Tube audio frequency amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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The proposed tube stereo amplifier, due to the availability of parts and ease of setup, can be an exciting step for a radio amateur into the world of tube radio electronics. It does not contain transient capacitors, it is distinguished by an extended band towards low frequencies and stability to supply voltage drops.

Even the simplest audio frequency power amplifiers (UMZCH) must provide natural sound. Full-fledged reproduction of low frequencies is especially necessary because even at an average volume, the sensitivity of hearing to low frequencies is significantly reduced relative to medium frequencies.

A feature of the frequency response of a transformer amplifier is a decrease in the gain at the lowest frequencies, which is explained by a decrease in the inductive resistance of the primary winding of the transformer [1]. Frequency and phase distortions can be, among other reasons, the result of the use of transient capacitors, since their reactance is frequency dependent, and this is most noticeable at low frequencies.

Most tube single-cycle UMZCH, for various reasons, have a drop in frequency response in the low-frequency region, starting from 60..80 Hz. The proposed UMZCH has a more even frequency response with high-quality signal amplification even at low power. This UMZCH works on the principle of a DC amplifier with the exception of the output transformer. The difference from, for example, Gubin's tube amplifier [2] lies in the unusual way of connecting the load and applying feedback from the UMZCH output to the screen grid of the first pentode. As a result, the subjective features of the reproduced sound are "bass" and "transparency".

The amplifier described in the article has an output power of about 3 W with a dynamic head resistance of 4 ohms (it is allowed to use a load of up to 12 0m), a sensitivity of about 300 mV. Operating frequency range - 30 Hz...20 kHz. The UMZCH output stage operates in class A mode.

Fig. 1

On fig. 1 shows a diagram of one of the stereo amplifier channels. In a two-stage amplifier, a negative feedback loop (NFB) is formed between the load and the cathode of the VL2 lamp, which equalizes the frequency response of the amplifier, tracking changes in the speaker impedance in the operating band, and reduces non-linear distortion. It is known that the use of a general OOS requires a multilateral consideration of the design features of amplifiers [3]. At the same time, relatively simple UMZCH without a common environmental protection often differ in a more natural sound [4].

Here, the circuit L2C3 (L2 is the inductance of the secondary winding of the output transformer T1) can be considered as an oscillatory circuit that performs a corrective function in the low-frequency region, shunting the cathode resistor R6 of the local OOS. Assume that C3 = 220 uF, L2 = 0,1 H. then the resonant frequency of the circuit

To prevent overload of the dynamic head at low frequencies, an additional resistor R7 was used, which sets the quality factor of the circuit

The frequency response of this amplifier (Fig. 2, solid curve) is closer to linear, especially in the LF section, than the frequency response of the amplifier without feedback (dashed curve).

Fig. 2

In addition, the amplifier has a PIC through a resistor connected between the cathodes of the VL1 and VL2 lamps. The divider of resistors R3, R4 sets the depth of this connection and allows you to abandon the oxide capacitor in the VL1 cathode circuit. At the same time, the OOS on the VL1 screen grid stabilizes the voltage at the lamp anode, reducing the dependence of the mode on changes in the supply voltage.

Resistor R5 and capacitor C1 are a decoupling power supply filter. Capacitors C2 and C4 prevent self-excitation at high frequencies. Resistor R2 sets the gain of the first lamp and the voltage on the control grid of the output lamp VL2. The output stage operation mode is stabilized by the auto-bias cathode resistor R6 and the capacitor C3. By selecting the anode resistor R2, the voltage at the anode VL2 is set, which is usually measured with a high-resistance voltmeter, which is not available to every radio amateur. In this amplifier, it is permissible to adjust the UMZCH lamp mode for a voltage of +50 V at the VL2 cathode.

A novice radio amateur can be recommended to assemble the UMZCH first on the circuit board, taking precautions. It is advisable to put shielding caps on the VL1 lamps of the stereo amplifier. After assembling the amplifier, make sure that the secondary winding of the transformer T1 is connected correctly. In case of excitation of the amplifier (sharp whistle), it is necessary to swap its ends.

The amplifier is tuned with measuring instruments, at least with a multimeter, and preferably with an oscilloscope. Using a tuning resistor R3, a voltage of +2 V is set on the VL50 cathode. If an oscilloscope is available, instead of a loudspeaker, an equivalent resistance corresponding to this type of transformer is switched on, for example, for a TVZ-1-9 transformer, a 4 m resistor with a power of 0 W. A sinusoidal signal of 5 kHz is fed to the input of the amplifier. Increasing the input signal level with a variable resistor R1, adjusting the resistor R1 achieve uniform limitation of the sinusoid from above and below (by ear - minimum distortion). 3 kOhm), without violating the uniformity of the frequency response and the shape of the sinusoid. The values ​​of the selected resistances of the resistors R4 in the right and left channels may differ from each other due to the spread of the real parameters of the radio elements.

If the sound of speakers with PA is impeccable, without low-frequency "mumbling", then it is permissible to put a jumper instead of resistor R7; otherwise, its value is chosen in the range of 20 ... 300 ohms. For example, for speakers with a dynamic head 4A-28 (12 0m), the resistance R7 = 0 This completes the amplifier setup.

Fig. 3

The power supply can be taken from outdated lamp equipment or assembled independently according to the diagram in fig. 3, given that the power of the network transformer should be 60 ... 90 watts. To increase the service life of the lamps, it is provided to turn on the glow first (switch SA1), and then after 20 ... 30 s - the anode power supply of the device (switch SA2). Instead of D226 diodes, you can use a diode bridge with a nominal voltage of at least 400 V. It is advisable to put a 2 kOhm (220 W) resistor in parallel with the capacitor C2 of the power supply to discharge capacitors C2 and C3 after turning off the power.

As an output, you can use transformers from household lamp equipment, for example, TV-2A, TVZ-1-9, TVZ-1-1 (for a load resistance of 8 ohms). In extreme cases, the TVK-110 LM transformer is also suitable (for the vertical scan of the TV). Detailed recommendations on the assembly of such amplifiers can be found in the article [5].

Together with such an UMZCH, we recommend using speakers with an open case and high-sensitivity dynamic heads.

Literature

1. Adamenko MV Secrets of low-frequency tube amplifiers. - M.: NT Press, 2007, p. 172
2. Three-tube amplifier Gubin. - .
3. Ageev S. Issues of designing amplifiers with a common environmental protection. - Radio, 2003. No. 4. p. 16-19.
4. Orlov A. UMZCH with a symmetrical input without a common environmental protection. - Radio, 2002, No. 4. p. 12-14.
5. Komarov S. Push-pull terminal amplifier for 6N23P and 6P43P. - Radio, 2008, No. 8. p. 49, 50; No. 9, p. 45-47.

Authors: S. Akhmatov, D. Sannikov

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