How to do exercise vacuum

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. In the amplifier, only finished hitch units (mains and output transformers, choke) from household lamp equipment of the last century are used.

A diagram of one of the amplifier channels is shown in Fig. 1. The input signal is fed to the volume control R1 and then through the capacitor C1 to the control grid of the lamp VL1. Pentode 6Ж1П was chosen for reasons of economy (the lamp filament current is 180 mA), non-deficiency and decent parameters (this lamp was used in professional microphone amplifiers).

Fig. 1. Diagram of one of the amplifier channels

From the load resistor R3 through the capacitor C4, the amplified signal is fed to the control grid of the output stage VL2. The VL2 lamp works with automatic bias, but the current source on the KR142EN5V microcircuit (voltage regulator + 5 V) is switched on in its cathode circuit. The stabilizer input is connected to the output of the cathode of the lamp, and a current-setting resistor R9 is connected to its output. When the resistance of the resistor is 100 Ohms, the anode current of the lamp VL2 is 50 mA. The use of a current source has improved the linearity of the frequency response of the amplifier.

The switch SA1 in the power supply circuit of the screen grid VL2 allows you to select the lamp on mode – 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 on the level of -3 dB. In triode switching, the amplifier power is at least 1 W at a load of 8 Ohms and a 40 Hz band. 12.5 kHz in the -3 dB level. In addition to differences in the electrical characteristics of the modes, the nature of the sound is significantly different.

How to do exercise vacuum

The amplifier is assembled on the chassis, which is a steel frame with dimensions of 240x150x x54 mm, soldered to a steel sheet with dimensions of 240x150x0.7 mm, in which the necessary openings for the installation of radio tubes, network and output transformers for ventilation are pre-cut. The soldering joints of the sheet and frame are thoroughly rinsed, cleaned and puttied with polyester car putty, after which the entire chassis is primed and painted with car enamel (Fig. 2).

Fig. 2. Elements of the amplifier

Output transformers are placed in steel screens. It is rather aesthetic than technical necessity. For this purpose, suitable metal boxes were found, inside of which M3 screws with previously sawn-off heads were soldered at the corners. The screws should protrude outwards beyond the edge of the box by 6. 8 mm, with their help the boxes are fastened to the chassis from the bottom side with four M3 nuts. The screens of the transformers are also primed and painted with auto enamel.

The front panel of the amplifier is cut from aluminum sheet AMG-5 with a thickness of 1.5 mm, polished with fine emery paper under a stream of warm water and covered with a colorless automobile lacquer. The panel has a backlit power switch and channel volume controls. The panel has dimensions of 240×65 mm, its upper edge protrudes 10 mm above the chassis.

On the back of the chassis, there is a mains connector with a fuse, a push-type WP4-18FB terminal for connecting speakers and two RCA RS-104 or RS-115 jacks (tulip type) left and right channel inputs.

Installation of the amplifier begins with the laying of the power supply circuit of the lamps of an intertwined pair of wires. All parts of the amplifier are mounted in the basement of the chassis by a mounted method at the terminals of the lamp panels and at the contact pads made of strips of textolite foiled on one side. Capacitors C6-C8 are fixed on plexiglass plates (Fig. 3). Also in the basement chassis along its front panel solder copper tinned wire with a diameter of 1.5. 2 mm, which serves as a common wire.

Fig. 3. Installation of the amplifier

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

Output transformers – TV-3SH or similar from the output LF-cascades of TVs on lamps 6P14P. Network transformer T2 – from tape recorder "Comet 201" or any other that provides 190. 200 V at a current of 150 mA at the secondary winding. Power filter choke – D-16, D-17.

RS407 diode bridge rectifier anode selected 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.

How to do exercise vacuum

The amplifier assembled from serviceable parts starts working immediately and practically does not need to be adjusted. From the inside of the protruding part of the front panel, tinted glass with a thickness of 5. 6 mm is glued onto the silicone colorless sealant (Fig. 4). The ends of the glass polished belt grinder. If you do not have a power tool, but you have time, you can manually polish the glass on a hard, flat surface, putting sandpaper on it. Glue the glass should be in the last turn, after the entire amplifier is finally assembled.

Fig. 4. Power assy assembly

Author: O. Platonov, Perm

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