Amplifier for the speaker "3L"

Czesław Klimczewski "Amplifier for the speaker '3L'"
Radioamator 1953/01

In the 11th issue of our magazine, an amplifier with two tubes operating in a push-pull system using a transformer that controls these tubes was described. Since this transformer can not always be purchased on the market, its winding presents a certain difficulty - currently the description of the amplifier is given, in which the function of the phase inverter is performed not by the transformer but by means of a radio tube. The amplifier does not have a low frequency choke for current rectification but a resistor with a resistance of 300 Ω and a power of about 5 watts. The resistor is easy to buy or wind up on a porcelain or glass rod.

Thanks to the elimination of the transformer and choke, the cost of the described amplifier is relatively low and its assembly is easier. The amplifier made exactly according to the given schemes will work perfectly and can power one or several speakers with a total power of about 25 watts.

Fig. 1 shows the schematic diagram of the amplifier. It is adapted to cooperate with a radio receiver having sockets to connect an additional loudspeaker. These sockets should 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 the "b" sign. If there is no sign on the receiver's casing, the additional loudspeaker sockets are connected in such a way that the amplifier can be connected freely without paying attention to the connection's compatibility. The amplifier can also be combined with another low frequency amplifier, so-called "preamplifier" to get more power for the speakers. For this purpose, the W1 switch was used to switch off the resistance R1 = 5000ohms, so that the amplifier can be connected to another amplifier with low output resistance - 500ohms or a large - of 5000-6000ohms. The radio equipment has the output resistance of an additional loudspeaker, usually large - in the order of 6000ohms.

Regardless of the possibility of amplifying radio broadcasts, the amplifier can be used to amplify broadcasts transmitted by a carbon microphone, which has a small build-in  audio transformer that increases the obtained acoustic signals.

This amplifier (so-called adapter) has three tubes. The first of these is the "double triode" type (6SN7) which controls the next two tubes (6L6). These tubes work in the push-pull system. One triode of the first tube controls one of the tubes of the push-pull system, the second triode - the second tube of this circuit.

The first tube (6SN7) acquires voltage from a radio apparatus (amplifier or microphone) through an input circuit consisting of resistors with a resistance of 5000ohms and 500ohms (or one only 500ohms resistor in the case of a low-noise connection), 0.1μF and 0.5μF capacitors  and potentiometer P1=0.5M, which regulates the volume of amplified radio programs - connected in such a way as shown in the schematic diagram.

Fig. 1.

The voltages received from the P1 potentiometer slide are sent to the control grid of the first triode of the tube (6SN7) through the resistor Rs=50K which is a protection for possibly occurring parasitic currents. This grid is also connected to a 10,000pF capacitor, which through a potentiometer P2=100K connects to the earthed amplifier base ("ground"). The potentiometer regulates the "sound color" of amplified radio programs. After passing through the first triode of the 6SN7 tube, the voltages obtained from the anode and generated on the resistor R5=30K are transmitted through the capacitor C4=0.1μF to the control grid of the first tube of the push-pull system. The leakage resistance of this tube (6L6) connected between the grid and the ground of the amplifier is equal to 500K (0.5M) and consists of three resistors: a 450K resistor, a P3 potentiometer = 20K and a 30K resistor. The potentiometer slide (P3) is connected to the grid of the second triode of the 6SN7 tube. Thanks to the P3 potentiometer, it is possible to select control voltages of such magnitude 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 generated at the resistance of the second voltage triode produced at the resistance R6=30K are transmitted via the capacitor C5=0.1μF to the control grid of the second tube of the push-pull system. These voltages are identical in frequency and value with the voltages received from the first tube triode. They have only the opposite phase in relation to them (they are inverted in relation to them), and so they are a mirror image which is needed for the work of the push-pull system. The leakage resistance connected to the grid of the second 6L6 tube also has a value of 500K. In this way, both tubes of the push-pull system receive identical control voltages but are shifted by 180 degrees in phase. It allows for their proper operation and obtaining non-distorted. These control voltages received from the first tube (double triode, eg type 6SN7) are larger than those supplied to the amplifier through the input circuit, because the tube also strengthens them (in the case of using the 6SN7 tube, the gain is about 13 times).

After passing the push-pull circuit, the currents obtained in the circuit are directed to the primary winding of the "output" transformer. From the secondary winding of this transformer, voltages for controlling the speakers are obtained. Thanks to the made taps, you can power one or a dozen small speakers by attaching them to the winding.

The amplifier's power supply is adapted to the alternating current obtained from the 220V or 110V electric network. This power supply consists of a rectifier and a smoothing filter. The grid voltage is connected to the primary winding of the transformer of the power supply through the B1-0.3amp fuse and the W2 switch. The secondary windings provide the voltage for the rectifier tube and amplifier tubes as well as the voltage for feeding the anodes of the rectifying tube. The value of the heater voltage depends on the types of tubes used in the amplifier; voltage for the rectifier tube anodes should have a value of 350-360V.

In the anode winding of the power transformer there is a second B2-0.3 ampere fuse protecting the transformer against possible short-circuiting. The extreme ends of this winding are also connected with C8 and C9 capacitors with a capacity of 10000pF with earthed ground to prevent amplification circuits of industrial disturbances generated in the network.

Fig. 2.

The rectifier tube (5Y3) converts the alternating network voltage received through the transformer into a pulsed one-way voltage. The filter located behind the rectifier's power supply, consisting of a resistor Rf=3K and two capacitors C10 and C11 of 32μF each - smoothes the pulsating voltage similar to the DC voltage, which can be used to power anodes and tubes auxiliary grids.

The triodes anodes receive constant voltage from the power supply through the resistor R15=5K blocked to ground by a capacitor C1=100nF. The resistor and the capacitor constitute an additional filter to smooth the voltage to a greater degree, and the required voltage drop is obtained to obtain a voltage of the same value as that required by the anode of the first tube (6SN7). Likewise, the screen grids of the push-pull tubes ("s2") are powered by a resistor R14=2K blocked to ground by a C7 capacitor with a capacity of 1μF. Both resistances are connected to the power supply after the Rf resistor located in the filter, due to which the received voltages are exceptionally well smoothed. The anodes of the push-pull tubes receive voltage directly from the power supply (before the filter), because thanks to the properties of this system, the currents flowing through both halves of the loudspeaker transformer transform the magnetic streams in its core against each other, resulting in pulsations canceling each other and not being reproduced through the loudspeaker so they do not disturb the broadcast.

It is also worth paying attention to the resistors in the cathodes of tubes, which are not blocked to ground with capacitors, thus creating some kind of feedback in amplifying circuits, which increases the fidelity of broadcasts played by the loudspeakers.

Similarly, resistors R10 and R11 with 1K resistance connected to the control grids of the pushs are designed to prevent parasitic couplings, as a result of which the amplifier is working properly and the programs are not disturbed by various side noises, whistles, etc.

So much for a brief description of the operation and schematic of the amplifier. Now the method of its assembly will be discussed.


The installation of the amplifier begins with the mounting on the chassis: transformers, tube sockets, electrolytic capacitors, switches, potentiometers, holder for signal lamp, holders for tube fuses, screws with washers for attaching wires to them, which are to be changed according to the scheme, and sockets. These sockets should be attached to appropriate plates made of bakelite (possibly thick and hard pressboard) and only then should they be fixed to the chassis of the amplifier but in such a way that none of them touches the chassis as this will cause the amplifier notto  work or it will even be damaged after switching on the power supply.

It should be noted, however, that one of the sockets marked with the letter "Z" is attached to the base directly, without a pre-panel (or bakelite) base. It is used to connect the base of the snap to ground (see Fig. 3).

Fig. 3.

Elements on the amplifier chassis should be arranged and laid out as shown in the assembly diagrams in Fig. 2 and Fig. 3.

Installation is carried out with rubber or igneous insulation. The diameter of these wires (wires) should not be less than 1÷0.8mm. The ends of the wires should be carefully cleaned from the insulation and soldered or screwed down firmly. Solder should be a tin with the use of rosin (it can be dissolved in pure spirit). The so-called "acid" for soldering radio connections should not be used, because over time it creates a precipitate that erodes the connection by breaking electrical contact and causes faulty operation of the amplifier.

The assembly starts with the power supply. One of the amplifier's power sockets is connected through the W2 switch with the beginning of the primary winding of the power transformer, the other - the mains socket - through the B fuse connects to the corresponding end of the same winding (220V or 110V terminal - depending on the mains voltage).

Then, the secondary winding for the rectifier tube heater are connected to the corresponding springs (marked with "Z") in the tube base. Similarly, the wires of the spring in the bases corresponding to heaters of the reinforcing tubes and the socket of the signaling bulb are connected - with the ends of the second heating coil intended for the amplifier tubes. These springs are also marked with the letters "Z" in the diagrams. These connections should be made in such a way that the heaters of all tubes are connected  "parallel" to the transformer's winding. The connection wires should be screwed together as shown in the assembly diagrams. The twisting of these wires will protect them from the influence of the interference field they generate on the very sensitive circuits of the amplifier's control grids, which could cause the audible buzz from the loudspeakers and harmful feedback.

The ends of the heater winding of tubes should be connected not only to the bases but also with the adjustable R15 resistor, which after launching the amplifier will adjust once, so that possible noise heard from the speakers - disappeared and the programs were played cleanly. After adjusting the amplifier you do not need to regulate anymore this resistor. The heater wires are connected to the extreme ends of the resistor, and its slider is earthed by connecting to the screw on the metal base of the amplifier.

The extreme ends of the anode winding in the power transformer are connected with the springs corresponding to the anodes (a1, a2) of the rectifying tube and with one of the ends of capacitors C8 and C9 with a capacity of 10000pF. The center of this winding is connected via fuse B2 to the amplifier's chassis. The other ends of the C8 and C9 capacitors are also connected to the chassis.

The negative pole of the rectified voltage is located in this way on the chassis of the amplifier, while the positive one - on the wire connected to one of the springs, filaments in the base of the rectifying tube.

The positive pole of the rectified voltage connects to the resistor Rf of the filter which smooths the voltage and the positive pole of the electrolytic capacitor C10 with a capacity of 32μF and the center of the primary winding of the loudspeaker transformer through which the anodes of the push-pull tubes are fed. The end of the resistor Rf connects to the positive pole of the second electrolytic capacitor C11 also with a capacity of 32μF and with one end resistor R14= 2K, through which auxiliary nets (s2) of the push-pull circuit tubes and - with one end of the resistor R13) are fed, through which the anodes (a1, a2) of the first triode of the tube receive voltage. The negative leads of the electrolytic capacitors (usually 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=5K and one terminal of the circuit breaker W1. The other end of this resistor and the second contact of the circuit breaker connect to one end of the capacitor C1 with a capacity of 0.1μF and one end of the resistor R2=500ohms. The other end of capacitor C1 is connected to one of the extreme springs of the potentiometer P1=0.5M. The other end of this potentiometer connects to the mass of the amplifier and one end of the capacitor C2=0.5μF. The other end of this capacitor connects to the other end of the resistor R2=500ohms and the second input socket of the amplifier ("b").

The middle spring of potentiometer P1 connected to its slider, is connected through the resistor R3=50K with the spring in the tube base of the first triode of the 6SN7 tube, belonging to the control grid (s1) of this tube. This spring is also connected to the capacitor C3=10000pF, which connects to one extreme spring of the potentiometer P2=100K. The second extreme spring of this potentiometer must be connected with its central spring and the ground of the amplifier. The springs in the tube base belonging to the cathodes (k1 and k2) of this tube should be connected together and through the resistor R=1K5 also with the ground of the amplifier.

The anodes (a1 and a2) of the discussed tube are then connected to one end of resistors R5=R6=30K. The other ends of these resistors are connected together with the other, the remaining end of the resistor R13=50K, and also with one end of the capacitor C6=1μF. The other end of this capacitor is grounded by connecting to the ground of the amplifier.

There is still a connection to be made to include the control net (s2) for the SECOND TRIODE of the first tube.

To do this, the springs belonging to the anode of the first triode (a1) are connected to one end of the capacitor C4=0.1μF. The other end of the resistor R7=450K. This resistor in turn connects to one of the extreme springs of the potentiometer P3=20K. The second extreme spring of this potentiometer is connected to one end of the resistor R8=30K. The other end connects to the amplifier ground, with one end of the R12 resistor = 250ohms and with one end of the resistor R9=500K (0.5M). The other end of the resistor R9=500K connects to the resistor R11=1K (through which the control voltages for the aforementioned tube from the anode circuit of the second triode (a2) of the 6SN7 tube are directed.

The center spring of the potentiometer P3=20K is then connected to the control grid (s2) of the second triode of the first amplifier tube.

The other end of the capacitor C5=0.1 μF connects to the anode of the second triode (a2) and to the end of the resistor R6=30K already connected to it. The end of the resistor R12=250ohms, which has not been connected so far, connects with both cathodes (k) of the push-pull tube system. Resistor R14=2K, through which the grids of the push-pull circuit tubes receive voltage, connecting from the side of these grids to the ground of the snap through a capacitor C7=1μF. Then, both anodes of these tubes are connected respectively to the two ends of the primary winding of the loudspeaker transformer, and the ends of the secondary windings of the transformer - to the corresponding output sockets of the amplifier.

The installation of the attachment is completed on the above.

Fig. 4

Chassis of the amplifier should be made of galvanized iron sheet or zinc sheet 1mm thick - according to the dimensions given in fig.4 and assembly drawings 2 and 3.

Transformers can be purchased or made by yourself. For the implementation of the TR.S power transformer, use a core made of iron plates, insulated on one side with lacquer or shellac, with a central section of the column of approximately 15 cm2. The primary winding should be wound in 0.6 or 0.5mm enamel wire and have 760 windings with a tap from 420 turns. The 220V electric network connects to the ends of the winding and 110V - to the part consisting of 420 windings.

The high voltage secondary winding should be wound in a wire with enamel of 0.15 or 0.12mm in diameter. It must have 2,600 windings with a detachment from the center of the winding, i.e. from 1,300 coils.

The winding of the rectifier tube heater is wound with enamelled wire of 1.2 or 1.0mm diameter. It should have in the case of tubes type 5Y3, 5Z4 or 5U4 - 16 turns, while in the case of tubes AZ12 - 13 turns.

The winding of the amplifying tubes heaterss should be wound with the same wire with the number of coils equal to 20 in case of using 6.3V series tubes.

Speaker transformer TR.G. (output) should have a core made of similarly made metal sheets as the previous one, but the cross-section of the middle column will be around 12cm2.

The primary winding of this transformer consists of two sections with the same number of windings; these sections are next to each other to form two electrically identical windings. The total number of windings is 1800 (two sections of 900 turns each) wound with 0.3 or 0.2 wire in enamel. From the center of this winding, i.e. from the connection point of the two sections with each other, a detachment is derived.

The first secondary winding wound evenly on both primary sections has 130 windings wound in wire in enamel with a diameter of 0.8mm, then further 130 windings wound with a similar wire but with a diameter of about 0.5mm, and 260 windings wound with wire also in enamel with a diameter of 0.3mm. Each of the given winding parts has a detachment led outwards. The total number of windings of this winding is therefore 520 (130+130+260). At the ends of the first part of the winding, a voltage with acoustic frequencies of about 30V is obtained; between the first end (beginning) of the winding and the second tap - 60V; between the same winding and the end of the winding - 120V.

The second secondary winding wound on the first secondary winding has 85 windings with taps at 26 and 42 turns. The wire used to wind this winding should have a diameter of about 0.15mm. This winding is used to power dynamic loudspeakers.

It is noted that when winding the transformers described above, the same winding directions should be maintained in all windings.

At the end, drawings of tube bases with markings at the feet are given in Fig. 5.

Fig. 5.

These bases correspond to the tube bases visible from the bottom. When using radio tubes other than those specified in the description, the wires should be connected with those legs in the bases that correspond to the same markings in the tube bases used in the schematic and assembly diagrams. When using an EL12 type tube, resistance of R12=250ohms should be changed to approximately 90ohms.

List of assembly parts

  1. Resistors:
    R1=5000/1W; R2=500/1W; R3=50K/1W; R4=1500/2W; R5=R6=30K/1W; R7=450K/1W; R8=30K/1W; R9=500K/1W; R10=R11=1000/1W; R12=250/4÷5W (for EL12-90 tubes/4÷5W); R13=5000/2W; R14=2000/2W; R15=50 (regulated); Rf=3000/5W - wired.
  2. Potencjometers:
    P1=0,5M/log; P2=100K/log; P3=20K/log/lin.
  3. Capacitors:
    C1=0,1µF/1500V; C2=0,5µF/1500V; C3=10000pF/1500V; C4=C5=0,1µF/1500V; C6=C7=1µF/750V (minimum); C8=C9=10000pF/3000V; C10=C11=32µF/550V.
  4. Fuses:
  5. Switeches:
  6. Transformers:
    TR.S. - power. Primary 220 i 110V, secondary - heaters - 6.3V/5A (amplifying tubes); 5V (or 4V for AZ12 tube)/1A (rectifying tube); 350-360V/150mA - plate voltage.
    TR.G. - output. Primary dedicated to 6L6 tubes (push-pull configuration), secondary - 30V, 60V, 120V.
  7. Tubes:
    6SN7 lub 6N7, 6H8, 6H8C, 6H7 x 1, 6L6 or 66, 63, EL12/375, EL12/spec., EL12 x 2, 5Y3 or 5Z4, 5C4, AZ12 x 1.

Cz. Klimczewski