What is a single-ended tube amplifier? Single ended tube amplifiers

During my amateur radio career, I have assembled and tested more than a dozen various amplifiers on lamps - both push-pull and single-stroke, including those with parallel connection of several. Most often, the good old ones were used. However, circuits with horizontal output pentodes - 6p45s, 6p44s and 6p41s - have repeatedly appeared on the Internet. I decided to stop at the latter, since despite the lower power than the 6p45, it does not have an inconvenient and dangerous pimp on top where the high-voltage anode wire is connected.Interest was further fueled by conflicting reviews on audiophile forums - from praise to complete denial of its sound parameters. As you know, it is better to collect it yourself, and then make a final conclusion. I took it as a basis schematic diagram single-ended amplifier S. Sergeev, only slightly changed the ratings of the piping and the bias of the output stage.

The driver contains the usual 6p14p output - here its role is secondary, pre-amplification. The output stage is 6p41s with automatic bias, which has proven itself to be excellent for its simplicity and stability of lamp operating parameters. The only difficulty - a powerful resistor - was solved simply. Since a search in boxes with 10-watt green ceramic resistors did not produce results (everything is available except the required 450-680 Ohms), I had to solder a garland of three MLT-2s on a small scarf, 180x3 = 560 Ohms.

The cathode resistor of the second channel is also assembled on it. Since the estimated power is 2 watts, these 6 are quite enough. You would still have to think about how to attach 2 powerful tubular resistors.

Power to the ULF comes from a mains transformer, rectifier and inductor. Transformer TSSh-170 is from a tube TV; you can also install TS-160, TS-180 here. In general, anyone capable of providing 250-300 V 0.3 A anode and 6.3 V 3 A filament voltage. Rectifier diodes - IN4007, choke - Dr-0.1. It has 1000 turns of 0.25 mm wire (this is if you don’t find a ready-made one and wind it yourself or take a network transformer to replace it).

Despite the significant voltage and current in the output stage - about 0.06 A, I took the risk of installing the relatively weak TVZ-1, which is more appropriate in 6p14p amplifiers. As it turned out later, I did the right thing :)

It would not hurt to take a metal case for our single-ended ULF, as I always did before, but I decided to take a risk here too, using an unnecessary Chinese front speaker from a 6-channel computer amplifier. This number also went with a bang :)

We will gut the acoustic system, design the future location of the radio elements and cut out the necessary windows.

Naturally, the lamps should be on top; we install them on a metal base - a sheet of two-millimeter aluminum, with cut-out round windows for the panels.

Then this sheet is covered with self-adhesive metallic color to match the main body. After gluing, the holes for the lamps are carefully cleared using a blade.

The lower part of the case is also reinforced with metal - so that the heavy network transformer does not fall out. It was also planned to install an electronic power filter on it, but in the end it was abandoned. The voltage at the power supply output is already not enough (only 260 V), so losing 20 V to the EF is wasteful.

At the back we cut out a rectangular window for a textolite panel of sockets and connectors - network, audio input and audio output to speakers.

We also cover this panel with self-adhesive tape.

Then we insert all the contact elements and screw it to the pre-cut AC window.

Large electrolytic capacitors were installed on a single aluminum base. There are 4 of these dimensional electrolytes - three for the power supply filter and oneat 300 uF 63 V, installed in the 6p41s cathode.

The case material - chipboard - turned out to be very easy to process, and electromagnetic interference from devices, which I was so afraid of, was absolutely inaudible. But this article is about assembly, configuration and testing of the circuit.

I bring to the attention of TV viewers an article on the topic of building a single-ended tube amplifier. Perhaps this is the only article like this here. In my deep conviction, single-ended amplifiers do not deserve attention. Those. For me, the answer to the question of what constitutes an amplifier exists. The article by Alexander Torres is written skillfully, with an understanding of the issues and technical aspects implementation of such a complex project. The author demonstrates high culture, only slightly indicating sarcasm, in relation to part of the TV viewers called udophiles. However, in my opinion, Alexander’s display of such restraint and tolerance towards obvious stupidity (about the coolness of a 4 W amplifier) ​​is excessive.

Two-stage single-cycle on 6SZZS without feedback. There are many amplifiers in the world. Which one is better, which one is worse – there is no clear answer. Some prefer transistor or microcircuit “powerful op-amps”, others prefer only single-ended ones, others faint if they find at least one semiconductor element in the amplifier (even if it’s just an indication LED - and instead they strive to install a neon light bulb or a “green eye” ). Four people are turned inside out if there are parallel lamps, transistors, capacitors or even resistors, but it turns out that they do not understand the difference between a transformer and a choke (a real case). Fifth, they try to solve all the problems by selecting the correct direction of the silver network wires and the “correct” solder. The described amplifier does not claim the title of “super-duper” or “all times and peoples”. I am well aware that the 6SZZS lamp, although good, is not the best. But it was interesting to design an amplifier based on some of the concepts. Although “the best concept is the absence of any concept” (C) paraphrased by A. Klyachin, nevertheless, the following wishes were expressed: 1. To do without feedback, even local ones. 2.Minimum amplification stages. 3. Do without electrolytic capacitors in the signal circuit (except for those on the power supply - they are also in the signal circuit). Get a high enough power for a single-ended circuit (15-18W) to provide sufficient overload capacity and a low level of distortion at normal room volume (4-5W on acoustics, with a sensitivity of 88-92dB). You can get by with a minimum of winding products, and those you cannot do without are as simple as possible.

The powerful stabilizer triode 6SZZS differs from most other triodes in its huge anode current. This is why there is a lot of love for building transformerless, or OTL, amplifiers with this tube. Unfortunately, I haven’t been lucky enough to hear a single normal-sounding OTL yet, but maybe I’ll get lucky in the future. However, its disadvantage, in addition to the high filament power, is high thermal inertia and temperature instability, especially with high leakage resistance in the grid circuit. This is manifested in the fact that when using a fixed bias (figure below left) due to changes in temperature, voltage and large thermal inertia - with maximum use of the lamp (i.e. close to the maximum power at the anode - 55-60 W), an avalanche self-heating of the lamp is often observed . There are many statements like “all this is nonsense, I did it and nothing happened.” But, as a rule, either a 6SZZS with an anode power of 40-45W was used, or it was a Loftin-White (directly coupled amplifier), or “just got lucky.” There are also individuals who use this lamp with half incandescence and a large “underload”. They don’t “peddle” it either, but I always wanted to ask them - why do you need 6SZZS? There are many other lamps.

To be fair, I note that I also came across lamps with a fixed bias (especially 6SZZS-V) that functioned normally, even at a power of 70-80W at the anode, but I also came across quite a few that “went haywire” already at 50W. I have one unique lamp that goes into avalanche self-heating as soon as the power exceeds 63-64W. Even with the use of the “autofix” described below, this lamp “flew away” into a current of 1 ampere, with a bias on the grid of minus 100V! Therefore, automatic bias is most often used (figure on the right), which provides excellent stabilization of the lamp operating mode. But, as in the “Golden Rule of Mechanics” - we win in strength, we lose in distance. Together with mode stabilization, we get a resistor in the cathode, on which high power is dissipated (about 20 W) and local feedback, to eliminate which the resistor must be shunted with a large capacitor. In the case of a 6SZZS operating at 300mA and 70V bias, the 230Ohm resistor dissipates 21W. And it requires an electrolytic capacitor whose impedance is no more than 1/10 of the resistor at the lower operating frequency. IN in this case- this is no less than 330 µF at 100 volts, but it is better to use 1000 µF at 100 V in combination with a 1-10 µF film capacitor.

What other options might there be? Direct coupled and step-down circuits can help, but they have their own disadvantages. The advantages of a fixed bias are, in addition to the absence of a resistor and capacitor in the cathode of the lamp, the absence of losses (heating) of this resistor and the ease of adjusting the bias with a simple low-power trimming resistor. In the case of auto-bias, the quiescent current of the lamp can only be changed by changing the value of the powerful resistor in the cathode of the output stage.

Many decades ago, a sequential autobias circuit was invented. It differed from conventional autobias in that the resistor was placed BEFORE the filter capacitor of the power supply. Since the voltage drop across it depends on the current through the lamp, stabilization occurs. It is only necessary to isolate the constant component, because A pulsating rectifier current flows through the resistor. Oleg Chernyshev (Yaroslavl) proposed taking the voltage from the resistor through a diode, thus constructing a peak detector, this managed to reduce the resistance of the resistor, the power released on it (by about 2-3 times), and reduce the ripple of the bias voltage. I went for a slight increase in the resistance of the resistor and the power dissipated on it to 11-12 W (but still, it is less than for conventional auto-bias) to increase the voltage removed from the resistor by adding a tuning resistor to the circuit. As a result, the resulting circuit has the following advantages: - the absence of a cathode resistor and capacitor, - ease of setting the desired lamp current using an ordinary small adjusted resistor. Stabilization of the mode, since it is not a fixed, but an automatic bias (Ucm depends on the lamp current). There is another advantage of the proposed circuit - the autofix resistor is located between the rectifier and the electrolyte, thereby limiting the charging current of the capacitor, both during switching on (InRush Current) and during operation.

There is another possibility - to use a current transformer installed in the alternating current circuit (in the secondary winding of the anode transformer, before the rectifier. It is also possible to install it in the primary winding.) This scheme further reduces power losses in the auxiliary circuits, but requires stronger filtering of the bias voltage , which can lead (and in some cases I have observed this) to self-excitation of the circuit at infra-low frequencies.

It should be noted that both the autofix circuit and the circuit with a current transformer, in the case of making a stereo amplifier rather than monoblocks, require separate anode windings and rectifiers for each channel. Let's move on to consider the complete amplifier circuit. The output stage is built according to the “autofix” circuit with an adjustable bias. The cascade operating mode is 210V at the anode at 0.28A. If desired, you can change it by adjusting the resistor in both directions (depending on the specific lamp). When the bias changes, both the current and the anode voltage change (due to a change in the voltage drop across the autofix resistor). The 1 Ohm resistor in the 6SZZS cathode circuit serves to measure current; after adjustment it can be short-circuited (although it does not bother anyone). Sectionalized output transformers - 4 sections of the primary winding (790 turns, in total, 0.85 mm wire), between which there are 3 sections of the secondary winding (36 turns each), which is wound with flat litz wire of large (2 sq. mm) cross-section - this made it possible to get by without paralleled sections and get away from equalizing currents. The secondary winding is tapped from one section, this allows the transformer to be turned on in three in various ways, obtaining with a load of 8 Ohm the value of Ra - 0.43 kOhm; 0.96kOhm and 3.8kOhm. The last value hardly has any practical meaning (although it fits entirely into the “concept” of Yuri Makarov - Ra/Ri = 20-30), but it can be interesting as an experiment, as well as when working with 4-ohm acoustics. The resistance of 430 Ohm is small at first glance, but on the other hand, “the Ra/Ri ratio should not be made more than 4-5, since the dynamics of the cascade deteriorate, and nonlinear distortions, when going above this ratio, decrease slightly (c) Anatoly Manakov." In reality, it all depends on the acoustic systems (AS), like many SEs without feedback, this amplifier is critical to the speaker impedance characteristics.

The core of the output transformer is “double C-Core” made of M5 iron, the cross-section of the central core is 18 sq. cm, the gasket is 0.3 mm. The transformer has an inductance of 4.5 H, the resistance of the primary winding is DC– 5.5 Ohm. The linear magnetization section of the transformer extends up to a current of 0.62A. With the secondary winding fully turned on, the frequency band of the transformer is 9Hz-75kHz, and the entire amplifier is 11Hz-53kHz (at a level of -3dB at a voltage of 10V at a load of 8 Ohms), the output impedance is about 2 Ohms, the sine wave distortion (according to the oscilloscope) at the output begins at power at a load of 15-18W. Gain factor – 13.

Since the goal was to build a 2-stage amplifier, the first stage (driver) must have a sufficient gain and a large margin in the output signal swing. The 6E5P lamp used, which was “discovered” for audio applications by Anatoly Manakov, with a power supply of 350-400 V allows you to obtain, in the absence of an output stage, a +120V peak-to-peak output signal swing.

This is approximately twice the maximum possible signal of +60-70 V pp, which depends on the bias voltage of the output stage. This tube can be connected as a tetrode or as a triode. In the first case, the gain is even excessive (100-130), in the second, on the contrary, it is not enough (30-40). In this regard, the so-called<ультралинейная>a tetrode connection circuit in which the second grid is connected to part of the anode load. With the ratings indicated on the diagram, this circuit has a gain of 60-70, which is most suitable for this case. IN original scheme A. Manakova has identical resistors in the anode, and the gain is 45-50. Driver bias can be done in several ways - traditional automatic bias (a resistor of about 100 ohms, shunted by a 2000 uF capacitor at the cathode, while the grid resistor sits on ground), fixed bias by a battery in the grid circuit, and fixed bias itself. The latter was chosen because it was necessary to do without capacitors in the cathodes of all lamps. Where the voltage comes from (negative source) for a fixed voltage doesn't really matter. And since there was none, “autofix” was used in the driver. Here its stabilizing properties of automatic bias are not so important, so the bias is chosen to be common to the two channels. Similar to the power supply of the output stage, the autofix resistor in the driver power supply also helps to reduce the peaks of the charging current of the electrolytes of the power supply.

The anode power supply of the input stage has a 3-stage filter, formed first by an autofix resistor and the first electrolytic capacitor, then by a series resistor and a second capacitor, and finally by an “electronic choke” on the mosfet and a large electrolytic capacitor installed parallel to the output stage, shunted by a film . The rectifier uses fast diodes and anti-interference filters (common mode, not shown in the diagram), which prevent the entry of “garbage” from the network. A similar “electronic choke” is used in the anode power supply of the driver. The filaments of all lamps are powered by alternating current; to reduce the background, all filaments are shifted upward by several tens of volts. The LED in the filament bias divider circuit is used for indication. With this design of the power supply, the background level at the output is about 3 mV, which is practically inaudible on speakers with a sensitivity of 90 dB, even if you “insert your ear into the speaker.” For the sake of experimentation, I tried, without changing anything in the power supply, to short-circuit the electronic chokes of the output stages. At the same time, a small background appeared in the speakers, inaudible from half a meter away, but I still recommend not abandoning them. When repeating the amplifier, it should be taken into account that some elements, not only lamps, also dissipate a certain amount of heat - these are autofix resistors and resistors in the anode circuit of the driver. They should be selected according to the power. Mosfets of electronic chokes heat up weakly; they do not need radiators. Screwing mosfets to a metal chassis is more than enough, but autofix resistors may also need a heatsink. The panels for 6SZZS are best ceramic, remember - they get very hot. The sound of the amplifier turned out to be quite interesting, you can feel a large reserve of power. Very clean and transparent high frequencies, perfectly transmitted midrange and soft, unobtrusive low frequencies, but of course - for transmitting “explosions” in cinema, this amplifier is less suitable than a powerful transistor push-pull. I thank Anatoly Manakov, Mark Feldsher and others for their help and advice.

P.S. After the article was published, a second version of the amplifier was made. Its main differences: The capacitance of capacitor C5 has been increased to 2000 μF. The number of turns of the primary winding of the output transformer is increased to 1200. Separate anode supply transformers (T2) are used for two channels. The remaining differences are not fundamental and are associated with a different mechanical design of the amplifier. Alexander Torres, Hong Kong.

Wonderful article. Clear goal, reasonable means. Prepared the publication and slightly edited it

Evgeny Bortnik, Krasnoyarsk, Russia, 2016

Andrey VRUBLEVSKY, Dmitry Chumanov

Supporters of single-ended amplifiers point first of all to their subjectively determined qualities: special sensitivity, melodiousness of sound, “musicality” (the last word has to be put in quotation marks, since it is not always clear what exactly is meant by it). The arguments of opponents, on the contrary, are based on the most objective data. This is, as a rule, low power, limited (both from below and from above) frequency range, and a high level of measured distortion. One could, of course, argue that WAVAC released a 100-watt single-ended amplifier; that the frequency range of the ML2 amplifier from LAMM Industries is 3-80000 Hz without OOS, but I’m afraid these arguments will seem unconvincing if you remember at what cost (30-35 thousand dollars) this was all achieved.

The photo shows a fashionable amplifier from WAVAC for tight wallets

Therefore, let’s come down to earth and try to answer questions that are relevant to most music lovers: what power is really needed for listening at home and what is the acceptable level of nonlinear distortion? It seems that it would be natural to say that the more power, the better, but the distortion, of course, is the opposite. Alas, in reality everything is not so simple. High power is achieved by transferring the output stage to class AB, which causes an inevitable increase in distortions of all types, and a noticeable reduction in these distortions, starting from a few percent, can only be achieved by resorting to deep negative feedback, about which has been written enough before us to understand that the sound quality cannot be improved with its help, you can only replace some distortions with others. A trivial example: 0.003% harmonics at 100 W of power - figures that are typical even for a relatively cheap transistor amplifier. So what is the reason for the poor, even wretched sound of most of these amplifiers, devoid of any emotionality?! Everything in this world comes at a price, and within one price range you will inevitably have to choose between power and quality.

But everything is not so hopeless. The question about the required power was convincingly answered by A. M. Likhnitsky in his article “Power”. According to his findings, a speaker system with a sensitivity of 90 dB paired with a 10 W amplifier is capable of creating in a room of 20 m2 the sound pressure necessary to fully reproduce the forte fortissimo of a symphony orchestra. Gentlemen, audiophiles, why more?

Now let's talk about the level of nonlinear distortion. We will have to turn to some conclusions of psychoacoustics, which assert, in particular, that the aural noticeability of nonlinear distortions for harmonics of different orders is not the same. Most researchers agree that 1% of the second harmonic will not be noticed even by professional experts, and the majority of subjects detect it at approximately 1.8-3.5%. Unfortunately, this is not quite the case with higher order harmonics. According to empirical observations, the aural noticeability of any harmonic is directly proportional to the square of its number. Based on this. 0.1%. say, the tenth harmonic and 2.5% of the second will cause a commensurate (albeit differently manifested) deterioration in sound quality. Moreover, some harmonics can mask the presence of others, so, in particular, the third harmonic becomes less noticeable in the presence of the second. The spectral combination of harmonics smoothly decreasing in level (the second highest, the third less, the fourth even less, etc.) is the most euphonious for our hearing. You can read more about the features of auditory perception in. We will only note that the information given in the passport about the total level of nonlinear distortions without indicating the spectrum of these distortions says absolutely nothing (!) about the sound quality.

Suppose we have convinced you, and a power of several watts at a few percent of harmonics suits you, but Why is it necessary for a single-cycle circuit? Let's take a look at an old textbook on tube circuitry. There the four main advantages of push-pull tube output stages are spelled out in black and white: - absence of permanent magnetization in the output transformer: - increased (in class A at least twice) output power; - compensation of even harmonics in the output signal; - reduced sensitivity to supply voltage ripples.

The textbook was written about half a century ago, and although the laws of physics have not changed during this time, it is worth thinking about what we are aiming for when applying these laws. Upon careful reading of the pages familiar from student times, one can notice that the main aspiration of circuit design in those years was to obtain more and more high power while reducing size and weight. Now we set ourselves the opposite task - obtaining the highest achievable sound quality, regardless of size and weight. So maybe we can try to go the opposite way - from class B to class A, from a push-pull cascade to a single-stroke cascade? Let's look at the so-called disadvantages of single-ended output stages.

1. Permanent bias in the output transformer. It not only reduces the inductance of the primary winding, but also forces the iron to work in a particular hysteresis cycle (to put it not quite technically correctly, in “pure class A”), that is, in a mode with increased linearity, especially on small signals, without that seemingly smoothed , but still there is a “step” present when crossing zero, which is inherent in the push-pull operating mode (maybe the main advantage of single-stroke devices is related to this - amazing microdynamics?). It is quite easy to compensate for the decrease in inductance - by increasing the dimensions and weight, we agreed that they are not the main thing for us.

2. The output power of a push-pull cascade operating in class A is equal to the output power of a single-cycle cascade built on the same pair of lamps, but connected in parallel. We do not consider class AB here due to the impossibility of its implementation without environmental protection.

3. Compensation of even harmonics in a push-pull cascade, due to the above-mentioned hearing characteristics, most often leads to a subjective deterioration in sound quality. It is no coincidence that some developers, both here and abroad, trying to bring the sound of their push-pull devices closer to the sound of single-stroke devices by skewing the bass reflex stage, mix a second harmonic into the signal. Unfortunately, this does not solve all the problems associated with two-strokes.

4. The increased sensitivity of single-ended cascades to supply voltage ripples is overcome by simply increasing the capacity of the filter capacitors of the power supply, which is not difficult given their current size and cost.

Here we come to unexpected conclusion: Most of the declared disadvantages of a single-ended amplifier, upon closer examination, turn out to be its advantages, while the rest today can be easily eliminated by increasing the dimensions, weight, and, as a consequence, cost. Low power with large dimensions, weight and cost - how acceptable this is, let everyone decide for themselves. It seems quite natural to us that a high-class amplifier, capable of delivering incomparable pleasure to a music lover in the appropriate circuit, has impressive dimensions and weight and costs more than a mediocre device, even with an order of magnitude higher power.

Of course, there are situations when high power is indispensable, for example, when scoring a disco, but at home, a well-designed single-ended triode amplifier without feedback with a power of 5 - 10 W with a typical distortion level of about 5-6% at full power (and accordingly about 1.0% at a power of 1 W), working at sound system sensitivity of 90 dB or more, is capable of providing very high sound quality, often unattainable for devices using any other circuitry.

The attentive reader may note that most of the above applies equally to transistor single-ended amplifiers. Absolutely right, there is such a trend in the world, its prominent representatives are the amplifiers of the “Pass Aleph” series by Nelson Pass. We are not against transistors, but we still note that today the tube triode is the most linear amplification element, and with its help it is, in any case, easier to obtain high sound quality. This is confirmed by the fact that in the highest price categories we see only single-ended tubes.

Well, okay, you say, let’s say. But what does all this have to do with the majority of music lovers, and our country, who do not have the opportunity to purchase not only a ready-made “Cary” or “Audio Note” amplifier, but also the components necessary for their construction - 300V lamps manufactured by “Western Electric”, transformers “ Tango, Black Gate and Multicap capacitors, Kirnber Cable silver wires? Here's what they are. For those of you who know which end to hold a soldering iron in your hand, we suggest that you independently assemble from available parts a tube amplifier that is easy to manufacture and configure, capable, nevertheless, of demonstrating all the advantages of single-ended tube sound that we have talked about so much.

Scheme. Single-ended tube amplifier made from affordable parts

From possible options We preferred the amplifier on the output beam tetrodes 6PCS in triode connection. This device was the prototype of the serial model “Avant Electric Nostalgia”, which differs from it in some modifications caused, in particular, by the technological requirements of the production series.

Basic technical parameters amplifier:
output power 7 W with non-linear distortion coefficient 6%,
sensitivity 0.4 V,
operating frequency band at full power is no worse than 12 Hz - 30 kHz without environmental impact.

We chose the 6CCD output lamp, firstly, for its availability and low (about 20 rubles on the St. Petersburg radio market) price. Secondly, due to the fairly high linearity in the triode connection, I have a promising harmonic spectrum (relatively high second harmonic and low third). Let us recall that this lamp, or rather its prototype 6L6, was developed specifically for use in audio paths. And thirdly, for its warm (this is the time to remember about the spectrum of harmonics) and - still, one cannot do without this word - “musical” sound, even in comparison with such serious rivals as the EL34 and 6550. Two relative disadvantages of this lamps in triode connection - we overcame the low output power (3.5 W) and a fairly high internal resistance (about 1.5 kOhm) by connecting two lamps in parallel. It should be noted that among Russian radio amateurs there is a widespread, in our opinion, unfounded opinion that parallel connection of lamps is inadmissible. Without wanting to go deeper into a discussion on this topic, let's give a simple example. One of the most expensive (after all, 330 thousand dollars) amplifiers from the well-respected Audio Note company, namely Gaku-On, has two lamps connected in parallel at the output, which does not at all prevent its lucky owners from enjoying music. One way or another, by connecting 6CCD lamps in parallel, we got an internal resistance of 750 Ohms and 7 W of triode power. Well, why not “three hundred”?!

Let's take a closer look at the diagram. The input stage, also known as the driver, is made according to a circuit with a dynamic load (SRPP) on one of the best domestic small-signal triodes 6N9S. The use of SRPP is explained not by any special preference for such stages, but by the fact that we tried different options (one triode with anode load, parallel connection of two triodes, etc.) and settled on SRPP. as providing the best, in our opinion, sound quality. The output stage, as mentioned above, is made of two 6CCD beam tetrodes in a triode connection. In order to minimize nonlinear distortion, the output lamps are selected in pairs based on anode current and slope with an accuracy of 1.5% and, if necessary, are also replaced in pairs. For those who do not have the opportunity to select lamps, we advise you not to get upset and use those lamps that you have (preferably from the same batch), since the spread in lamp parameters leads to an increase mainly in the second harmonic, which should not radically deteriorate the sound. The operating modes of the output lamps we have chosen may, at first glance, cause confusion. In particular, the voltage on the second grid is 100 V higher than the value specified in the reference book. In our justification, we will refer to the article, which proves the possibility of using pentodes and beam tetrodes in a triode connection exceeding some reference modes without significantly reducing the operating life of the lamps.

Our many years of experience working with lamps confirms this; moreover, the cost of 6CCDs is not so high (unlike, say, 300V), and replacing even the entire set of lamps once every few years is unlikely to have a noticeable impact on anyone’s budget. The load of the output stage is a transformer.

Output transformer, of course, is the most important element of the design. Perhaps no less depends on it than on the output lamp. In our version, it is made on an W-shaped core made of transformer steel with a thickness of 0.35 mm (an SH-core on E310-330 steel is quite suitable), the width of the middle rod is 25 mm, the height of the camp is 40 mm. The primary winding consists of four sections of 510 + 1190 + 1190 + 510 turns of PEV or PETV wire with a diameter of 0.28 mm. Between them there are three sections of the secondary winding of 216 turns of wire with a diameter of 0.71 mm. From the 130th turn you can tap for a 4-ohm load. All sections of the primary winding are connected in series, the secondary - in parallel. Capacitor paper (ordinary paper can also be used) 0.3 mm thick is laid between the windings. After winding, the reel is impregnated with technical wax (a mixture of paraffin and perlin). The core is assembled: W-plates and I-plates separately, with a gap of 0.25 mm between them using a plate of insulating material.

This is not the only possible output transformer design. It is quite acceptable to use other designs, for example, in recent years, a two-coil version with a PL core has become widespread, which has certain advantages (as well as disadvantages). In this case, you will have to calculate the transformer yourself. Let us remind you that you can find the information necessary for the calculation in, and we will also indicate the main parameters. First of all, this is the AC resistance of the primary winding of 2.5-3.0 kOhm, as well as the DC bias current of at least 120 mA. The only caveat: do not use cores with an average core area of ​​less than 10 cm2 (overall power less than 150 W), otherwise you are unlikely to get acceptable performance at low frequencies.

The power supply is assembled on the 5TsZS kenotron, which is no coincidence. Practice shows that kenotron power supply can significantly improve the sound quality of an amplifier, no matter what semiconductor diodes you have previously used. It is no coincidence that the most expensive models use kenotrons. For the power transformer we used a magnetic circuit Ш25×50, the primary winding contains 770 turns of PEV wire with a diameter of 0.63 mm, the step-up winding - 1340 - 1340 turns of wire with a diameter of 0.315 mm, the filament windings - respectively, 19 turns of wire 1.25 mm to power the kenotron, 24 turns of the same wire to power the filament of the output lamps, and 24 turns of 0.71 mm wire to power the filament of the input lamps. You can also use another magnetic circuit from a transformer with a power of at least 150 W, making the calculation yourself.

All parts are mounted on an aluminum chassis and interconnected using hinged mounting. Try to make maximum use of the terminals of the elements themselves: where they are lacking, use MGTF-0.35 wire, paying special attention to the “ground” circuits. Basic requirements for installation: the wires should be as short as possible and under no circumstances should closed circuits be allowed, otherwise you will end up with not an amplifier, but a radio receiver. A circuit assembled without errors does not require configuration. It is only advisable to check the voltages and currents at the indicated points using a tester. If the measured values ​​differ from those shown in the diagram by no more than 10%. - Everything is fine. Gross differences most likely indicate an installation error or a malfunction of any element.

Before using for the first time, check the installation carefully. This will relieve you from the thrill. If, when turned on, your amplifier did not give any alarm signals (burning smell, sparks, loud clicks, etc.), let it warm up for 10-15 minutes and start taking measurements.

With proper installation of ground circuits, the background level in your speakers should be quite low. With a speaker system with a sensitivity of 90 dB, it is only audible if your ear is placed close to the subwoofer B otherwise You will have to experiment with the arrangement of parts and wires, which can sometimes even take several days. But, one way or another, this is a solvable task, and, therefore, you can cope with it.

Now let's touch on such a sore point as types of elements used. Why sick? On this matter, we had to read and hear directly opposite opinions, starting with the fact that our domestic components are no worse (or even better) than the most expensive and prestigious foreign ones, and ending with the fact that without “Black Gate” and “Multicap” there is nothing even trying to get a decent sound. A detailed consideration of these issues is beyond the scope of the article, and we will limit ourselves to only some specific recommendations based on our personal experience.

The types of elements indicated in the diagram guarantee you a certain initial level of quality, quite comparable to that inherent in some expensive foreign models. And then, based on your tastes and capabilities, try to rise to a higher level. Just don't ask too much of this scheme and it won't disappoint. So, let's start in order.

The potentiometer at the input can radically affect the sound quality. Unfortunately, a worthy replacement for the expensive “ALPS” can only be a step attenuator, say, based on domestic reed switches with gold-plated contacts.

Replacing the transition capacitance has no less effect on the sound. If we have the opportunity, we recommend trying “Multicap RTX” or “Jensen”, known no less than “Audio Note”. They sound very different, but each of them, in our opinion, deserves its high reputation (and high cost). With all our patriotism, we cannot agree with those who claim that our K40U-9 (KBG, FT, FGTI and many others) are better (as an option - no worse) than the above-mentioned “Multicap”, “Jensen”, etc. etc., We assume that statements of this kind are not sufficiently caused high quality used when testing audio paths.

Our MBGOs (MBGV, MBGN, MBGCh, even better KBG-MN, etc.) have proven themselves to be excellent in the power supply, if you close your eyes to the fact that they will take up half the room. Despite our endless respect for the “Black Gate” series “WKZ”, we would not hesitate to recommend them due to their prohibitive cost. We advise you to save them for more advanced designs, and put something simpler here, for example “Rubicon” or “Nichicon”.

And finally, if for installation you use some OFC wire from a well-known company (to our taste) and “WBT” or “Audio Note” solder, it will not get any worse.

A few words about acoustic systems, which can be used with this amplifier. They say that only highly sensitive (95 dB and above) speaker systems can unlock the capabilities of low-power tube amplifiers. Undoubtedly, the higher the sensitivity of your speakers, the less power is required from the amplifier to create the same sound pressure level and the less distortion, accordingly, will be. But the problem is that a more sensitive acoustic system does not always turn out to be better in sound.
How can this be? In the home kit of one of the authors, the described amplifier worked for a long time with speaker systems based on Peerless dynamic heads with a sensitivity of 88 dB, reproducing music of various genres, including hard rock at high volume, and there were no problems with transmitting dynamic contrasts. At the Russian Hi-End 2000 exhibition, the Nostalgia amplifier was demonstrated complete with speaker systems with a sensitivity of 87 dB in a hall with an area of ​​at least 50 m2 and, to the amazement of many, including ours, on most phonograms it was able to provide the necessary volume, without going into clipping. So if maximum volume is not your main criterion for assessing sound quality, use the speaker system that you have, and you may be pleasantly surprised. In fact, you shouldn’t be surprised; the subjective perception of the sound volume of tube amplifiers is significantly different from the perception of the volume of transistor amplifiers. The most commonly referred to subjective power rating as "Nostalgia" is 35-40 watts. We hope that we have dispelled your doubts.

There is another problem, in our opinion, no less important. The combination of a high (3 ohm) amplifier output impedance with a high quality speaker system can sometimes lead to an unwanted boost at low frequencies, simply put, to humming. In such cases, the blame most often falls on the amplifier, although it seems to us that the speaker system is no less to blame. More precisely, this is a problem of mutual matching between the amplifier and the speaker system. There are several ways to solve this. The simplest is the introduction of shallow feedback, which reduces the output impedance of the amplifier to an acceptable level. How can this be, you say, since we just refused feedback for ideological reasons. Well, in this case we propose to make a compromise, given that in most cases an OOS depth of 2-3 dB is sufficient. But for convinced opponents of OOS, we will present a more radical solution - to independently manufacture an acoustic system with a reduced quality factor specifically for use with amplifiers without feedback. If such a prospect does not frighten you, we, for our part, are ready to publish one of the possible design options for such a system on the pages of the magazine.

Literature
1. Likhnitsky A. Power. Part 1. "AudnoMagazin".N" 2 (7) 96.
2. Frankland S. Single-Ended Vs Push-Pull. Part 1. “Stereophile” 12/1996.
3. Tsykin G. Electrical signal amplifiers. 1963,
4. Troshkin N. Triode from scrap materials. "Class A", October 1997.
5. Tsykin G. Low frequency transformers. 1955.

Specification
Amplifier
R1 - MLT 0.5 470 kOhm
C1 - 47 uF, 450 V
R2, R3 - MLT 0.5 1.5 kOhm
C3 - 1000 µF, 6ZV
R4 - MLT 1 20kOhm
C2 - 0.15 µF, 250V
R5 - MLT 0.5 220 kOhm
C4 - 300 pF (K78)
R6, R10 - MLT 0.5 1.0 kOhm
R7, R11 - MLT 1 100 Ohm
R8, R12 - MLT 0.5 22 Ohm
R9 - PEV 10 240 Ohm
R13* - MLT 0.5 30-120* kOhm
V1, V2 - 6Н9С
V3, V4 - 6PCS
S2 (K72 P6, K72 P9)
S1, SZ (K50-27, K50-37, K50-42, Rubicon, Nichicon, Jamicon)

Power unit
VI - 5TSZS
L1, L2 - 2.5H x 0.14 A
C1, C2, SZ - 220 µF, 450 V
C4 - 47 uF, 100 V
R1 - MLT 1 300 kOhm
R2 - MLT 1 - 43 kOhm
C1, C2, NW (K50-27, K50-37, K50-42, Rubicon, Nichicon, Jamcon)