Tube amplifiers have always been popular with amateur radio circles.. Construction described, not for sound professionals, but for fans, who just listen to music and like to create something on their own. Easily accessible lamps EF86 and EL34 were used in the design., as well as commonly available type transformers.
Principle of operation and scheme
The amplifier is built in topology SE (Single-Ended – with one reinforcing element). Main condition, which should have been consistent with the design, consisted of achieving power around 7. ..9 Watts per channel when using readily available items.
In each channel two lamps work. At the voltage amplification stage, a low-noise and low-power pentode EF86 or EF806 was used (domestic analogue 6Zh32P), and the pentode EL34 – on power. General view and pinout of lamps is shown in the figure. 1.
Schematic diagram of the amplifier shown in Figure 2.
Left channel elements are marked with the letter L, right channel – the letter R, and the elements of the anode power source – letter A, glow pattern – letter H. Input signal from J1L connector / R after separation of the DC components is shown on the V1L pentagonal grid. The voltage gain is classic and low noise. The pentode provides an appropriate level of control signal and allows you to save the gain for the global feedback loop of the amplifier using R6L, R5L, C2L. Negative feedback allows you to equalize the frequency response and reduce distortion of the amplifier. A pre-amplified signal is applied to the V2L lamp., working with automatic polarization, power supply R11L, CE4L. From the anode of the lamp, through the J3L connector, the signal is fed to the output transformer. The author's model uses a transformer LOSE25-9 Ra = 2,5 to / 8 ohm, other types Ra = 2,5-3,0 to / 8 Ohm can be used after minor experimental corrections R6L, C2R. UA is common to both channels and consists of a rectifier bridge on fast roofs D1 … D4 UF4007, filter capacitors CE1A / CE2A and active filter circuits based on Q1A transistor type P6NK60ZFP, CE3A capacitor provides “smooth” start-up of anode power supply, eliminating the need for anode voltage delay. Due to heat dissipation, transistor Q1 is mounted on the radiator. CE4A Capacitor Provides Extra Power Filtering, the initial stages are powered by their own R8L decoupling filters / C1L. Incandescent lamp uses alternating voltage, resistors R1, 2H set the potential of the heaters relative to the ground, R3H chain, D1H powers the DJ LED, signaling amplifier operation. Anode Voltage (240 VAC / 0,3 BUT) from the transformer is fed to the JA1 connector, glow voltage (6,3 VAC / 4 BUT) to JH1 connector.
The amplifier is made on a double-sided printed circuit board, containing all electronic components, except transformers. The arrangement of elements is shown in the figures. 3 and 4.
Power resistors need to be lifted as far as possible from the board surface, to ensure proper air circulation, install transistor Q1A on the radiator, contact area to be greased with thermally conductive paste.
The assembled board is shown in the photograph. 5 and photos at the beginning of the article.
Preparing the wiring inside the amplifier enclosure requires great care. Guides, that go to the filaments should have a cross section 2,5 mm and be twisted to avoid electromagnetic field scattering. Wires must have reinforced insulation. The electrical connection diagram is shown in the figure. 6.
Before final housing design, check the effect of the mains transformer on the speaker coils, orienting their center columns (perpendicular) or moving them as far apart as possible.
Particular attention should be paid to the accuracy of the filament voltage., with inaccurate winding happens, that the voltage is too high (even after more 7 AT), leading to overheating of the lamp, increase in hum level as a result of cathode radiator leakage and of course, reducing lamp life. In this case, two small identical resistors with the corresponding voltage, voltage corrective, must be included in a row with a filament winding. However, it is best to use a good quality mains transformer..
Start amplifier, after a visual inspection of the assembly and checking for the absence of short circuits and the correct location of the elements, happens in stages.
After loading the output transformers with a resistor 8 ohm / 20 W and connecting them to the outputs of the oscilloscope and to the inputs of the functional generator (minimum signal level) you must first apply the voltage. Its value should be 6,3 In ± 5% at current consumption 3,5 A ± 5%. Secondly, Anode voltage must be supplied from a regulated power source or autotransformer, at a value of about 50 AT, should be checked, Is the feedback amplifier connected correctly?, if the circuit is energized, replace the connection of one of the speaker transformer windings, for feedback to reduce gain. If the amplifier is stable, anode voltage can be further increased. at 240 In AC, it is necessary to counteract the voltage drop across the cathode resistors R11L / R lamp EL34, which should be around 13 AT. The voltage at the anode should be close to 300 AT, voltage, applied to the primary stage on the capacitors CE1L / R , about 260 AT, about 2,2 In cathodes EF86.
After amplification, it is worth checking the output signals for sinusoidal control at 1 W / 7 W for several frequencies, For example, 100 Hz / 1 kHz / 10 kHz. For lower power, it is worth evaluating the transmission of a rectangular signal, Based on this, need to adjust the values of R6L / R, C2L / R to obtain correct signal reproduction regardless of the output transformer used. signals, obtained during model measurements, shown in figures 7a, 7b, 7with, 7d.
In the model at 7 Watts THD distortion + N stay lower 4% in the range 100 Hz / 10 kHz. Bandwidth is 20 Hz … 20 kHz at rated load and power 7 W. The results may vary depending on the transformers used and the condition of the lamps.. If everything went well, can switch to audio, connect speakers and enjoy the warm tube sound.
Author: Adam tattoo