Two tube amps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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The article offers two options for tube audio frequency power amplifiers. A feature of the presented designs is the galvanic connection between the cascades. The authors made an attempt to find the optimal combination of simplicity, quality and repeatability of the UMZCH with a single-cycle output stage with a power of up to 8 W per channel.

Often, when evaluating a sound reproducing system, the listener consciously or unconsciously focuses on subjective sensations that determine the sound quality. In this case, such characteristics as naturalness, "transparency", "softness" of sound, "quickness" (distinctness) of bass, detailing of the reproduced composition, etc. are used. Of course, with a certain degree of conventionality, these characteristics can be associated with the objective parameters of this system - amplitude - frequency response (frequency response), coefficients of harmonic and intermodulation distortions, noise and background levels, speaker damping coefficient, etc. Each of the tube UMZCHs offered here, from our point of view, can be considered as a combination of acceptable sound quality, good technical parameters and comparative simplicity circuit solution.

The first amplifier is single-cycle, on a G-807 lamp (in Fig. 1, a diagram of one of the channels of a stereo amplifier). It is a modernization of the amplifier "Profundo" [1]. Here, an additional cathode follower was used, assembled on the triode part of the combined radio tube VL1 (6F1P). This inclusion allows you to coordinate the operation of the input and output stages in order to eliminate the drop in the frequency response in the high-frequency region and reduce non-linear distortions mainly in the low-frequency region that occur in such a circuit with a direct connection of the pentode anode and the G-807 control grid.

Rice. 1. Diagram of one of the channels of a stereo amplifier

As in the first version of "Profundo", all stages of the amplifier are covered by a chain of successive local feedbacks. Local positive feedback (POS) is necessary not only to exclude the oxide capacitor from the VL1.1 cathode circuit, but also to improve the reproduction of low frequencies ("fast" bass). A voltage divider R7R5 is formed in its circuit, to which a tetrode screen grid is connected. Capacitor C1 is optional, but can be used to eliminate possible noise when moving the slider of resistor R1. The output stage is assembled according to an ultra-linear scheme, which reduces its non-linear distortion and output impedance.

The UMZCH I power supply used a unified transformer TS-180 (from old TVs). The rectifier is made on semiconductor diodes VD1, VD2 according to a symmetrical voltage doubling circuit. The shallow depth of the overall feedback does not provide a significant suppression of the background from the ripple of the anode voltage, therefore, U-shaped filters with chokes are used in the power supply.

The establishment of UMZCH is carried out either according to the method described in the article [1], or (in the absence of devices) by adjusting the resistor R4 until the maximum undistorted signal is reached. The quiescent current of the anode of the G-807 lamp, equal to 70 mA, can be adjusted by selecting a resistor r8. The offset on the control grid of the output lamp relative to the cathode is about -20 V.

This UMZCH allows you to use an output transformer with a relatively small magnetic circuit without loss of low frequencies. As BA1, a broadband highly sensitive (90...100 dB/W/m) dynamic head can be used.

On fig. 2 shows a diagram of a single-cycle UMZCH II on 6C41C triodes in the output stage (one of the two channels of a stereo amplifier). The amplification of the first stage is carried out by the VL1 (6E5P) tetrode, from the anode of which the signal is fed to the grid of the output lamp VL2 (6S41S). The signal from the middle of the secondary winding of the output transformer T1 through the capacitor C2 enters the screen grid VL1, forming a PIC loop. It additionally increases the amplification of the low-frequency signal using the LC2 circuit (where L is the inductance of half of the secondary winding of the transformer T1), i.e., it performs a corrective function in the low-frequency region of the audio range. In this case, the resonant frequency of the circuit can be estimated as f_rez = 1/(2π√LC2). The OOS is formed by resistor R6 on the screen grid VL1. NFB reduces non-linear distortion and prevents self-excitation of the amplifier at low frequencies.

Rice. 2. Scheme of a single-cycle UMZCH II on triodes 6S41S

In the power supply of this amplifier, a semiconductor diode rectifier (bridge circuit) is used for the output stage, and for the first stage (on the VL1 tetrode), a half-wave rectifier based on diodes VD5, VD6 with capacitors C3, C5 is used. As a mains transformer in the power supply for both UMZCH, it is possible to use the TS-180 transformer (or its modifications, for example, TS-180-2) with a sufficient margin of power, by appropriately connecting the secondary windings to obtain the required alternating voltage (63 + 63 + 42 V).

The amplifier is adjusted by setting the quiescent current I for VL2₀ = 120 mA by selecting resistor R3. In this case, the bias voltage on the grid of the output lamp relative to the cathode should be about -75 V.

The magnetic circuits of the mains and output transformers should be placed in the case mutually perpendicular to minimize magnetic coupling through the stray field.

The parameters of all UMZCH are given in table. 1. They were measured using an S-107 oscilloscope, a V3-38 millivoltmeter, a G3-118 generator and a notch filter included in its kit.

Table 1. UMZCH parameters

^* К_г of all UMZCH at their rated output power (f = 1 kHz) does not exceed 1,6%.

On fig. 3 shows the frequency response of the two proposed amplifiers. For UMZCH I, the frequency response was measured at its rated power P_Mr. \u5d 1 W (hereinafter - at a frequency f \uXNUMXd XNUMX kHz), for UMZCH II - at power P_Mr.= 6 W.

Rice. 3. Frequency response of two amplifiers

In table. 2 shows the parameters of the output transformers for lamps used in UMZCH I and II.

Table 2. Parameters of output transformers for lamps used in UMZCH I and II

To extend the service life of radio tubes, it is desirable to install a switch (tumbler), through which voltage is applied to the anodes of the lamps approximately 20 s after turning on their glow.

Chokes L1 and L2 in fig. 1 and fig. 2 can be replaced by unified D31-5-0,14. If they are not available, chokes Dr-1,2 - 0,16 and the like can be used, however, in UMZCH II, the capacitance of capacitors C4, C6 and C7 should be increased to 300 microfarads.

In both UMZCH designs, variable resistors R1 with a type B control curve are used. The remaining resistors are MLT or imported. Powerful resistor R8 (2,4 kOhm) in the circuit of fig. 1 - for example, PEV-10 or imported higher power. Resistors spread tolerance - ±10%. Trimmer resistor - SP-2-2-0,5, SP-3-9, etc., preferably with an axis stopper.

Oxide capacitors - for example, K50-12, K50-17, K50-31 and similar (or imported). The capacitor at the UMZCH input can be selected from film (for example, the K73-9 series) or paper (K40U-9 series), although its effect on the sound is less noticeable than the interstage one (in both amplifiers, the connection between the stages is direct, without capacitors).

When assembling and debugging amplifiers, extreme care and caution should be observed (high voltage). The issues of eliminating the AC background are well described in [2, Ch. 3] and [3].

To design the amplifier, you can use the recommendations set forth, for example, in [2, Ch. 4]. We add that the UMZCH chassis can be made of aluminum or steel with a thickness of 1,5 and 0,5 ... 0,8 mm, respectively. RCA input connectors ("tulips"), output terminals - threaded. The trimming resistor in the cathode circuit is preferably located as close as possible to the input lamp. Its body is connected to a common wire or shielded. The wires of the filament circuits are twisted together.

The psychoacoustic characteristics of each of the described UMZCH have their own characteristics. In our opinion, the first UMZCH is characterized by the detail and transparency of the sound palette, the second - by the combination of the softness of the bass register with the clarity of the high-frequency components of the sound. A common characteristic of both designs is the "warmth" of the sound, as they say about the sound with tube amplifiers.

Literature

1. Akhmatov S., Sannikov D. "Profundo" - tube audio frequency amplifier. - Radio, 2012, No. 5, p. 16, 17.
2. Adamenko MV Secrets of low-frequency tube amplifiers. - M.: NT Press. 2007.
3. Simulkin S. Secrets of high-end lamp technology. - Radio hobby, 1999, No. 4, pp. 49-52.

Authors: S. Akhmatov, D. Sannikov

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