Speaker amplifier "3L"
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- Published: Monday, 08 March 2021 15:50
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Speaker amplifier "3L"
eng. Czesław Klimczewski
Radioamator, Rok III, Styczeń 1953r., Nr 1 (Radioamateur, Year III, January 1953, No. 1)
In issue 11 of our magazine there is a description of a loudspeaker pickup (amplifier) with two electron tubes operating in a push-pull arrangement using a transformer that controls these tubes. As it is not always possible to purchase such a transformer on the market, winding it presents a certain difficulty - a description of the amplifier is now given, in which the function of the so-called the "phase inverter" is not accomplished via a transformer but via a tube. The amplifier also does not have a low-frequency choke for filtering the rectified pulsed current, but a resistor with a resistance of 3000 ohms and a load capacity of about 5 watts. Such a resistor is easy to buy, or you can wind it yourself on a porcelain or glass tube.
Due to the non-use of a transformer and choke, the cost of the described amplifier is relatively low and its assembly is easier. This amplifier, made strictly according to the given diagrams, will work perfectly and can power one or more speakers with a total power of about 25 watts.
Fig. 1 shows a schematic diagram of the amplifier. This amplifier is adapted to work with a radio receiver with sockets for connecting an additional loudspeaker. These sockets must be connected to the amplifier in such a way that one of them, marked with a "plus" sign, should be connected to the socket marked with a "b" sign. If there is no marking on the receiver housing, then the sockets of the additional loudspeaker are connected in it in such a way that the amplifier can be connected freely without paying attention to the compatibility of connections. This attachment can also be combined with a low-frequency amplifier, the so-called "preamplifier" to obtain more power for driving the loudspeakers. For this purpose, the described amplifier uses a W1 switch that turns off the resistance R1 = 5000 ohms, so that it can be connected to a radio apparatus or an amplifier having a small "output" resistance - of the order of 500 ohms or a large one - of the order of 5000 ÷ 6000 ohms. Radio apparatuses have the output resistance of the additional loudspeaker, usually high - in the order of 6000 ohms.
The voltages received from the spool of the potentiometer P1 are sent to the control grid of the first triode of the tube (6SN7) through the resistor Rs = 50,000 ohms (50K), which is a eliminator for possible parasitic currents. This grid is also connected to the 10.000 pF capacitor, which is connected via P2 = 100K potentiometer to the grounded base of the device ("ground"). This potentiometer is used to adjust the "timbre" of the amplified programs.
After passing through the first triode of the 6SN7 tube, the voltages obtained from the anode and formed at the resistance R5 = 30K are directed through the fixed capacitor C4 = 0.1 µF to the control grid of the first tube of the push-pull circuit. The leakage resistance of this electron tube (6L6) connected between this grid and the mass of the amplifier is 500K (0.5M) and consists of three resistors: a 450K resistor, a P3 = 20K potentiometer and a 30K resistor. The slider of this potentiometer (P3) is connected with the control grid of the second triode of the discussed tube 6SN7. Thanks to the use of the potentiometer P3, the control voltages can be selected such that they will be equal to the control voltages obtained from the input circuit and applied to the grid of the first triode in the 6SN7 tube. The voltages obtained from the anode of the second triode, generated at the resistance R6 = 30K, are sent through the capacitor C5 = 0.1 µF to the control grid of the second electron tube of the push-pull circuit. These voltages are identical in terms of frequency and height to the voltages obtained from the first triode of the tube, they have only the "phase" opposite to them (they are inverted in relation to them), so they are a mirror image of what is needed for the operation of the push-pull system. The leakage resistance connected to the grid of the second 6L6 tube is also 500K but single. In this way, both electron tubes of the push-pull system receive the same control voltages, but shifted by 180 degrees in phase, which allows their correct operation and obtaining undistorted, full-power amplified programs. These control voltages obtained from the first vacuum tube (double triode e.g. 6SN7 type) are greater than those supplied to the amplifier by the input circuit, because the electron tube also amplifies them (in the case of using the 6SN7 tube, the gain is about 13 times).
After passing through the electron tubes of the push-pull circuit, the currents obtained in the circuit of their anodes are directed to the primary winding of the "output" transformer. Voltages for loudspeaker control are obtained from the secondary winding of this transformer. Thanks to the taps made, you can power one or a dozen small loudspeakers by connecting them to the winding.
The power supply of the amplifying device is adapted to alternating current obtained from an electric network with a voltage of 220V or 110V. This power supply consists of a "half-wave" rectifier and a filter that smooths the pulsating rectified voltage. The mains voltage is connected to the primary winding of the power supply transformer through the B1 fuse - 0.3 amps and the W2 switch. From the secondary windings, the voltage for the filament of the rectifier tube and the amplifying tubes, and the voltage for the supply of the anodes of the rectifying tube are obtained. The amount of filament voltage depends on the types of tubes used in the amplifier; the voltage for the anodes of the rectifier tube should be 350-360V for each of them.
There is a second B2-0.3 amp fuse in the anode winding of the mains transformer, protecting the transformer against a possible short circuit. The outer ends of this winding are also connected with C8 and C9 fixed capacitors with a capacity of 10,000pF to the ground of the amplifying device in order to prevent the amplification circuits from industrial interferences arising in the supply network.
The rectifying tube (5Y3) converts the variable mains voltage obtained through the transformer into a pulsating, unidirectional voltage. The filter behind the rectifying part of the power supply, consisting of a resistor Rf = 3000 ohms and two capacitors C10 and C11, 32µF each - smoothes the pulsating voltage, making it similar to direct voltage, which can be used to supply anodes and auxiliary grids of tubes.
The triode anodes receive DC voltages from the power supply through a resistor R15 = 5000 ohms blocked to ground with a constant capacitor C1 = 100 nF. This resistor and the capacitor constitute an additional filter smoothing the rectified voltage to a greater extent and the necessary voltage drop is obtained to obtain the voltage at the level required by the anodes of the first tube (6SN7). Similarly, the auxiliary grids of the push-pull tubes ("s2") are powered by a resistor R14 = 2000 ohms blocked to ground by a capacitor C7 with a capacity of 1µF. Both resistances are connected to the power supply after the Rf resistor located in the filter, thanks to which the obtained voltages are extremely smooth. The anodes of the push-pull tubes receive voltages directly from the power supply (before the filter), because thanks to the properties of this system, the currents flowing through both halves of the speaker transformer generate magnetic fluxes in its core directed against each other, as a result of which the pulsations cancel each other and are not reproduced by the speaker, so they do not disturb the broadcast being played.
It is also worth paying the Reader's attention to the resistors in the cathodes of the electron tubes, which are not blocked to the ground by capacitors, thus creating a kind of feedback in the circuits which amplify the fidelity of the broadcast reproduced by the loudspeakers..
Similarly, the R10 and R11 resistors with a resistance of 1000 ohms included in the cables connected to the control grids of the push-pull tubes are designed to prevent parasitic feedback, so that the described amplification device works properly and the broadcasts are not disturbed by various side noises, whistles, etc.
So briefly about the description of the operation and the schematic diagram of the amplifying system, now the method of its assembly will be discussed.
The assembly of the system begins with mounting on the base of transformers, sockets for tubes, electrolytic capacitors, switches, potentiometers, bulb holders, holders for tubular fuses, screws with washers for attaching to them the wires that are to be changed according to the diagram, and sockets for " inputs "," outputs" and networks. These sockets should be attached to appropriate plates made of bakelite (possibly made of thick and hard pressboard) and these should be attached to the base of the device, but in such a way that none of them touches the metal of the base, as this will prevent the system from function or it will even be damaged when it is connected to the electricity supply.
However, it should be noted that one of the sockets marked with the letter "Z" is attached to the chassis directly, without the pressboard (or Bakelite) washer. It is used to connect the amplifier's base to ground (see fig. 1).
The fixed mounting parts on the amplifier base should be positioned and arranged as shown in the assembly diagrams in Figs. 2 and 3.
Installation is carried out with rubber or plastic insulated cables, the diameter of these cables should not be less than 1 ÷ 0.8 mm. The ends of the connecting wires should be carefully cleaned of insulation and the joints should be soldered or tightened with screws. Solder with tin with rosin (it can be dissolved in pure spirit). The so-called "acid" for soldering radio connections must not be used, because with time it forms a deposit that eats the places of connections, spoiling the electrical contact and causes the amplifier to malfunction or prevents it completely.
Installation begins with the power supply. One of the amplifier's mains sockets is connected through the W2 switch with the beginning of the primary winding on the mains transformer, and the other mains socket - through the B fuse, is connected to the appropriate end of the same winding (the terminal is 220V or 110V - depending on the mains voltage).
Then the ends of the secondary winding intended for filing the electron rectifier tube are connected to the appropriate springs (marked with the letters "ż") in the base of this tube. Similarly, it is connected with the wires of the springs in the sockets corresponding to the electronic filaments of the amplifier tubes and the signal bulb holder - with the ends of the second filament winding intended for the electronic amplifier tubes. These springs are also marked with the letters "ż" in the diagrams. These connections should be made in such a way that the filaments of all electron tubes are connected to the transformer winding "in parallel", while the connecting wires should be twisted together as shown in the assembly diagrams. Twisting these wires will prevent the interference they generate on the very sensitive circuits of the control grids of the amplifier's electron tubes, which could cause hum audible from the loudspeakers and harmful feedback to the normal operation of the system.
The ends of the winding for the heating of the amplifier tubes should be connected not only with the tube sockets but also with an adjustable resistor R15 (the so-called "entbrummerk"), which, after activating the amplifier, will be adjusted once so that any hum heard from the loudspeakers disappears and the broadcasts are cleanly reproduced. After adjusting the resistor adapter, it does not need to be adjusted anymore. The glow wires are connected to the extreme contacts of the resistor, and the middle contact (connected with the slider) is earthed by connecting to the screw on the metal base of the amplifier.
The outer ends of the anode winding in the network transformer are connected to the springs corresponding to the anodes (a1, a2) of the rectifier electron tube and to one of the ends of the fixed capacitors C8 and C9 with a capacity of 10000 pF. The center of this winding is connected via an electric fuse B2 to the base of the amplifier. The remaining ends of the capacitors C8 and C9 are also connected to this base.
The negative pole of the rectified voltage is thus located on the metal base of the device (ground), and the positive pole - on the wire connected to one of the filament springs in the electron base of the rectifying tube.
In turn, the conductor, being the positive pole of the rectified voltage, connects to the resistor Rf of the filter that smooths this voltage and to the positive pole of the electrolytic capacitor. C10 with a capacity of 32µF and with the center of the primary winding of the loudspeaker transformer through which the anodes of the push-pull tubes are supplied. The end of the Rf resistor connects to the positive pole of the second electrolytic capacitor C11, also with a capacity of 32μF, and to one end of the resistor R14 = 2000 ohms, through which the auxiliary grids (s2) of the push-pull electron tubes are supplied and - with one end of the resistor R13), 5000 ohms, through which the anodes (a1, a2) of the first triode tube receive voltages. The negative poles of electrolytic capacitors (most often located on the metal housing of the capacitor) are connected to ground.
Then the assembly of the reinforcing part begins. The input socket marked with the letter "a" connects to one end of the resistor R1 = 5000 ohms and to one terminal of the W1 switch. The other end of this resistance and the second contact of the switch connect to one end of the capacitor C1 with a capacity of 0.1 µF and one end of the resistance R2 = 500 ohms. The other end of the capacitor C1 is connected to one of the external springs of the potentiometer P1 = 0.5M. The other extreme tip of this potentiometer connects to the ground of the amplifier and one end of the capacitor C2 = 0.5 µF. The second end of this capacitor connects to the second, remaining end of the resistor R2 = 500 ohms and the second input jack of the amplifier ("b").
The middle lead of the potentiometer P1 connected to its slider is connected through a resistor R3 = 50K with a spring in the lamp base of the first triode of the 6SN7 electron tube belonging to the control grid (s1) of this tube. This spring is also connected to the capacitor C3 = 10000pF, which connects to one external output of the potentiometer P2 = 100K. The second external output of this potentiometer must be connected with its center and with the mass of the amplifier. Springs in the tube base belonging to the cathodes (k1 and k2) of this electron tube should be connected together and through a resistor R = 1500 ohms also with the mass of the amplifier.
Then the anodes (a1 and a2) of the tube in question are connected to one end of the resistors R5 = R6 = 30K. The other ends of these resistors are connected together and to the other, remaining end of the resistor R13 = 50,000 ohms, and also to one end of the capacitor C6 = 1 µF. The other end of this capacitor is grounded through the amplifier ground connection.
There is still the connection that needs to be made to switch on the control grid (s2) of the SECOND TRIODE of the first electron tube to work.
To do this, the springs belonging to the anode of the first triode (a1) are connected to one end of the fixed capacitor C4 = 0.1 µF. The other end of this capacitor is connected through the electron tube resistor of the push-pull circuit and to one end of the resistor R7 = 450K. This resistor, in turn, connects to one of the extreme springs of the potentiometer P3 = 20K. The second outer spring of this potentiometer is connected to one end of the resistor R8 = 30K. Its other end connects to the ground of the amplifier, with one end of the resistor R12 = 250 ohms and with one end of the resistor R9 = 500K (0.5M). The other end of the resistor R9 = 500K connects to the resistor R11 = 1000 ohms (through which the control voltages for the electron tube are directed from the anode circuit of the second triode (a2) of the 6SN7 tube.
The middle spring of the potentiometer P3 = 20K is then connected to the control grid (s2) of the second triode of the first vacuum tube in the amplifier.
The other end of the capacitor C5 = 10nF connects to the anode of the second triode (a2) and to the end of the resistor R6 = 30K already connected to it. The hitherto unconnected end of the resistor R12 = 250 ohms connects to both cathodes (k) of the push-pull electron tubes. Resistor R14 = 2000 ohms, through which the auxiliary grids of the push-pull electron tubes receive voltage, connects from the side of these grids to the mass of the pickup through a fixed capacitor C7 = 1 µF. Then both anodes of these electron tubes are connected, respectively, to the two outer ends of the primary winding on the output transformer, and the ends of the secondary windings of this transformer - to the corresponding output sockets of the amplifier.
In the above description, the assembly of the amplifier is completed.
The base of the device should be made of galvanized iron sheet or 1 mm thick zinc sheet - according to the dimensions given in fig. 4 and assembly drawings 2 and 3.
Transformers can be purchased or made by yourself. To make the TR.S mains transformer, use a core made of iron plates, insulated on one side with varnish or shellac, with a central column cross-section of approximately 15 cm2. The primary winding should be wound with enamel wire with a diameter of 0.6 or 0.5 mm and have 760 turns with a tap from 420 turns. Electric network with a voltage of 220V is connected to the ends of the winding, and 110V - to its part consisting of 420 turns.
The high-voltage secondary winding should be wound with an enamel wire with a diameter of 0.15 or 0.12 mm. It must have 2600 turns with a tap from the middle of the winding, i.e. 1300 turns.
The filament winding of the rectifying tube is wound with enameled wire with a diameter of 1.2 or 1.0 mm. It should have 16 turns for tube types 5Y3, 5Z4 or 5U4, and 13 turns for tube AZ12..
The filament winding of the amplifier tubes should be wound with the same wire with the number of turns equal to 20 when using tubes of the 6.3V series.
Loudspeaker transformer TR.G. (output) should have a core made of similarly made plates as the previous transformer, except that the cross-section of the middle column will be about 12 cm2.
The primary winding of this transformer consists of two sections with an equal number of turns; these sections are next to each other to form two electrically identical windings. The total number of turns is 1800 (two sections of 900 turns each) wound with wire 0.3 or 0.2 in enamel. A tap is led out from the center of this winding, i.e. from the point where both sections connect to each other.
The first secondary winding evenly wound on both primary sections has 130 turns wound with enamel wire with a diameter of 0.8mm, then 130 turns wound with a similar wire but with a diameter of about 0.5mm, and 260 turns wound with a wire also in enamel with a diameter of 0.3mm . Each of the given parts of the winding has a tap led out to the outside. The total number of turns of this winding is thus 520 (130 + 130 + 260). At the ends of the first part of the winding, an acoustic frequency voltage of about 30V is obtained; between the first end (beginning) of the winding and the second tap - 60V; between the same winding start and winding end - 120V.
The second secondary winding wound on the first secondary winding has 85 turns with taps on the 26th and 42nd turns. The wire used to wind this winding should have a diameter of about 0.15 mm. This winding is used to power dynamic speakers.
Attention is drawn to the fact that when making the described transformers, the same winding directions should be maintained in all windings.
At the end, there are drawings of tube bases with markings at the feet in Fig. 5.
These bases correspond to the tube sockets visible from the bottom. When using radio tubes other than those specified in the description, the wires should be connected to the feet in the sockets that correspond to the same markings in the tube sockets used in the developed schematic and assembly diagrams. When using an EL12 tube, the resistance R12 = 250 ohms should be changed to about 90 ohms.