Preamplifier with two ECC83 tubes. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Preamplifier with two ECC83 tubes. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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The proposed preamplifier is a so-called combined ULF, that is, an amplifier in which semiconductor devices are used along with electrovacuum devices. This design is made on two lamps of the ECC83 type, and a J-FET field-effect transistor is used in the final stage.

The circuit of this combined preamplifier is based on proven circuit solutions that have been used by almost all developers of low-frequency tube technology for decades. Schematic diagram of the pre-amplifier is shown in the figure.

(click to enlarge)

The preamplifier has two inputs (INPUT 1 and INPUT 2), each of which is intended primarily for connecting an electric guitar and other instruments. However, these same inputs can also be used to connect other signal sources such as a microphone. Both inputs can be used at the same time, while the tone correction will also be common to the signals in both channels. To simplify the design, resistor dividers are excluded from the amplifier circuit, usually connected to the contacts of jack connectors. Naturally, if desired, these dividers can be set, but this is not necessary.

Low-frequency signals arriving at the contacts of the input connectors are fed through resistors R2 and R4 to the grids of triodes E1A and E1B of the first lamp of the ECC83 type, which is a double triode. Frequency compensation of the influence of the integrating element formed by the resistor connected in series and the input capacitance of the lamp triode is not necessary. On the contrary, this RC element contributes to the suppression of high-frequency interference outside the acoustic range. The triodes of the ECC83 lamp are connected according to the classical amplifier circuit. Different values of the cathode resistors and capacitors provide a high frequency shift in the signal applied to the high input.

From the anodes of the triodes E1A and E1B through load capacitors with different capacities (C1 = 22 nF, and C2 - 68 nF), the signals pass to the potentiometers P1 and P2 (GAIN 1 and GAIN 2), which set the level of the signal entering the next amplifying stage. By moving the sliders of these potentiometers to the highest pins in the circuit and using a conventional guitar pickup, the signal in the subsequent stages is limited, which provides a “sustain” effect to the guitar tone. At the same time, we are not talking about any significant limitation, the sinusoid is only noticeably rounded. Through the amplifying stages, made on the E1A and E1B triodes (before the GAIN potentiometers), the 500 mV input signal passes with virtually no noticeable distortion.

The signals passing from the potentiometer sliders are mixed on resistors R9 and R10. Capacitor C9 is connected in parallel with resistor R6, which provides a shift at higher frequencies, this shift also depends on the position of the second input potentiometer slider. In addition, a low-capacitance capacitor C1 is connected between the upper output and the potentiometer P5 engine, which provides a shift in the high-frequency components of the signal coming from the upper input. As a result, the signal passing through the amplification channel of the first input is richer in high-frequency components than the signal passing through the amplification stage of the second input. If desired, the capacitance of the capacitors can be changed or completely eliminated the compensation scheme and assemble the channels according to the same scheme. As a result, both channels will work in the same way, however, there will be a natural suppression of the high-frequency components of the signals applied to both inputs.

The mixed signal from both channels is fed to the next amplifying stage, made on the second lamp of the ECC83 type. On the first triode E2A of this lamp, a conventional amplifying stage is assembled, and on the second triode E2B, a cathode follower. Such inclusion in tube amplifiers is quite common.

From the cathode of the E2V triode, the signal is fed to a passive three-band tone control unit, which is made according to the classical scheme. Potentiometer P4 regulates high frequencies (TREBLE), potentiometer P5 - low (BASS), and potentiometer P6 - medium (MIDDLE). After the tone control block, a volume control is installed. This is a 2 MΩ/LOG potentiometer, which has almost no effect on the operation of the correction circuits.

Matching the total output impedance of the preamplifier and the terminal amplifier connected to its output is provided by a cascade made on a BF245B J-FET field-effect transistor connected according to the follower circuit. It is powered by a 12 V voltage source. The amplification of this stage, even with a low supply voltage of 12 V, is quite sufficient, since it is turned on after the volume control, and the effective output voltage required to excite the final amplifier is approximately 1,5 V. From the output of the preamplifier, the signal is fed to the input of the final transistor amplifier.

From the cathode of the E2A triode, through the P13 potentiometer, indicated on the diagram as EFX, the signal is also fed to the EFEKT output for external effects units or for other purposes. However, the EFEKT output can also serve as a linear input, so a separating resistor RXNUMX is installed in front of the RXNUMX potentiometer, which determines the complex resistance of this input / output and signal matching.

The components of the pre-amplifier under consideration are also power circuits. The anode voltage for lamps is formed by a full-wave rectifier from an alternating voltage taken from the secondary winding (280 V / 30 mA) of a toroidal network transformer using a diode bridge D1 (1 A / 400 V). The rectified voltage is filtered by a chain of RC elements, consisting of resistors R17-R19 and capacitors C12-C15 with a capacity of 22 to 47 microfarads with a nominal voltage of 400 V. When assembling and working with this amplifier, for safety reasons, special attention should be paid to circuits with a voltage of 400 V and charged capacitors.

The constant heating voltage is also formed by a full-wave rectifier from the alternating voltage taken from the secondary winding (18 V / 0,5 A) of the transformer, filtered by a capacitor C17 with a capacity of 2000 uF and stabilized by an integrated stabilizer IC1 type mA7812 (12 V / 1 A). The filaments in each ECC83 lamp are connected in parallel, while one extreme terminal is always grounded. The voltage of 12 V is also used to power the matching stage with the J-FET transistor TJ, as well as to power the control LED (not shown in the diagram). The rectifier and filament voltage regulator can be placed on the preamplifier board, paying particular attention to proper grounding. Stabilizer IC1 with an input voltage of about 24 V and a current consumption of 300 mA must be placed on a radiator.

See other articles Section Tube Power Amplifiers.

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