functional characteristic B. Oxide capacitors - K50-35 or their imported analogues, the rest - MBM (C1) and BMT-2 (C3, C4). Ratings may differ by ± 20% from those indicated in the diagram.

Output transformers - TV-3Sh or similar from the output low-frequency stages of TVs on 6P14P lamps. Network transformer T2 - from the tape recorder "Kometa-201" or any other, providing a voltage of 190 ... 200 V at a current of 150 mA on the secondary winding. Power filter throttle - D-16, D-17.

The diode bridge RS407 of the anode rectifier was chosen with a large margin. It can be replaced by KTs401 -KTs405 or any other with a reverse voltage of 500 V and a current of 300 mA. Switch SA1 - MT1, SA2 - MIRS-101-8C3 with backlight.

An amplifier assembled from serviceable parts starts working immediately and practically does not need to be adjusted. Tinted glass 5...6 mm thick is glued to the protruding part of the front panel from its inner side on a colorless silicone sealant (Fig. 4). The ends of the glass are polished with a belt grinder. If you don't have a power tool but have the time, glass sanding can be done by hand on a hard, flat surface with sandpaper on it. The glass should be glued last, after the entire amplifier is finally assembled.

Rice. 4. Amplifier assembly

Author: O. Platonov

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