Powerful tube amplifier with deep OOS 80 watts on 6P3C tubes. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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When designing tube audio frequency power amplifiers (UMZCH), many authors use output stages operating in class A. They argue their decision with the minimum coefficient of nonlinear distortion of such stages. However, cascades operating in class A have a fairly decent initial anode current (the operating point lies in the middle of the linear section of the lamp characteristic). Therefore, the efficiency of the lamp will be very low. The direct current flowing through the lamp will heat up its electrodes. If forced cooling of the lamps is not provided for, then their electrodes will be intensively destroyed. It should be noted that when building class A amplifiers with an output power of 10...20 W, it is still possible to create a compact cooling system. But if the amplifier is designed, for example, for 100 W, then a very bulky "cooler" will have to be built.

Therefore, it is more profitable to use a more economical mode of lamp operation in class B. The disadvantage of this mode is an increased level of non-linear distortion. This is due to the fact that in this mode the operating point of the lamp lies in a more nonlinear initial section of the lamp characteristic. With a push-pull circuit for switching on lamps, this causes distortion in the form of a "step". There is a very simple way to compensate for such distortions. To do this, the amplifier must be covered with deep negative feedback.

The proposed amplifier is powered by a two-transformer power supply (Fig. 1). Transformer TZ provides power to the anode circuits of the entire circuit and the grid circuits of the output lamps of the amplifier, T4 generates filament voltages, bias voltages on the grids of the output lamps and voltage to power the fans that cool the amplifier. To reduce the background level, the preamp lamps are heated from a direct current source.

Rice. 1. Two-transformer power supply

The circuit diagram of the amplifier is shown in fig. 2. A preamplifier is assembled on a small-sized double triode VL1. The input signal levels are regulated by variable resistors R1 and R2. The left and right channel signals are fed to three-band tone controls. Further, the signals through the compensating amplifier on the double triode VL2 are fed to the phase inverters on the double triode VL3. Correcting RC circuits connected to the cathodes of the VL2 triodes reduce the nonlinear distortion of the amplifier and prevent its self-excitation at infra-low frequencies. On VL3 anodes, anti-phase signals are obtained, which are necessary for the operation of push-pull output stages. Anti-phase signals are "swinged" by preamplifiers on double triodes VL4, VL5 to the levels necessary to excite the output lamps VL6...VL9. Both tetrodes in each lamp are connected in parallel to increase the output power. The lamps are loaded with output transformers T1, T2.

Rice. 2. Schematic diagram of the amplifier (click to enlarge)

Transformers match the high impedance of the lamps with the impedance of the speakers.

The amplifier is assembled in a duralumin case. Fans M1 and M2 are positioned so that they blow over the output lamps. XS1 - socket "JACK" or "miniJACK". R1, R2, R11, R13, R15, R17, R19, R21 - any variable resistors of a suitable type. SA1 must withstand a current of up to 6 A at a supply voltage of 220 V. For T1 and T2, E-shaped cores with a cross section of 32x64 mm are used. Windings I, III contain 600 turns of wire PEVTL-2 d0,4 mm, and windings IIa and IIb - 100 turns of the same wire. Winding IV contains 70 turns of wire PEV-2 d1,2 mm. TZ and T4 are wound on toroidal cores with a section of 65x25 mm (T3) and 40x25 mm (T4). T3 has a primary winding consisting of 600 turns of PEVTL-2 wire d0,8 mm, and a secondary winding consisting of two windings of 570 turns of the same wire each. The primary winding T4 consists of 1600 turns of PEVTL-2 wire d0,31 mm, winding II - 500 turns of the same wire, III and IV - 52 and 104 turns of PEVTL-2 wire d0,8 mm. The winding order of the windings for T1 and T2 is shown in fig. 3.

Rice. 3. The order of winding windings for T1 and T2

Setting up an amplifier begins with a power source. Remove the lamps VL6 ... VL9 from the panels and turn on the power. In this case, HL1 should light up, and M1 and M2 should work. Constant output voltages are measured, which should differ from those indicated in the circuit by no more than ± 10%. The volume sliders are set to the far right, and the tone controls are set to the middle position. Temporarily turn off the environmental protection circuits (R52, C46, ​​C47, R75, C38, C51). Sinusoidal signals with a frequency of 1 kHz and an amplitude of 250 mV are fed to the inputs of the LC and PC. A two-channel oscilloscope controls anti-phase signals at the anodes of the VL4, VL5 lamps (their amplitudes must be the same, and the shape must be undistorted). Install VL6 ... VL9 in place, and either acoustic systems or (better) load equivalents (8 Ohm x 150 W resistors) are connected to the outputs. An undistorted signal should also be observed at the output. Restore the chain of environmental protection. If the amplifier is self-excited, you should choose the capacitances C38, C47 or resistors R52, R75. In this case, it is impossible to greatly reduce the OOS, since the coefficient of nonlinear distortion will increase accordingly. This completes the amplifier setup.

In order to properly operate the amplifier, it should be remembered that turning on the amplifier without load is strictly prohibited. Failure to comply with this requirement will result in failure of output lamps and transformers.

Author: V. Fedorov, Lipetsk; Publication: cxem.net

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