Lamp UMZCH entry level. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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Over the years, sound amplification technology has accumulated a huge number of technical solutions that allow obtaining excellent results, however, in spite of everything, many designers (not only radio amateurs, but also serious firms) again and again return to their roots - as simple as possible from the point of view of circuitry, but at the same time the same time for the most effective solutions that allow you to get high-quality sound. One of these areas of design is the construction of UMZCH on vacuum tubes. (UMZCH - Audio Frequency Power Amplifier).

However, here we must pay tribute - despite the seeming simplicity of electrical circuits, not everyone succeeds in getting a "decent" sound. But if an experienced radio amateur fails to bring only one more coin to his piggy bank of experience, then for a beginner this problem, being unsolvable on his own, can permanently deprive him of his desire to engage in design. However, this is already from the field of psychology ... :)

The attention of novice constructors is offered a very simple to repeat, and most importantly, a non-capricious and fairly high-quality tube UMZCH, which uses common lamps and parts that were widely used in their time in televisions and radios.

The amplifier was designed as a terminal amplifier (that is, it does not include any tone controls or any other nodes, such as switches, corrective preamps, etc.) and was originally intended to amplify the signal coming from the computer's sound card, however, very good (subjectively) characteristics allow it to be used to amplify the signal from other, more "serious" sources (CD player, vinyl disc player, tape recorder, etc.)

A schematic diagram of one channel of the amplifier is shown in fig. one

(click to enlarge)

The amplifier is two-stage. The first stage is built on one half of the 6N3P (VL1) double triode and is a classic voltage amplifier stage. The second half of the lamp is used in the second channel of the amplifier.

Lamp pinout 6N3P

On resistors R4, R5, due to the cathode current flowing through them, a bias voltage is created, which sets the lamp operation mode. The absence of a capacitor in the cathode circuit (which is usually present in industrial designs and is connected in parallel with the cathode resistor) is not without meaning - this allows you to get local OOS in the cascade, due to which, although the gain is somewhat reduced, the linearity of the cascade is increased. The depth of such a local OOS is small and is determined by the ratio of the resistance values ​​of the resistors R4 and R6. This technique also allows you to "kill" the second rabbit - it is very convenient to apply a voltage of the general OOS to the cathode circuit, which is done in our case - the signal from the amplifier output through the divider formed by resistors R5 and R4 is fed directly to the cathode.

The type of the lamp and the operating point were chosen based on the desire to obtain a regime in the linear section of the CVC (voltage-ampere characteristic) of the lamp, while the appearance of the grid current would be unacceptable (the current in the grid circuit occurs when the voltage on it becomes positive relative to the cathode, as a result, there are strong signal distortions) in any mode of operation of the amplifier, and a small output impedance of the stage with sufficient amplification, which will allow you to "ignore" the parasitic capacitances of the installation and the lamp, and the inductance of the resistors of the subsequent stage. But with all this, the anode current must be small enough to ensure the longevity of the lamp. As a result, the resistance in the anode circuit was 47 kOhm and the anode current was 3 mA (with the anode current regulated by the reference book 8 mA for the 6N3P lamp) - at this point, the I–V characteristics are quite linear for an input signal with a swing of up to 3 volts. The voltage gain of the cascade is 16,5.

The second stage also does not differ in originality - this is a typical single-cycle cascade built on a powerful output pentode 6P14P (VL2). The cathode resistor R9 sets the operating point of the lamp (anode current 48 mA, second grid 7 mA), and also organizes a local shallow OOS. The resistor in the grid circuit is chosen with a relatively low resistance to reduce the influence of parasitic capacitances of the installation and the leakage current of the first grid (lamps in general always have a leakage current in the circuit of the first grid, even when the voltage on it is negative with respect to the cathode, but it is most noticeable for high-power lamps. The magnitude of this current is of the order of several μA. The negative effect is the "departure" of the lamp mode), but it is important that its resistance be significantly greater than the output resistance of the previous stage.

The lamp of the second stage is loaded on the output transformer - it is necessary to match the high output resistance of the lamp (about 4,5 kOhm) with a relatively low-resistance load. The principle of choosing a transformer for this design - "cheap and cheerful" - transformers of the TVZ-1-9 type were used, which were used both in televisions and in some radio receivers. You can use other types of output audio transformers, it is only important that they are designed specifically for use in single-ended output stages. You can even experiment with TVK type transformers (used in vertical scan output stages), but you must be aware that the output transformer is perhaps the most important detail in a tube amplifier - its quality for the most part will determine the quality of the amplifier as a whole.

Output stage voltage gain 0,85 (measured at 4 ohm load)

At the input of the amplifier, a filter is used that does not pass the lower frequencies of the audio range to the input of the amplifier (from about 40 Hz and below). The need for such a filter is caused by the following considerations: a) most middle-class household acoustic systems have lower operating frequencies from 40 to 60 Hz and, in principle, are not capable of reproducing a signal with a frequency below this threshold - the signal supplied to the acoustic system is obviously lower than its minimum operating frequency only generates significant additional distortion due to the displacement of the loudspeaker cones by this signal; b) domestic premises are small in size and, as a result, at low frequencies in such premises there are many resonances that cause the effect of "mumbling" during playback, and the smaller the room, the more pronounced this effect, the higher frequencies the resonance manifests itself; c) with decreasing frequency, the power of the amplifier required for playback should increase (this is true for the entire frequency range) - for example, if 100 W is enough to reproduce a signal with a frequency of 3 Hz at a normal volume, then to reproduce 50 Hz with the same volume, it is already necessary 12W amplifier output power; d) the lower operating frequency of most industrial audio transformers is 40-50 Hz - at lower frequencies, the transformer, as well as the acoustic system, loses efficiency (this is due to the finite value of the inductance of the primary winding), and in combination with the higher power of the lower frequency signal also generates significant distortion. Taking into account all this, as well as the fact that the output power of the single-cycle amplifying stage on a 6P14P lamp is limited to 4,5 W, it was decided to use such a filter. Of course, if you use high-quality transformers and acoustic systems, then there is no need for such a filter. In this case, you can not mount it by removing R2 for this and replacing C2 with a jumper.

Looking ahead, I would like to note that when comparing the sound of an amplifier with a filter and without, subjective preference was always given to the variant of an amplifier with a filter - bass, contrary to forecasts, is more "elastic" due to the elimination of output stage overload and a significant reduction in the "mumbling" of the room.

Power supply unit The amplifier is quite simple - it is a transformer, also taken from an old tube TV, with an anode voltage rectifier (Fig. 2). The capacitance of the filter capacitor C7 is chosen relatively small - this is due to the desire to reduce the peak current through the rectifier diodes (it is no secret that the rectifier diodes operating on a capacitive load are open only for a short period of time compared to the duration of the half-cycle, and at this time current flows through them , significantly exceeding the average consumed by the load). But since the voltage ripples are quite significant on a small capacitance, the R1 C10 filter is used in the amplifier (Fig. 5), where the capacitance C5 can already be quite large in order to effectively suppress them. The first stage is also fed through the same R7 C3 filter, which additionally protects it from supply voltage ripples caused by the operation of the second stage.

The R11-R14 chain (Fig. 1) is one common for both channels of the amplifier and is designed to create a positive potential of the filament circuit relative to the cathodes of the lamps. This is necessary to reduce the AC background - a highly heated filament and the cathode form some kind of vacuum diode, and if there is a positive voltage on the cathode relative to the filament at some time, a small current will flow from the filament to the cathode. This current will also flow through the cathode resistors, causing a voltage drop across them, which will then be amplified by all subsequent stages in the same way as the useful signal.

The R11 and R12 connected in series perform another function - the capacitances of the power filters are discharged through them when the amplifier is turned off.

The total current consumed by the incandescent lamps is 1,85 A. The filament winding of the transformer must be designed for this (or more) current, otherwise the filament winding of the transformer may overheat.

Construction and details

Both channels of the amplifier, except for the power supply, are entirely mounted on one printed circuit board (Fig. 3). Since the lamps dissipate a lot of heat, it makes no sense to strive to obtain a high mounting density. For the same reason, it is desirable to use foil fiberglass as a material for a printed circuit board - this material is more temperature resistant than textolite or getinaks, and does not deform when heated, which often happens with boards based on getinaks.

Resistors can be types BC or MLT. R1-R5, R13 and R14 can be of any power (the printed circuit board is designed to install resistors such as BC-0,5 and MLT-0,5), R6, R7, R8, R11 and R12 is better to take a power of at least 0,5 W (for R7 and R8, this is due not so much to the power dissipated on them, but to the possibility of "shoot through" between the turns of the thread at the moment the power is supplied to the amplifier). R9 must be at least 1W, R10 - 2W. R10 is best to take a wire - also because of the possible breakdown at the time of switching on, but in extreme cases, MLT-2 is also suitable.

The resistances of resistors R1, R11-R14 can differ significantly from those indicated in the diagram: R1 can be from 100 kOhm to 1 MΩ; R13, R14 from 1 to 100 kOhm, but preferably the same resistance; resistance R11 can vary from 100 to 470 kOhm, and the resistance R12 should be 5-15 times less than the resistance R11. R7 can be from 2 to 8,2 kOhm. The resistance R10 should not be increased, but any resistors in the range from 100 to 220 ohms can be used. The resistance R6 can also vary - from 22 to 75 kOhm, however, it must be taken into account that with an increase in the resistance R6, it is necessary to increase the resistance R4, as a result of which the depth of the feedback will change somewhat, and therefore the sensitivity of the amplifier will change. To set the required sensitivity, you will need to select the resistance R5. The resistance R9 should not be changed - only as a last resort, you can install a resistor with a resistance of 130 ohms.

The printed circuit board has two places for the resistor R12 (marked as R12 "on the wiring diagram), connected in parallel, so two resistors with a resistance greater than the nominal can also be used as R12.

Resistors R4, R5 and R9 for both channels do not hurt to pick up in pairs with the closest resistance values ​​- this will make it easier to tune the amplifier.

Capacitors C1, C2 and C4 are film capacitors. C1 and C2 type K73-9, C4 - K73-17. Capacitance C4 can be from 0,47 to 1,5 uF. The operating voltage of capacitors C1 and C2 is not critical (capacitors with a voltage of 100 V are used), the voltage of capacitor C4 must be at least 250 V. Other types of capacitors can be used, however, it must be taken into account that, for example, metal-paper or mica capacitors will have much larger dimensions, and the use of ferroelectric capacitors in audio circuits is unacceptable due to the significant piezoelectric effect. The use of unsealed capacitors (such as BMT, MBM) is also unacceptable due to the presence of leakage currents in them. Electrolytic capacitors are absolutely not suitable.

Power filter capacitors - any suitable size electrolytic with an operating voltage of at least 300 V. The capacitance C3 must be at least 10 microfarads (however, in this case it is desirable to increase the resistance R7 to 5,1-6,2 kOhm), it is undesirable to reduce the capacitance C5 ( in extreme cases, you can put 220 microfarads). It is also undesirable to reduce the capacitance of the C7 filter capacitor in the power supply.

The rectifier bridge diodes can also be replaced with any others, it is only important that when the amplifier is turned on, they can withstand the charging current of the filter capacitors (up to 2 A), and are designed for a reverse voltage of at least 400 V. D226G is quite suitable.

Socket PL9-2	Panel PLC9
Modified PLC socket9

PL9-2 sockets were used to place the lamps. Other sockets that can be installed on a printed circuit board are also suitable. In the absence of such, you can use panels that are not suitable for printed wiring. To install on a board, you can solder pieces of a thick single-core wire to their terminals, with the help of which the socket will be installed on the board. However, it would be preferable to modify directly the conclusions of the panel, biting off part of the conclusion with sharp side cutters (nippers) (see photo).

Jumpers JP1 are used from failed computer motherboards. The pins of the connector through which the signal is fed to the input of the amplifier are of the same type. Pins are also mounted on the board to connect the output transformer and power supply - they are used from the unified connectors used in TVs. The wires to these pins are soldered, although the use of connectors is not excluded.

During installation, special attention should be paid to connecting to a common wire - all circuits of a common wire must be connected either at one point or in a strictly defined sequence. On the printed circuit board, this sequence is observed - it is only necessary to make sure that there are no "extra" connections.

The rated output power of the amplifier is 3 W, the maximum is 4 W, the rated input voltage is 0,75 V. This power is enough for comfortable listening to audio programs in a room of 30 m² (acoustic systems 6AC-224 are used, from the Cantata-205 radiogram kit).

The appearance of the amplifier mounted on the board is shown in the photo

Establishment amplifier is easy. First of all, make sure that the power supply is working. The voltage '+275' can be between 250 and 300 V (depending on the type of transformer used). An alternating voltage of 6,3 V is considered within the normal range if it is not lower than 6,0 V, but not higher than 6,5 V. Then the amplifier board is connected to the power supply. Lamps are not yet installed.

Table 1 - voltage on panels without lamps

Having connected the board, you need to check the incoming voltages on the lamp panels. Table 1 shows the voltage values ​​for this case.

Very carefully referencing the voltage measurement on the 2nd knife of the VL2 socket - there should be an absolute "0". The slightest positive DC voltage will only mean one thing - capacitor C4 is leaking and must be replaced to turning on the lamps. The voltage "+49" is the voltage that is obtained at the R11-R12 divider, and if you changed the values ​​\u11b\u14bof these resistors, then it may differ from the specified one, but in any case it must correspond to the voltage at the connection point R275-R3. The absence or significant discrepancy between the voltage "+5" on any leg indicates a malfunction in this circuit, usually an open circuit. Of course, C7 or C10 may still be faulty, but in this case the consequence of their fault will be expressed by charring the resistors RXNUMX or RXNUMX, respectively.

Table 2 - voltage on the legs of the lamps

If everything is in order, turn off the power, connect speakers or a load equivalent (which can be a resistor with a resistance of 3,9 to 8,2 Ohms and a power dissipation of at least 2 W), remove the JP1 jumper and install the lamps. We supply power to the amplifier and immediately again control the voltage at the legs of 3 VL2 lamps. As the cathodes warm up, it should gradually increase to +6,0..6,1 V and then remain so - this will indicate that the lamps have reached the normal operating mode. A voltage higher than 6,3 V indicates a strong wear of the lamp (the steepness of the characteristic has decreased, as a rule, a consequence of gas contamination inside the lamp bulb), an underestimated voltage (from about 5,8 and lower) is also characteristic of long-running lamps (loss of emission) - these bulbs need to be replaced. The voltages on the other legs of the lamps are shown in Table 2. The voltages on the anodes and cathodes VL1 are indicated for the case of an open JP1 - when it is put in place, the voltages on the anodes will drop to 110..120 volts, and on the cathodes to 1,7..1,8 AT.

If the voltages are within the allowed range, you can try to apply a small amplitude signal to the amplifier input (about 25-50 mV, since JP1 is removed and the sensitivity is maximum). If successful, it remains only to make sure that the overall feedback is negative. To do this, carefully install JP1 in place. If in this case the self-excitation of the amplifier occurs, accompanied by loud noise, howling or whistling in the speaker system, in this case it is necessary to change the ends of the secondary winding of the output transformer between themselves.

On this, the adjustment can be considered complete.

Safety measures

1. During any installation work, the device must be de-energized. Since the amplifier uses high-capacity storage capacitors, it is necessary to wait for their discharge, which occurs within 30-40 seconds after the amplifier is turned off. When testing the power supply separately from the amplifier, be careful - in this case, the capacitor C7 is able to store a charge for a very long time (up to several days). To ensure the discharge of the capacitor, a resistor with a resistance of 100 kΩ to 1 MΩ and a power of at least 0,5 W should be temporarily soldered in parallel to it. It is strongly not recommended to discharge capacitors by short-circuiting their terminals (for example, with a screwdriver or tweezers) - this can lead to both failure of the capacitor and injury.
2. Tube amplifiers, unlike transistor ones, are not afraid of a short circuit in the load, but an open circuit in the load circuit can disable the output transformer. It is strongly not recommended to turn on the amplifier in the absence of a rated load connected to its output (nominal load resistance 4 ... 8 Ohm) - this threatens to breakdown the insulation of the primary winding of the output transformer due to its significant inductance. If you are going to operate the amplifier together with headphones, you must take this into account and, while connecting the headphones, ensure parallel connection of a load dummy, which can be a conventional resistor with a resistance of 3,9 to 8,2 ohms and a power dissipation of at least 2 watts. Any load switching, in which even a short-term break in its circuit is possible, must be performed only when the amplifier is turned off.
3. Output pentodes 6P14P are very hot during operation. Don't get burned :-)

Literature

1. D.S. Gurlev. Handbook of electronic devices. - "Technique", Kyiv, 1966
2. M. Kireev. Amateur Radio High-End. 40 best designs of lamp UMZCH for 40 years. "Radioamator", Kyiv, 1999

Author: Andrey Kovalev (Tyumen); Publication: cxem.net

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