We have long been accustomed to the fact that we are surrounded everywhere by microelectronics and transistor technology. In televisions, players, receivers, tape recorders everywhere we hear sound in the speakers, amplified by special microcircuits that are powered by low voltage and produce a very loud sound.
But not so long ago - several decades, these same transistor amplifiers, and then microcircuits, just appeared. Fashionistas proudly wore receivers that were powered by special batteries - anode batteries and batteries for incandescent lamps; it was then simply a miracle that it was possible to receive and hear the radio on the go.
Lamps were very widespread. Cinemas had powerful tube amplifiers, the output of which was usually two G-807, 6R3S, or less often GU-80 tubes.
And the famous mobile film installations "KINAP" made in Odessa for an alternating voltage of 110V, which were powered from a standard network through an autotransformer, at the output of the amplifier there were the famous 6P3S lamps - lamps that were used in home-made transmitters on medium waves and it was a couple of trifles to make it, having also a lamp receiver , a microphone and a wire antenna stretched in the yard, through which it was possible to communicate over the air with a friend from a neighboring street.
But time passed and new electronic devices appeared, which began to slowly displace lamps, but it is not yet possible to completely replace lamps with transistors, because lamps have an advantage in powerful output cascades of transmitters and radar technology, but nevertheless the technical process moves forward.
What attracts a tube amplifier??
The first and most important thing is high-quality reproduced sound. The amplifier has, first of all, low distortion and a high signal slew rate.
What is a good system? According to Alexander Chervyakov, “they put on a record and you can’t hear it, the better the amplifier, the less you can hear it,” that is, you can hear the music, in the smallest subtleties, every instrument is the music around you, you have merged with it and nothing else exists, nervana.
Construction scheme
According to the construction scheme, amplifiers can be divided:
1. primarily single-ended or push-pull - in the ULF output stage one lamp or two lamps are used in the so-called push-pull connection. In the push-pull version, it is possible to obtain more power at the output, with good quality of the reproduced undistorted signal.
2. Mono amplifiers or stereo amplifiers.
3. Single-band or multi-band, when each amplifier reproduces its own frequency band and is loaded onto the corresponding acoustic system - speakers.
An amplifier consists of several successive stages, usually:
A diagram of a tube amplifier is shown. Input amplifier on a pentode, ECF80 tube (6BL8, 6F1P, 7199), 6AN8A triode, output stage on a KT88 or KT90 or EL156 beam tetrode, 5U4G kenotron as a rectifier. Output transformer for Tanso XE205 single-ended tube amplifier. The power transformer in the anode winding has taps that switch depending on the applied output tube.
Basic specifications tube ULF, an example is shown in parentheses - amplifier parameters on the famous 300B tube.
Power - W, at load in Ohms. (20)
Reproducible frequency band - Hz, kHz (5 -80,000)
Load resistance - Ohm (4-8)
Input sensitivity, mV (775)
Signal to noise ratio (no noise) dB (90)
Nonlinear distortion coefficient, no more than % (less than 0.1 at a frequency of 1 kHz, at a power of 1 W)
Number of channels
Supply voltage, V
Power consumption from power supply - W (250)
Weight, kg
Overall dimensions, mm
Price
Accessories for tube amplifier
Output transformer. One of the most important elements of high-quality audio sound design is the output transformer used. Used high quality audio output transformers for Hashimoto, Tamura, Elektra-Print, Tribute, James Audio, Lundahl, Hirata Tango, AUDIO NOTE, etc.
Capacitors. To create the required amplitude-frequency response, the parameters of the component elements are important. Music lovers attach a very important role not only to the brands used, but also how they are included in the circuit: if the capacitor is located between the stages of the amplifier, then the outer lining is connected to a lower impedance, i.e. to the driver, if as a blocking one, then the outer lining is connected to ground, in the picture the outer lining is marked with a stripe.
The photo shows capacitors for low-frequency sound amplifiers Jensen audio capacitors; aluminum, copper, and silver are used as foil; accordingly, the price varies widely. Manufacturers of audio line capacitors: Audio Note, TFTF, Mundorf, Jensen, Duelund CAST and others. Frequency characteristics vary depending on the design: paper case - copper foil, copper case and copper plates, staniol - mylar in oil, aluminum foil in an aluminum case and silver-plated terminals, so fans of high-quality audio make various measurements of the characteristics of parts to determine the best price ratio - quality. Electrolytic capacitors have a wide range of choices: Black Gate, etc. For cathode circuits, Caddock is preferred.
Switches
Resistors. Various resistors are used for manufacturing: tantalum resistors Audio Note, metal film Beyschlag, Allen-Bradley, etc.
Lamps. Since we are talking about tube sound lovers, one of the main elements for construction is the tube. Domestic lamps 6n2p, 6n8s, 6P3s, 6p14p, 6s33s, 6r3s. Passionate about perfect sound, true lovers of tube sound prefer only NOS tubes - these are completely new tubes that were released a long time ago, examples are the 6AC5GT, 45 tubes (the tube was produced from the late 1920s in the USA until the end of the 50s), 2A3 , 300V, etc. A large number of well-known lamps PX4, PX25, KT-88, KT-66, 6L6, EL-12, EL-156, EYY-12, 5692, ECC83, ECC88, EL34, 5881, 6SL7 have been and are used. But many people prefer vintage lamps.
Manufacturers of vacuum tubes.
German - Telefunken, Valvo, Siemens, Lorenz. Europe - Amperex, Philips, Mazda. England - Mullard, Genalex, Brimar. America - RCA, Raytheon, General Electrics, Sylvania and others. Tubes for the amplifier are purchased directly from abroad or through the websites www.tubes4audio.com, www.kogerer.ru, www.cryoset.com/catalog/index.php?cPath=22&osCsid=d721583766160686aa0fa118d03b88fd, www.groovetubes.com, www. iconaudio.com.
There are (have been) many high-quality amplifiers produced in the world.
Audio amplifiers load the speaker system, but there are quite a few who sometimes want to listen to music on headphones, for example MrSpeakers Alpha Dog.
On the picture. Stereo amplifier MB520 20 W, price £ 950 or more, bandwidth 15Hz~35kHz, S/N ratio 82dB, load impedance 8/16 Ohm, size 412x185x415 mm. Preamplifier on EF86, 12AU7 tube used as bass reflex, rectifier for each channel on 5AR4, output tubes EL34. Stainless steel is used. Motor driven attenuator controlled by remote control, position indicated by green LED.
The MB805 is a monoblock amplifier, priced at £5,999. Power per channel (8 ohm load) 50W, signal-to-noise level is -90db.
MB81. Mono amplifier based on GU-81, cost £12,500. The signal-to-noise ratio is -100dB, ripple in the frequency band 20 Hz - 20 kHz - 1dB, load 4Ω - 16Ω. Input sensitivity 600 mV, input impedance 100k. Power consumption from the network 220/240/115 volts average 450watts, 750w max. The output is 200 W into an 8 Ohm load. Input amplifier on a 6SL7, 6SN7 tube, drivers on two EL34.
SE (single-end) - single-ended output, meaning amplification of the signal unchanged.
Eimac 250TH Audio Amplifier
Video of a tube amplifier in operation, demonstrating music playback.
The article, with many photographs, describes the process of creating a miniature tube head based on the Fender Champ.
The cabinet for the modified 4GD-28 speaker was made in the same style as the head.
The source of inspiration, spare parts and original nameplates were the Yunost-301 tube electrophones, raised from oblivion.
I provide in my article a detailed description of the process of converting two inexpensive transformers TP-112-8 into one separating one for anode power supply.
On the vast expanses of the Internet, such an amplifier has been discussed many times in various circuit design options. At the end of the article there is a list of a dozen Datagor articles on the topic of SRPP. Not having deep knowledge of electronics, I simply repeated one of the circuits.
The article is primarily aimed at beginner tube audio enthusiasts who want to build a decent-sounding system with minimal labor and material costs. The circuit and design allow you to choose your own UMZCH sound option from the 4 proposed ones.
Guys, the photo shows the possible result of your work! Get to work!
Let’s put aside the “husk” in the form of: creative and conceptual technical solutions, the use of all kinds of “harmonizers,” the directionality and straightness of the conductors, the dependence of the sound on the exorbitant price of the elements used, and so on.
Let's formulate our ideology.
We will proceed from positive and physically explainable high-end options:
Option #1. You can turn to our Chinese friends on Aliexpress for help. China has everything!
This is what the new Chinese lamp panels look like.
Snow-white glazed ceramics. After all, China is the birthplace of porcelain and they know how to make it better than anyone else.
— most connoisseurs of high-quality music, who know how to handle soldering equipment and have some experience in repairing radio equipment, can try to assemble a high-class tube amplifier, which is usually called Hi-End, on their own. Tube devices of this type belong in all respects to a special class of household radio-electronic equipment. Basically, they have an attractive design, with nothing covered by a casing - everything is in plain sight.
After all, it is clear that the more visible the electronic components installed on the chassis are, the greater the authority of the device. Naturally, the parametric values of a tube amplifier are significantly superior to models made with integrated or transistor elements. In addition to this, when analyzing the sound of a tube device, all attention is paid to the personal assessment of the sound rather than to the image on the oscilloscope screen. In addition, it has a small number of used parts.
If there are no particular problems when choosing a pre-amplifier circuit, then when choosing a suitable final stage circuit, difficulties may arise. Tube audio power amplifier may have several versions. For example, there are single-cycle and push-pull devices, and also have different operating modes of the output path, in particular “A” or “AB”. The output stage of single-ended amplification is, by and large, a sample, because it is in mode “A”.
This operating mode is characterized by the lowest nonlinear distortion values, but its efficiency is not high. Also, the output power of such a stage is not very large. Consequently, if it is necessary to sound an internal space of medium size, a push-pull amplifier with the “AB” operating mode will be required. But when a single-cycle device can be made with only two stages, one of which is preliminary and the other amplifying, then a driver is needed for the push-pull circuit and its correct operation
But if single-cycle tube audio power amplifier may consist of only two stages - a pre-amplifier and a power amplifier, then a push-pull circuit for normal operation requires a driver or cascade that forms two voltages of identical amplitude, shifted in phase by 180. Output stages, regardless of whether it is single-ended or push-pull, require the presence of output transformer. Which acts as a matching device for the interelectrode resistance of a radio tube with low acoustic resistance.
True admirers of “tube” sound argue that the amplifier circuit should not have any semiconductor devices. Therefore, the power supply rectifier must be implemented using a vacuum diode, which is specially designed for high-voltage rectifiers. If you intend to repeat a working, proven tube amplifier circuit, then you do not need to immediately assemble a complicated push-pull device. To provide sound in a small room and obtain an ideal sound picture, a single-ended tube amplifier is fully sufficient. In addition, it is easier to manufacture and configure.
There are certain rules for installing radio-electronic structures, in our case these are tube audio power amplifier. Therefore, before starting the manufacture of the device, it would be advisable to thoroughly study the primary principles of assembling such systems. The main rule when assembling structures using vacuum tubes is to route the connecting conductors along the shortest possible path. The most effective method is to refrain from using wires in places where you can do without them. Fixed resistors and capacitors must be installed directly on the lamp panels. In this case, special “petals” must be used as auxiliary points. This method of assembling a radio-electronic device is called “mounted mounting”.
In practice, printed circuit boards are not used when creating tube amplifiers. Also, one of the rules says - avoid laying conductors parallel to each other. However, such a seemingly chaotic layout is considered the norm and is completely justified. In many cases, when the amplifier is already assembled, a low-frequency hum is heard in the speakers; it must be removed. The primary task is performed by the correct choice of the ground point. There are two ways to organize grounding:
The location for the grounding point must be verified by experiment, listening for the presence of background. To determine where the low-frequency hum comes from, you need to do this: Using a sequential experiment, starting with the double triode of the pre-amplifier, you need to short-circuit the lamp grids to ground. If the background decreases noticeably, it will become clear which lamp circuit is causing the background noise. And then, also experimentally, you need to try to eliminate this problem. There are auxiliary methods that are required to be used:
Tube audio power amplifier, or rather, the filament circuit of the pre-amplifier lamp can be powered with direct current. But in this case, you will have to add another rectifier assembled using diodes to the power supply. And the use of rectifier diodes in itself is undesirable, since it breaks the design principle of manufacturing a Hi-End tube amplifier without the use of semiconductors.
The paired placement of the output and mains transformers in a lamp device is quite an important point. These components must be installed strictly vertically, thereby reducing the background level from the network. One of the effective ways to install transformers is to place them in a casing made of metal and grounded. The magnetic cores of transformers also need to be grounded.
Radio tubes are devices from ancient times, but they have become fashionable again. Therefore it is necessary to complete tube audio power amplifier with the same retro elements that were installed in the original lamp designs. If this concerns permanent resistors, then you can use carbon resistors that have high stability of parameters or wire resistors. However, these elements have a large scatter - up to 10%. Therefore, for a tube amplifier, the best choice would be to use small-sized precision resistors with a metal-dielectric conductive layer - C2-14 or C2-29. But the price of such elements is significantly high, so instead of them, MLTs are quite suitable.
Particularly zealous adherents of the retro style get an “audiophile’s dream” for their projects. These are carbon resistors BC, developed in the Soviet Union specifically for use in tube amplifiers. 
If desired, they can be found in tube radios from the 50s and 60s. If according to the circuit the resistor must have a power of more than 5 W, then PEV wire resistors coated with glassy heat-resistant enamel are suitable.
Capacitors used in tube amplifiers are generally not critical to a particular dielectric, as well as to the design of the element itself. Any type of capacitor can be used in the tone control paths. Also, in the rectifier circuits of the power supply, you can install any type of capacitors as a filter. When designing high-quality low-frequency amplifiers, the coupling capacitors installed in the circuit are of great importance.
They have a special influence on the reproduction of a natural, undistorted sound signal. Actually, thanks to them we get exceptional “tube sound”. When choosing coupling capacitors that will be installed in tube audio power amplifier, special attention must be paid to ensure that the leakage current is as small as possible. Because the correct operation of the lamp, in particular its operating point, directly depends on this parameter.
In addition, we must not forget that the separating capacitor is connected to the anode circuit of the lamp, which means that it is under high voltage. So, such capacitors must have an operating voltage of at least 400v. One of the best capacitors working as a transition capacitor are those from JENSEN. It is these capacities that are used in top-end HI-END class amplifiers. But their price is very high, reaching up to 7,500 rubles for one capacitor. If you use domestic components, then the most suitable ones would be, for example: K73-16 or K40U-9, but in terms of quality they are significantly inferior to branded ones.
The presented tube amplifier circuit consists of three separate modules:
The preamplifier is manufactured using a simple circuit with the ability to adjust the signal gain. It also has a pair of separate tone controls for low and high frequencies. To increase the efficiency of the device, you can add an equalizer for several bands to the design of the preamplifier.
The pre-amplifier circuit presented here is made on one half of a 6N3P double triode. Structurally, the preamplifier can be manufactured on a common frame with an output stage. In the case of a stereo version, two identical channels are naturally formed, therefore, the triode will be fully involved. Practice shows that when starting to create any design, it is best to first use a circuit board. And after setting it up, assemble it in the main building. Provided that it is assembled correctly, the preamplifier begins to operate synchronously with the supply voltage without any problems. However, at the setup stage you need to set the anode voltage of the radio tube.
The capacitor in the output circuit C7 can be used K73-16 with a rated voltage of 400v, but preferably from JENSEN, which will provide better sound quality. Tube audio power amplifier not particularly critical of electrolytic capacitors, so any type can be used, but with a voltage margin. At the setup stage, we connect a low-frequency generator to the input circuit of the pre-amplifier and apply a signal. An oscilloscope must be connected to the output.
Initially, we set the input signal range to within 10 mv. Then we determine the output voltage value and calculate the amplification factor. Using an audio signal in the range of 20 Hz - 20000 Hz at the input, you can calculate the throughput of the amplification path and display its frequency response. By selecting the capacitance value of the capacitors, it is possible to determine the acceptable proportion of high and low frequencies.
Tube audio power amplifier implemented on two octal radio tubes. A double triode with separate cathodes 6N9S connected in a parallel circuit is installed in the input circuit, and the final stage is made on a fairly powerful output beam tetrode 6P13S connected as a triode. Actually, it is the triode installed in the final path that creates exceptional sound quality.
To perform a simple adjustment of the amplifier, an ordinary multimeter will be enough, but to make precise and correct adjustments you need to have an oscilloscope and an audio frequency generator. You need to start by setting the voltage at the cathodes of the 6N9S double triode, which should be within 1.3v - 1.5v. This voltage is set by selecting a constant resistor R3. The current at the output of the 6P13S beam tetrode should be in the range from 60 to 65 mA. If a powerful constant resistor 500 Ohm - 4 W (R8) is not available, then it can be assembled from a pair of two-watt MLTs with a nominal value of 1 kOhm and connected in parallel. All other resistors indicated in the diagram can be installed of any type, but preference is still given to C2-14.
Just like in the preamplifier, the important component is the decoupling capacitor C3. As mentioned above, the ideal option would be to install this element from JENSEN. Again, if you don’t have them at hand, you can also use Soviet film capacitors K73-16 or K40U-9, although they are worse than overseas ones. For correct operation of the circuit, these components are selected with the lowest leakage current. If it is impossible to carry out such a selection, it is still advisable to buy elements from foreign manufacturers.
The power supply is assembled using a 5Ts3S direct-heated kenotron, which provides AC rectification that fully complies with the design standards for HI-END class tube power amplifiers. If it is not possible to purchase such a kenotron, then you can install two rectifier diodes instead.
The power supply installed in the amplifier does not require any adjustment - everything is turned on. The topology of the circuit makes it possible to use any chokes with an inductance of at least 5 H. As an option: using such devices from outdated TVs. The power transformer can also be borrowed from old Soviet-made lamp equipment. If you have the skills, you can make it yourself. The transformer must consist of two windings with a voltage of 6.3v each, providing power to the amplifier radio tubes. Another winding should have an operating voltage of 5v, which is supplied to the kenotron filament circuit and the secondary one, which has a midpoint. This winding guarantees two voltages of 300v and a current of 200 mA.
The procedure for assembling a tube audio amplifier is as follows: first, the power supply and the power amplifier itself are made. After the settings have been made and the necessary parameters have been installed, the preamplifier is connected. All parametric measurements with measuring instruments should be done not on a “live” acoustic system, but on its equivalent. This is in order to avoid the possibility of expensive acoustics being decommissioned. The load equivalent can be made of powerful resistors or thick nichrome wire.
Next you need to work on the housing for the tube audio amplifier. You can develop the design yourself, or borrow it from someone. The most affordable material for making the body is multilayer plywood. The output and preliminary stage lamps and transformers are installed on the upper part of the housing. On the front panel there are tone and sound control devices and a power supply indicator. You may end up with devices like the models shown here.
The amplifier is made on the basis of industrial units UPV-1.25 (power 1250 W). It provided sound broadcasting in small towns or in areas of large cities. The proposed amplifier, intended for sounding a discotheque hall, achieves a soft amplitude limitation characteristic and small harmonic distortions.
Modern audio amplifiers with an output power of 1000...2000 W are built on transistors. A tube amplifier of such power has a total weight of 150...200 kg and its dimensions are much larger, which makes it inconvenient for transportation. But if it is used permanently in one room, this drawback is less noticeable.
A tube amplifier made for a club disco, with its relative simplicity, provides high-quality sound through a speaker system distributed throughout the hall. The sound path is entirely made using tubes, and the power supply is made according to a classic transformer circuit. Only two powerful GU-81 M lamps with a direct filament cathode were used as output lamps.
The amplifier is made on the basis of amplifier components developed in the 70s for wired broadcasting - UPV-1.25 (power 1250 W). It was installed in regional communication centers and provided sound broadcasting in small regional towns or in areas of large cities. The design features of this amplifier made it very reliable and durable in operation: it was turned on in the morning at 6 p.m. and turned off at 24 p.m. when the broadcast ended. Thus, he worked for years 18 hours a day.
I had to make changes to the amplifier design to improve its parameters and match the output voltage to the load, and make it more convenient to service and move. First, I rewound the secondary winding of the output transformer, since the factory output voltage was 240 V. Then I changed the design, assembling the amplifier in two blocks (photo in Fig. 1) connected by a cable to the connector (amplifier unit and high-voltage power supply). The power supply circuit has been changed. Measures have been taken to expand the bandwidth, and the transistors used in the pre-amplifier driver have been eliminated. The preamplifier is also built on tubes with a two-input mixer and a microphone amplifier. The result is an amplifier with good performance for a high output power UMZCH.
Amplifier specifications:
Description of the amplifier and power supply
Pre-amplifier (Fig. 2) consists of a microphone amplifier on a VL1 tube, two identical stages on VL2, VL3 tubes, tone and gain controls and a mixer on a VL4 tube. The amplifier does not have any special features, but the pre-amplifier lamps are heated with direct current.
Pre-terminal amplifier UMZCH (Fig. 3) contains three lamps - VL5 - VL7. Using VL5 triodes, an amplifier with a load in the form of a T1 transformer is assembled, creating paraphase signals. Separating capacitor C27 eliminates magnetization of the transformer magnetic circuit. Next follow two amplification stages, assembled according to a push-pull circuit using VL6, VL7 (6N8S, 6N6P) lamps.
To obtain the required anode current pulse at low voltage on the screen grids, a voltage of about 700 V is applied to the pentode grids of lamps VL8, VL9. The negative feedback voltage (NFV), which is introduced to the input of the push-pull stage of the pre-final amplifier, is removed from the divider, which consists of resistors R71, R69 and R72, R70. Capacitors C28-C31, C34-C37, C40-C45 provide the necessary correction of the frequency response of the stages covered by the OOS. To increase the stability of the amplifier outside the passband, the primary winding of the output transformer is shunted by circuits C41R67 and C42R68; For the same purpose, resistors R60 and R64 are connected in series with the control grid circuits VL8 and VL9. From the high-voltage power supply, through the primary winding of the output transformer, a voltage of 3500 V is supplied to the anodes of powerful lamps VL8, VL9, and 700 V is supplied to the screen grids. The +700 V and +70 V power circuits are supplemented with blocking capacitors 0.25 μF at 1000 V and 1 µF at 160 V, respectively.
The pre-terminal amplifier, together with the final stage of the power amplifier, is covered by OOS, the depth of which reaches 26 dB. Deep OOS provides sufficiently high quality indicators of the amplifier, low sensitivity to changes and variations in the parameters of individual elements. There is virtually no response to load shedding (insensitive to load shedding). This is due to the very low output impedance of the amplifier.
To ensure the stability of the amplifier over the entire operating frequency range, frequency-phase response correction circuits are introduced into the OOS loop. In the HF region, correction is carried out by capacitors S28-C31, in the LF region - by circuits S35YA51 and S36B52. For deeper suppression of common-mode interference (and even harmonics), chokes L1 and L2 are included in the cathode circuits, and the necessary bias on the lamp grids is created by resistors R47, R48 and R55. The signal from the output stage of the pre-final amplifier through capacitors C38 and C39 is supplied to the control grids VL8, VL9.
“Low-voltage” power supply (its diagram with the continued numbering of elements is shown in Fig. 4) built with a network transformer from which the filaments of all lamps are powered, and the filament windings of the output lamps are wound in two sections separately. To heat the pre-amplifier tubes, the alternating current is rectified by diodes VD1, VD2 with capacitor C46.
The preamplifier tubes are supplied with stabilized voltage. To power the anode circuits, a stabilizer is assembled on VL10 - 6H13C. Relays K1-KZ serve to delay the supply of anode voltage to unheated lamps; this increases the life of the lamps. The relay is turned on using a time relay or manually with a toggle switch. Two dial indicators are connected in parallel to resistors R65, R66 to control the anode current of the GU-81.
The background and noise can also be caused by the anode supply circuits, so voltage stabilizers are used on the VL10 lamp and a group of zener diodes. It is advisable to additionally bypass the anode supply circuits of the amplifier stages with paper capacitors (the larger the capacitance, the better).
Dear radio amateurs! We present to your attention a tube 2-cycle power amplifier. The diagram of radio engineer E. Vasilchenko is taken as a basis. Features: Output transformers are wound on the TS-180 base (a separate circuit is attached). Three transformers were used as power supplies; a delay circuit for turning on the anode voltage was used (smooth turn-on: filament power, warming up, then supplying the anode power). Industrial chokes from a TV were installed in the anode power circuit; FT-3 was taken as C2-C3 after some experiments (as the most realistic, K78-2, in particular, embellished the sound). The markings of the components used are indicated in the diagrams. During production, surface mounting with contact blocks and shielded Luxman-audio wire were used. Facing material: tinted mirror, MDF. Input-output sockets are made of yellow non-oxidizing metal, the frame is made of metal from under the MPK “Olymp-005” remote control. There is no background or hum. The resistors were selected with the maximum required accuracy using a multimeter. There is no excitation, the sine wave is clean. The parameters are indicated, read the description carefully with the necessary changes and additions made during the setup. In my opinion, the circuit is not too complicated to repeat. Good luck!
Preliminary remarks about the purpose of the development.
The motto of this work was the rejection of uncompromisingness in favor of balanced, expedient decisions. The amplifier was radically redesigned many times, but in the end, although it cannot be called new, it was possible to make a small home ULF with good sound quality with the maximum use of “off-hand materials” and available parts.
The lamps were chosen for several reasons. They cannot but be attracted by the initially high linearity, ease of modification of the circuit, selection of components, simplicity of calculations, as well as the clarity and conciseness of the circuits. The next point is that there is no “tube sound”. The so-called “tube sound” is a persistent myth into which everyone puts their own understanding. For some, this is a limited-range sound with a clear predominance of mid-frequencies - evidence that the transformer core is too small. For others, tube sound is associated with “transparency,” high resolution, and detail. For others, it is a “soft, comfortable” sound. Let us take the liberty of asserting that none of the above characteristics is an indispensable attribute of tube equipment, just as “impartial, monitor” sound is of transistor equipment. The special characteristic features of the sound of a particular amplifier, no matter whether transistor or tube, are determined mainly by the structure of the circuit and the components used. In this sense, it can be considered that“tube sound” is the absence of tedious “transistor”, “plastic” sound,which is well known to owners of music centers and domestic amplifiers.
After testing and listening to a number of amplifier designs and measuring objective parameters, it was found that most related topologies give comparable results:
The frequency response of the amplifier is mainly determined by the output transformer and a 5 Hz -25...30 kHz band at a level of 1-2 dB can be realized without any problems. The coefficient of nonlinear distortion (THD) of amplifiers with an open circuit OOS ranges from one to ten percent of the maximum levels and tenths at small levels. However, the sound character of such amplifiers is noticeably different, despite their identical parameters.
In this regard, it was decided not to take into account the SOI value. This is nothing more than an indicator of the presence or absence of gross design and implementation errors. A typical indicator of a working tube amplifier is a few tenths of a percent with a power of several watts.
A certain opinion has been formed about OOS with adjustable depth. : Its presence and depth are a matter of taste and habit.Deep OOS was rejected immediately- it is very difficult to replicate the sound of old QUAD and Leak on modern components. Some topologies accepted the introduction of shallow OOS well, in particular, the circuit of a pentode single-ended amplifier on the EL-34 with SRPP boost on the 6N9C. When voltage was applied from the secondary winding of the output transformer to the cathode of the “lower” SRPP lamp through a resistor of several kilo-ohms, the gain decreased slightly (by 2-4 dB), and the slightly pronounced “telephone” timbre disappeared. This timbre is due to poor damping of speaker systems, high output impedance of a single-ended pentode amplifier, and, more often, insufficient quality of the output transformer.
The depth of the environmental feedback should be experimentally selected to the minimum of one’s own unpleasant sensations, since when some parameters are improved, for example, the subjective perception of the linearity of the LFC. others deteriorate, such as the naturalness of the timbres of voices and instruments and spatial characteristics. In this case, the amplifier must have some margin of gain and stability. As a rule, there are no problems with amplification. Tube circuits have a very large dynamic range and allow you to work in any part of it. This property is widely used by tube circuit enthusiasts. The fact is that the magnitude and degree of nonlinearity of the amplitude characteristic of the lamp depends on the direct and alternating current mode, and this is clearly audible. In addition, the lamps themselves have different properties.Lamps with low slope, such as 6N1P, 6N8S, give less distortion and have greater flexibility in choosing the operating point.Tubes with high slope or gain have no competitors in guitar and other amplifiers with a specific sound character. In addition, the initially high degree of identity of the lamp parameters allows the use of compensation (or multiplication, if necessary) of nonlinearities.
With some experience, a wide field opens up for selecting the sound character to your own taste. In this aspect, the designer of transistor amplifiers is very limited in the means of influencing the sound of the device. The transistor cascade has incomparably greater nonlinearity, and the choice of the cascade operating point is related to the mode of the entire amplifier. It is not for nothing that experts award the title “legendary” mainly to tube amplifiers and, in isolated cases, to truly outstanding transistor amplifiers. To be fair, it should be noted that in transistor circuitry there are also methods for changing the character of sound that are beyond the scope of this article. To the completely reasonable remark of the reader thatthe amplifier must be completely neutral and contribute nothing to the sound,The author has prepared a routine explanation that the sound of an amplifier still means the sound of the entire path, including the audio
material, loudspeakers and listening room, abstracted as far as possible from the inherent characteristics of these components. The listener usually has no difficulty in distinguishing, say, whether certain formants are emphasized by the amplifier, the speakers, or the resonance of the room. Any amplifier, even the most “monitor” one, introduces changes into the amplified signal. To check this fact, we can recommend a comparison with a “straight wire”. It's not just tubes or transistors that make these changes. Components that are considered linear - resistors and capacitors - also change the character of the sound.
The PA cannot be designed in isolation from the acoustic systems and the signal source. There are no universal amplifiers, just as there are no ready-made recipes for creating amplifiers “for rock” or “for vocals”. There are only some obvious patterns, abundantly described in the literature. We note only those that relate to the subject of our development. An amateur designer who creates equipment for himself has a noticeable head start over his professional colleague. As a rule, he needs the amplifier to “sound” the selected specific, not very extensive, musical material, in a specific room and with a specific acoustic system. In our case, the musical material was easy enough for amplifiers - rock and roll of the 60s , jazz, sometimes simple classics. The peculiarity of this music library is the wide representation of natural musical instruments, the absence of hard (in terms of spectrum), aggressive genres. Quite a large part of the music library consists of recordings made in a laconic style, with small compositions, even duets. Such music is often chosen as background music and, as a rule, is not listened to loudly. It is quite possible that such a repertoire largely influenced the choice of tube circuit. The preliminary choice was between the following options:
Fully transistor amplifier with current dumping and deep 00C (current dumping amplifier, similar to QUAD- 405 );
Transistor without common 00C;
Hybrid without common 00C (input voltage amplifier on a lamp, output emitter follower on bipolar transistors);
Push-pull tube with transformer output.
Based on a set of preferences, the latter was chosen. It was inferior to transistor and hybrid ones in volume and when transmitting powerful bass lines. Some versions of the hybrid amplifier were more transparent in the upper range (a clear sign of low intermodulation distortion). But in terms of reliability of transmission on the small volumes of the mid-frequency range, so important for jazz and classical music, the tube turned out to be the leader. It is quite possible that the reason is not only in the different spectrum of distortions, but also in the value of the output resistance.Amplifiers without general feedback have a relatively high output impedance(triode tube, about 1-3 Ohms). This undoubtedly affects the frequency response of the PA-AS combination, especially in the region of the resonant frequencies of the speakers and crossover frequencies. On the other hand, the nonlinearity of acoustic transformation decreases when operating from a source with high output impedance. Tube amplifiers have traditionally been used with single-way speaker systems. In this combination, the “disadvantages” of the amplifier: limited power in the lower range, high output impedance - did not deteriorate the sound. In other words, not all modern speakers will work well with tubes. Moreover, it would be logical to begin designing a sound reproduction complex with the selection of adequate acoustic systems.
In our case, the speakers turned out to be quite omnivorous, which was confirmed by testing their use with different amplifiers. In one case, these were three-way floorstanding speakers in a “closed box” design with a traditional design. MF and HF were reproduced by silk dome speakers, and LF by a large, 35-cm “wheel” with a paper diffuser. In the second - two-way speakers produced by the Ferropribor plant (St. Petersburg) type S-153 (15 0АС-0 0 3ФГ1) with a Heil emitter and an imported midrange-woofer with a diameter of 25 cm. It should be noted that in both cases the speakers were “inconvenient” load due to the rather large unevenness of the impedance module in frequency regions that are important for the perception of many instruments, and/or low sensitivity.
In connection with the above, it was decided to make the output stage using triodes.This circuit has a comfortable sound over the entire frequency range and fairly good damping.The low sensitivity of the speakers (87 and 8-9 dB) forces the use of a push-pull circuit.To retain all the advantages of triodes, the output stage must operate in class A, that is, without anode current cutoff.
Lamp type 6P1P | ULF power W 4 |
6P6S | |
6P14P | |
6PZS/G8 07 | |
EL34 | |
GU-50 | |
6P36S | |
6P45S | |
6S1EP | |
6N5S | |
6HI3C |
In table 1 shows what powers can be obtained from common domestic lamps, triodes and pentodes in triode mode.
Directly heated triodes have the best properties in terms of sound amplification.The distortion spectrum of this class of amplification elements contains a minimum number of harmonics, usually the second and third. Tetrodes and pentodes in a triode connection are inferior to true triodes in this indicator. They have a wider and more powerful range of distortions, regardless of the connection method (meaning the fashion for ultra-linear circuits). The output impedance of a triode transformer stage without 000 is usually about 0.3Rh. Cathode followers and Circlotrons have this parameter an order of magnitude lower, but they have their drawbacks, in particular, the difficulty of obtaining a high drive voltage on the output tube grids. Obtain a signal amplitude of 300-4 00 V with a small number of harmonics and a distortion level of less than 0 .5% is a very difficult task, and practice shows that in PAs built according to the UA-UT circuit (voltage amplifier - current amplifier), the nature of the sound is determined mainly by the UA. Thus, when choosing a method for implementing the plan, the developer is guided by a whole complex of objective indicators and subjective preferences and sometimes unconsciously.
After weighing all the pros and cons, it was decided to use the most available ones at the moment 6PZS-E lamps, representing analogue widely knownsound tetrodes 6L6 and 5881. This lamp has specific current-voltage characteristics (Fig. 1.1), allowing it to be used in mode with grid currents, both in triode and tetrode connection.
Figure 1.1. Graph of the current-voltage characteristic of a 6PZS-E lamp in triode connection
As can be seen from the graphs, at a grid voltage of +10 V, the anode characteristic does not yet have a pentode “elbow”. The lines corresponding to the grid voltages +10 and -10 V are located at the same distance from the zero voltage line. This means that in this section of the load straight line the slope does not change, in contrast to the section with low anode currents. The internal resistance of 6PZS-E at low anode currents increases greatly, and the dependence of the anode current on the grid voltage, that is, the slope, decreases. This feature is well known to tube designers and is widely used in push-pull amplifiers. Thanks to it, the boundary between modes A and AB is practically absent, since due to the drop in transconductance, the current through the lamp practically does not stop even at high blocking voltages, and switching distortions are of a low order. Something similar is implemented in transistor amplifiers designated “class AA” with the help of some circuit tricks.
Another one feature of this lamp,also known to experienced amateurs, isits high overload capacity for anode voltage.After training, it works great with an anode voltage of 600-700 V and a voltage on the second grid of 450 V and even up to 500 V. In terms of its power capabilities, it is only slightly inferior to the EL-34. In triode mode, the lamp operates for months without any visible problems at an anode voltage of 400-450 V. This abnormal mode allows the use of a relatively high-resistance anode load, which has a beneficial effect on the level of distortion. By high-resistance here we mean a load significantly exceeding Ra = 2Ri, at which maximum amplification efficiency is achieved. It is enough to accept a load equal to (5-10)Ri. Of course, under no circumstances should the maximum permissible cathode current conditions be exceeded, and it is undesirable to exceed the power dissipation at the anode. All these features make 6PZS-E a very attractive lamp for experimentation, but in terms of sound it often loses to its “classmates” and even more so to the 6C4C. Experiments with 6PZS-E were stopped at the stage when further modifications became impossible in the old housing, and the potential capabilities of the lamps were almost completely used. By this time, the circuit was a three-stage push-pull amplifier operating in class A2, with a maximum output power of about 20 W. It should also be noted that the calculated current-voltage characteristics used in the programs may differ from the real ones, especially in the region of positive grid voltages.
Amateur calculation of the output stage:
Select the type of radio tube, find the current-voltage characteristics graphs.
Select a switching circuit: in our case, a circuit with a common cathode, with a fixed bias (Fig. 1.2).
Rice. 1.2 Single-ended output stage circuit.
Evaluate the level of distortion and output power with various anode load and operating point positions.
Go to the push-pull circuit: double the resulting anode load, power consumption and output. The output impedance will be halved.
Based on the data obtained, proceed to the calculation of the output transformer, power supply, and pre-amplifier stages.
List of symbols:
Uc is the voltage on the lamp control grid;
Ra is the resistance of the anode load;
Ri is the internal resistance of the lamp;
Ua, la - anode voltage and current;
Rh - load resistance;
Un - actuation voltage.
Calculation of DC mode graphically
The family of current-voltage characteristics of 6PZS-E in a triode connection is shown in the graph in Fig. 1.3.
Select the anodic load resistance Ra. Reference data for 5881 and 6V6 tubes states about 1.7 kOhm. The measured values for 6PCS-E are about 0.9-1.2 kOhm, we will stick to these values.
Rice. 1.3. Safe operation area 6P3S-E Select Ra = 2.5 kOhm.
We construct a hyperbola of the maximum permissible power dissipation at the anode: Ra max = Ua 1a. Instantaneous modes during lamp operation should not be above this curve.For 6PZS-E, the permissible power dissipation at the anode is 21 W.For similar sized and configured electrodes 5881 and 6V6, 25 or 30 W are usually given, depending on the lamp version. This difference is due to the fact that forensuring increased durability of the domestic lamp (as indicated by the “E” index),The manufacturer limits the maximum permissible electrical and temperature conditions. This reduces gas emission from the electrodes. Hobbyists often operate lamps in their designs under very harsh conditions, when the only reliable indicator of the intensity of the mode is the red-hot anodes. Analysis of amateur designs shows that6PZS-E can work for years with power dissipated at the anode up to 25-30 W, unlike 6PZS, which has a different design. The longevity of the lamp is greatly influenced by the leakage resistance of the grid. According to the specifications, this resistance should not exceed 100 kOhm with a fixed bias, and 150 kOhm with automatic bias. In this case, the deterioration of vacuum as a result of gas separation does not lead to a noticeable change in the operating mode. Failure to comply with this point of the technical specifications leads to consequences that are well known to the owners of “Priboev” and other 6PCS devices suffering from the “red anode disease”. In our calculations, we will limit the permissible power to 23-25 W. At the same time, we take into account the specifics of the application: in our circuit, the leakage resistors are very low-resistance. In addition, usually in high-quality audio equipment the lamps are replaced with new ones long before noticeable leakage and reduction in slope occur. A lamp operating in class A dissipates maximum power when there is no signal. Currents and voltages on it should also not exceed permissible values. To remind you of this, we will construct two corresponding segments, limiting the possible modes to the safe operation area (ROA) of the lamp.
When the signal is amplified, the operating mode of the lamp, that is, the anode current and voltage, draws a straight line. When operating on a reactive load, the straight line turns into an ellipse, and the instantaneous power may exceed the permissible one. However, the average dissipated power will still remain less than the rest power.
We select the operating point of the cascade - current and quiescent voltage. Let's set the left boundary of the operating modes so that the voltage on the grid does not exceed 10 V (U 10 V). The right boundary is usually set by the maximum permissible anode voltage, and in the case of triode connection of pentodes and tetrodes, by the voltage on the second grid. Since in our case this voltage does not exceed the tested 550 V, this is not very relevant. Much more important is the drop in steepness and the increase in internal resistance. Therefore, we will limit the range of operating modes on the right not by the maximum permissible voltage, but by the minimum permissible current, to be specific, 15-20 mA. In this case, Ucmin = -70 V. The rest point is almost in the middle of this segment.
Thus, the grid voltage in rest mode turned out to be -30 V, and the required amplitude of the excitation voltage was 80 V from peak to peak or 28 V effective value. We find the intersection of the -30 V line with the load straight line and the corresponding modes: 350 V and 70 mA. From here you can obtain the required voltage of the anode power source: it should be greater by the amount of the voltage drop on the primary winding of the output transformer. This drop can be estimated even before it is calculated. The most typical output transformer efficiency values are 0.85-0.87. This means that the value of the active resistance of the winding is 0.13-0.15 Ra, that is, in our case it is approximately 350-400 Ohms. As a result, the supply voltage should be about 380 V at full load.
After selecting the operating point, distortion and energy parameters are usually calculated. We are interested in the effect of the choice of operating point on distortion. Let's turn to Fig. 1.4, obtained using the SE Amp Cad report generator.
Rice. 1.4 Selecting the operating point.
It is clearly seen from the figure that a symmetrical change in the grid voltage relative to the resting point corresponds to an asymmetrical change in the anode current and voltage.
The ratio of the lengths of segments OA and OB is a measure of distortion. Using the three-ordinate method, you can calculate the magnitude of the second and third harmonics. Let's give the numbers - 111 and 2% for the second and third harmonics, respectively. These are typical values for any single-ended stage operating at maximum power.
Such a high level of distortion should not be alarming. The fact is that in a push-pull amplifier class A the lamps are connected in counter-parallel alternating current, and ideally there is no second harmonic at all, and the level of the third decreases quite quickly as the power decreases. At half power it is already an acceptable 0.1%. In addition, the mathematical model in the positive bias region rarely matches the actual behavior of the lamp. In fact, segment OA is slightly shorter than the program draws it. Let us note the useful fact that as the load increases, the level of distortion decreases: when Ra = 4 kOhm segments OA! and OB" are almost equal. The output power of the cascade, as it is customary to represent it, is equal to the area of the shaded triangles. It can be calculated both analytically and directly from the graphs. We will take the finished value from the report compiled by the program - 11 W. This is almost three times the power , which can be obtained from a cascade in class A1 (without grid currents) at the same level of distortion. Let's focus on the following mode:
Iа = 50 mA - quiescent current;
Ua=365 V - voltage on the anodes at the resting point;
Uc=-33 V - grid bias voltage;
Upp=75 V (peak to peak) - excitation voltage corresponding to maximum power;
Pa=22 W - power dissipated at the anode at the resting point;
Pa=16 W - average power dissipated at the anode at maximum signal;
Pout =11 W - maximum output power;
Rout=3.5 Ohm - output resistance;
Distortion 2nd = 11% - second harmonic level;
Distortion 3rd = 2% - third harmonic level.
The transition to a push-pull circuit gives us data for further calculations:
Ra=5 kOhm;
Rmax=22 W;
Iav=100 mA;
Uc =26 V (rms).
The input impedance of a stage operating with grid currents is nonlinear, so the driver must be built according to the circuit of a power amplifier, and not a voltage amplifier. Powerful industrial PAs usually use a transformer connection between the driver and output stages. In our case, the excitation voltage is only 26 W, so it is quite possible to get by with a cathode follower (CF) with direct coupling (Fig. 1.5).
The output impedance of the cathode follower is approximately Rou t * Ri /y for a double triode 6N8S (analogue 6SN7) this will be 370 Ohms, which is quite enough to provide a grid current of about 1 mA. Using the TubeCAD program, we get the cascade modes:
Fig" 1.6. Selecting the operating point of the cascade on 6N8S
Umax out = 40/+39.8 B - maximum possible output signal level;
Uc = -3.56 V - bias voltage;
Ia = 11 mA - quiescent current;
Upit =280 V - supply voltage;
Kus = 0.9 - voltage gain;
Pa = 1.87 W - power dissipation at the anode.
These values can be obtained from the current-voltage characteristic (Fig. 1.6), considering the supply voltage of the cascade E0 to be the sum of the positive Uri and negative Uc supply poles.
The voltage transfer coefficient of the cathode follower is 0.8-0.9 depending on the load size. Therefore, the sensitivity of the amplifier at the CP input is 28/0.8 = 35 V (rms). This gain distribution allows us to limit ourselves to only three stages, including those already described. In many cases, the output of this stage has sufficient amplitude to feed directly into the output tube grids. The need for manual selection of the divider should not be considered a disadvantage of this circuit, since most so-called auto-balance circuits are either asymmetrical in modes or also require adjustment. The calculation of this phase inverter differs little from the calculation of a conventional rheostat cascade.
Rice. 1.8. Simulator screen with calculation results
Despite its simplicity, the presented simulator provides satisfactory accuracy.
In Fig. Figure 1.8 shows a screen with calculation results and modes for direct and alternating current. Capacitors Sb, C7 model the input capacitance of the next stage, C1 - the previous one, as well as the mounting capacitance. Without these elements, the calculation of the frequency response will be incorrect. C2 is necessary to equalize the frequency response of the shoulders. A small local feedback loop through R3, which is not shunted by a capacitor, makes it easier to adjust the bass reflex. The cascade gain is 42.5 and exceeds the required one with a small margin. At a frequency of 20 kHz it drops by 1.5 dB relative to 1 kHz - this is the price for using 6N9S, which has fairly large interelectrode capacitances. The calculated THD is 0.4% with an input signal of 0 dB = 0.775 V; 0.17% - at -20 dB and 1% - at +6 dB. These values are of interest only in comparison with other methods of implementing the circuit, since the triode model Ic + Ia = K (Ua + y Uc)3/2 in all simulators does not take into account the design features of the lamp.
The diagram of one amplifier channel is shown in Fig. 1.9, power supply - separate circuit
Fig, 1.9. Schematic diagram of one of theamplifier channels
A common power transformer was used for both channels. The anode voltage of +37O V is rectified using a full-wave circuit using high-quality capacitors and industrial chokes from the television industry. The -125V negative voltage is taken from a separate transformer via a well-filtered full-wave rectifier. The lamps of the output and preliminary stages are heated from different windings of a separate powerful transformer TN-54. To minimize the background, the filaments of the input lamps are powered according to a circuit using 100 Ohm resistors, the connection point of which is “tied” to ground. A delay (time relay) was used to turn on the anode voltage, after applying the filament voltage, with an interval of ~37 seconds - to preserve the life of the lamps. The output transformers are wound on the basis of industrial TS-180 (circuit windings are included). The amplifier useshigh-quality polystyrene (K71-7), polypropylene (K78-2) and fluoroplastic (FT-3) capacitors, including those from RIFA, KBG-MN, MBGO-1.Resistors are selected with extreme precision (units of Ohm). The anode supply voltage is+363 V. V Polypropylene capacitors K78-2-0.1 µF at 315 V were initially tried as pass-through audio capacitors, but they strongly color the sound in the high frequency region,with fluoroplastic FT-3 - the sound is realistic. The output stage of each channel consumes +370 V, 100 mA from the source; 20 mA is required for the drivers and 2 mA for the bass reflex. In total this is 122 mA, and taking into account the traditional reserve - 140 mA. Each pair of output lamps is 1.8 A, 6N8S/9S consume 300 mA. Approximate total electrical power for two channels Ri = 220 W.
Amplifier settings.
This procedure begins by setting the quiescent current of the output lamps of one of the channels. It is better not to insert lamps of an unused channel. Before turning on, it is necessary to set the sliders of trimming resistors R9, R10 to the position of maximum resistance. The 6N9S lamp is not needed yet. A milliammeter with a measurement limit of at least 500 mA is connected to the break in the anode power wire, and a voltmeter with a measurement limit of 500 V is connected to the connection point between R11 and R12.
Immediately after turning on the amplifier to the network through a step-start resistor, you need to make sure that there is a negative bias of at least 100 V. After this, the voltmeter can be connected to the anode power source and make sure that the voltage on the filter capacitors gradually increases, and the current in the anode power circuit does not exceed several milliamp.
After a few seconds, full mains voltage can be applied. The anode voltage should be increased. Connect a voltmeter to the grid of one of the output lamps. Gradually reducing the resistances R9 and R10, set the voltage on the grids-33 V This operation requires a lot of patience, since after each change in the position of the motors, the consumption from the power source changes and therefore the supply voltage also changes. Therefore, you need to turn the variable resistor sliders simultaneously in both arms and at a small angle.The consumption of the entire amplifier channel should be about 120 mA. At an anode voltage of more than 300 V, a characteristic blue glow appears in bPZS-E cylinders.This is their “calling card”, a completely normal, safe situation. By the intensity of this glow one can judge the degree of load on the lamp. If the lamps in the arms glow differently, then most likely they have different parameters and modes. If the glow begins to pulsate in time with the music, this means a transition to AB mode or overload.
Driver quiescent currentmust make upat least 10 mA per arm.
If at this current it is not possible to set the bias voltage-33-34 V on the output lamp grids, you will need to select resistor R14. The voltage on capacitor C5 should be about 125 V, at the anode of the drivers is about 150 V. The quiescent current of the output lamps can be set to 50-60 mA.After setting the required voltages and currents, you need to turn off the amplifier and turn it on again after a while. After a 20-minute warm-up, you can adjust the modes. The final setting of the modes can be done only after the second channel has been adjusted, since the supply voltages may decrease slightly after connecting the second channel. If the lamps have been pre-trained, the next check of the modes can be done after a week if desired.
A few words about balancing the bass reflex. It should be carried out both on a sinusoidal signal and on a rectangular one. It is advisable to select a lamp with the same slope of the triodes in the cylinder. R6 consists of two resistors connected in parallel with values equal to R2 and R4. Thus, the AC load of the arms and the gain are equalized. By changing R3 you need to achieve the same signal range on the driver grids. The voltage on R5 will have the form of a sinusoid with double the frequency. By observing the fronts of the rectangular signal, you can align the behavior of the shoulders at HF. To do this, you need to select a capacitor with a capacity of several tens of picofarads parallel to R4.The capacitor must be high quality and not ceramic.In general, the issue of using certain passive components is quite controversial. What is certain is that they greatly influence the character of the sound. The type of components used is indicated in the diagram.Measurements.
After assembly and preliminary configuration, you can check the amplifier parameters. Due to the above, objective parameters were of interest to us only as an indicator of the correctness of the implementation of the underlying idea. A CD test disc and a 3H SURA generator were used as a signal source. The signals were observed on the screen of an oscilloscope S1-68, S1-94. Voltages and currents were measured with digital multimeters VICTOR VC-9807, 9808, 97.
In transistor amplifiers, the maximum power is determined by the clipping limit when the signal reaches the power supply level. In this case, signal distortion increases sharply. In conventional tube amplifiers, distortion increases monotonically until grid currents appear in the output tubes. At this moment, distortions increase from a few to tens of percent. The signal limitation is “soft”, without kinks. A characteristic feature of a class A2 amplifier is the absence of pronounced clipping, since the main factors limiting the output power are the driver current and, ultimately, the power of the power supply.
Therefore, it is impossible to track the achievement of the maximum power level on the oscilloscope screen. In this case, you have to use the POST method, which defines the maximum power as the power at which the distortion level reaches 10%.When measuring at an equivalent load, the following were obtained:
Output power - 20 W; max - 24 W
Frequency range with roll off at the edges -3 dB, 5Hz-19kHz.
The most interesting data was observed when working under a real load. The amplifier was connected to the speakers, and a music signal from the CD player was supplied to the input. The volume control set the level at which phonograms are usually listened to, the so-called comfort level. After this, the sound card input was connected to the amplifier output (via a 1:10 resistive divider), and the CD was replaced with a CD-R with test signals.
Frequency response of the system
In Fig. Figure 1.16 shows a fragment of the frequency response, the scale division value is 10 dB. This unexpected behavior of the system compared to a resistive load becomes understandable if we recall the input impedance module of a three-way speaker.
Rice. 1 16 Fragment of frequency response in the process of measuring amplifier parameters
By ear there is no increase in the frequency response in the region of 3-4 kHz. To check, the frequency response of a transistor amplifier with a similar tonal balance was measured. Due to lower output resistanceunevennessin this area amounted 0.5 dB, mainly around 1.5 kHz. The character of the timbre of the upper middle of the sound range was transmitted identically to the tube one. The coefficient of nonlinear distortion was measured at frequencies of 1 and 3 kHz (Fig. 1.18 and 1.19).
As you can see, the distortion of the amplifier at low power is represented exclusively by the second harmonic; this is a clear sign of an unbalanced bass reflex. The third harmonic is masked in the first case by device interference, in the second by noise.The measured SOI is 0.09% at 1 kHz and 0.08% at 3 kHz. These are values worthy of very high-class equipment.
Things are somewhat worse with intermodulation distortion (Fig. 1.20). When applying frequencies to the input 10 and 11 kHz equal amplitude difference tone 1 kHz has a level of -50 dB or 0,3%. The most likely reason is the increased asymmetry of the phase inverter arms at HF, since the amplifier under study did not have a capacitor in the VL1.1 anode.
Hearing examination.
Listening confirmed the high quality potential of the amplifier. Despite the very modest configuration, it fully justified all the efforts spent on it. Of the sound features, we notesoft, non-aggressive top, while maintaining a fairly high level of detail. Bass transmission is juicy, but not booming,as one would expect from a high output impedance amplifier; Most likely, the amplifier will be sensitive to changing speakers.The middle of the audio range is best transmitted.The sound character changes noticeably when replacing tubes and passive components. The MELZ ones turned out to be the best 6Н8С and 6Н9С 1952-1953 with metal bases. The signal source was a Harman Cordon-39 DVD player with an audiophile sound processor, and Yamaha-NS-8900 acoustics. Rн=6 Ohm. Ideally reproduces music styles: jazz, blues, wind instruments, guitar. I was also surprised that the amplifier believably, with characteristic duration, depth, and frequency, reflected the low-frequency component of one of the compositions of the above-mentioned styles, in contrast to the Yamaha-RV-557 receiver. This already reveals one of the most important advantages of a transistor-based tube amplifier: expressing in detail each of the instruments. In other words, we listen to music, songs, and the ear does not get tired of doing this even after prolonged or relatively loud listening, as if “pulling” us into the need to listen further. There is practically no background. Sometimes it just needs to be heard, and, as for design, it needs to be seen. Excellent Hi-End sound must be matched by excellent appearance! A line filter using ceramic capacitors and a ferromagnetic choke is used at the AC input. LUXMAN cables containing oxygen-free copper are used in the input audio circuits.
