Lamp preliminary unch. Tube audio power amplifier

I haven't written anything here for a long time ... Somehow, everything didn't stick.

But finally, something was found that could really be of interest to someone else besides the author.

Frankly, I pondered this topic for a long time ... I rummaged through everything on the internet that could be found about this and only realizing that there was very little really sane and useful about the topic voiced in the title, I decided to crown my efforts with an epistolary report, for which, before all armed with a camera to capture the process in all details, trying not to miss a single important moment.

It so happened that for more than 30 years of practice of my radio engineering "creativity", I have never had a chance to make a completely tube amplifier.

There were a lot of reasons for this!

I will not list them all. I can only say that I have dealt with lamps, and quite successfully and productively. But this was due to the pre-amplification cascades and made it possible not to get involved with hemorrhoids, due to the need to mount a bunch of pieces of iron, in the form of chokes, big trances and others like them.

But now I wanted, at least once in my life, to make a classic (moreover, just a classic !!!) lamp lamp, with lamps beautifully glowing in the dark, mounted outside ...

Not that I did not understand what it would result in for me ... But, I confess honestly, I did not realize that, unlike the design of semiconductor ("stone") equipment, the manufacture of a lamp apparatus should rather be attributed not so much to electronics as to plumbing work.

But I am getting ahead of myself ...

To begin with, as I said above, without further ado, I blasted in the search engine line: "DIY tube amplifier."

However, having reached (no lies !!!) the tenth page of the search engine, I realized that the main motive of those who had already managed to tell about their experience in creating tube amplifiers with their own hands was not the desire to teach others something, but rather the desire to show off their own achievements without sharing the secret of such "success" with others.

There is very little real information about HOW to do this, and if it exists, it is very scattered and sparing of details.

Actually, at that moment I realized that I was favorably left a place in this clearing. J

So why, in fact, a lamp tube?

I will not rant about fashion trends, such as Hi-End. It is clear that this is both fashionable and prestigious, and the sound of a tube really compares favorably with a transistor sound. What? ... - With this question not here! If you just want to "decide for yourself" - raise your brain to your acquaintances who have such devices, or managers in salons, such as the Purple Legion.

And if you decide that you want this, but spend on this "miracle" of the money that those who sell it usually ask for this kind of equipment are not ready (and who touches, for whatever reason you are not ready! ..) , then, probably, this article will be useful to you ...

So where do you start?

Perhaps, in this case, you can easily determine the sequence of actions!

In the case of "stone" devices, everything was somewhat different. At first, the filling was assembled there, and only then did we think about cases for our creations.

In the case of tube amplifiers, everything is exactly the opposite, since for these machines the amplifier body is, first of all, a structure that carries all the main elements. So, first of all, decide on how you would like your amplifier to look as a result, that is, decide on the case!

I must say (I know from my own practice) that this is the most difficult question in our "fatherland". Alas, in Russia, finding a decent housing for radio equipment is almost impossible to solve. L

I'm not that lucky ... But at one time I brought a lot of such iron from the "celestial". Therefore, I was lucky to avoid this problem. And I will even say more! Perhaps I can help some of you deal with this problem as well! ;) Well, yes, this is all just in private ...

In the meantime, having decided on how our creation will look like, it is worth solving the second, of the most important tasks - to decide which one to assemble from amplifiers?

There are a lot of schemes, ideas, not to mention opinions!

And figuring out on the fly which of the ideas to grasp is incredibly difficult.

In such cases, it is worth starting with the simplest and, in the same place with the one, worked out not even for years - but for decades of material ...

But such, as the practice of studying the issue has shown, there is a lot.

And here, perhaps, it is worth starting to share your own experience.

There are a lot of established stereotypes in our minds. So, for example, high-speed driving in our country inevitably evokes an association with Michael Schumacher, and the racing car itself inevitably with a red Ferrari ...

Similarly, when it comes to tube Hi-End, the first thing that comes to mind for people who have already come into contact, at least to a minimal extent, with this topic is, of course, Audio Note.

For more than a dozen years, it is the Audionot sound that is almost a religion among a large part of the "sophisticated hi-end"

At one time, many copies were broken in the field of discussions about what, in fact, is this secret of sounding the creations of Peter Kvortrup (dad and one of the main designers of Audio Note).

I remember that this casket was as easy to open as most of the others.

A relatively small number of experiments made it possible to find out that the first cascade, usually built according to the so-called SRPP (cascade) scheme, introduced the bulk of the colors into the Audinotovsky sound.

I did not philosophize either, having determined that it should be he and nothing else at the entrance, although something else could be simpler, but not much.

The output stage is even easier!

Here it is worth starting from the principle of accessibility. Speaking about accessibility, I mean, first of all, the element base, on the basis of which you can build something that sounds quite decent.

In this it is worth relying on the "experience of the ancestors" in abundance that have come down to us in the form of the remains of old tube TVs and radio (Hello trash heap !!!).

In extreme cases, this junk, in the form of weekend (TVZ-Sh) and power (TS-180) transformers, is usually in abundance at local flea markets that take place on weekends in all areas and areas of our "immense" ...

And in conclusion, the problem of choosing an output lamp boils down to understanding that these very output transformers TVZ-Sh were designed to work with almost the only one developed in the socialist fatherland, a light bulb created specifically to amplify sound. Of course, we are talking about the legendary 6P14P or its more modern counterparts 6P15P or 6P18P.

However, your will! You can also supply a "proprietary" analogue in the form of EL 84. How much the result will be worth is up to you to judge. Here I will just note that these replacements should not entail any constructive or schematic changes. Even the modes of these lamps are almost identical and, most likely, you will not have to adjust anything with such a replacement on an already made and working amplifier.

Since we are talking about lamps, perhaps it is worth mentioning the light bulb for the first stage.

I am not afraid of the shits of malicious remarks of the "dissent", but IMHO there is simply no better candidate for the first stage than 6N23P-EB. However, I will immediately warn you that the number of people who agreed with me will be approximately equal to the number of those who objected. I will only say that if we are striving for the Audionot sounding, then this is the very thing! J

Well, actually, we have almost drawn our scheme ourselves.

To all that has been said above, it is worth adding unless only the fact that speaking about the output stage, I meant exactly and exclusively triode switching on 6P14P. It is in this inclusion that this lamp is able to touch the strings of the soul in a way that few others do.

Yes! This will result in a loss of power. But, perhaps, I should have said this before ... Hi -End is not for scoring discos. Moreover! In Hi-End, the quality of the device is usually inversely proportional to the power (read loudness) at which the amplifier fully reveals its capabilities.

In addition, I will reassure you that the very 1.5 - 2 watts per channel that we can get with 6P14P in triode switching, in terms of subjective sound volume, will seem adequate to 10 watts per channel, obtained from a typical silicon-transistor wux.

So, just trust those thousands of people who have already walked this path before you and, believe me, were completely satisfied with the result. ;)

Moreover! I also have much more "serious" devices, which, of course, are objectively better than this creation. But this simple and seemingly completely uncomplicated machine has its own soul, gentle and kind ... It is capable of touching and warming human souls with its very warm voice. J (Evan pulled me away! .. Sorry again for the pretentious syllable.)

The only question of the circuitry of our wuxia, perhaps, was the question of "proper and healthy nutrition." And this, I must say, is a matter of paramount importance when it comes to sound! For the sound that we hear as a result is, in fact, nothing more than the power supply of your amplifier modulated by the input signal.

Hence the conclusion - the power supply of the tube amplifier should also be tube-powered! So this is a kenotron! And if we absolutely remain adherents of the classics, then the choke ...

And if everything is simple with the kenotron (by summing up the anode currents of all the lamps, we get the total consumption, based on which the required kenotron is selected), then with the choke, a problem can really arise ...

However, I was lucky. In my bins I found a real choke from some old tube TV. But even if not, then the simplest and most effective solution to this problem would be to buy at the nearest construction market for 120 wooden banal 18 Watt choke for old fluorescent lamps. Their inductance of 2 Henrys (usually something like that ...) is quite enough for our purposes.

How long or short, but on the spaces of the Runet I managed to find two whole schemes that almost completely meet all the aspects sounded above. The first of them is built exactly on the idea that was described by me above. The second differs only in that it has a pair of output lamps in parallel at the output, but it has a beautifully painted power supply that fully meets all my requirements.

These diagrams are:

In fact, oddly enough, the essence of my article is not directly related to the amplifier circuit ... In any case, this is not the main thing for me in this case. The main thing is to talk about how to collect all this?

It is worth noting that the classic approach to building a tube amplifier, in contrast to transistor devices, usually assembled on printed circuit boards, is the so-called surface-mounted assembly.

Frankly, for me this has always been the most repulsive factor when it comes to assembling tube circuits. For me, who was used to making a separate printed circuit board even for a freestanding volume level changer so that everything was correct and neat, the very thought of loosely dangling parts in the amplifier case, fastened together only by soldering and, sorry, dangling on snot, was frightening ... And , starting to build this machine, I had to overcome some internal barrier and practically on the go figure out how to fix everything so that in the future I would not worry about what, and not about…. is there something there at one fine moment ?. ..

First, it is worth carefully separating those commutations that we will need later. With your permission, I will omit this stage, since it is specific and does not imply many solutions.

I'll just present the result as a given. In my case, it was the wiring of the input switch, ALPS for the volume control, and the input, output and power connectors themselves.

It is characteristic that at this stage we remove the upper and lower panels of the case. The lower one just interferes with us, and we will need the upper panel as the basis of our design.

Here's what we have at this stage:

Looks like I missed one important point... The fact is that before proceeding with the assembly of the amplifier, you must first select at least the basic elements of the future car. They are needed to determine the design of your device.

We are talking primarily about light bulbs, sockets for them, output and power transformers and chokes. About the very elements that are attached directly to the body.

And only having completely selected everything we need, we can arrange it the way you like, determine the places for these elements and mark the top panel.

Here's how I decided to arrange the elements of my amplifier:

I confess, I had the idea to plagiarize the layout of the elements from one of the most popular Audio Note amplifiers, but, overcoming this temptation, I decided to arrange the elements according to the classical scheme. The idea of ​​this topology, in this case, is not fundamental. The fact itself is important as a stage. This must be done very carefully, thinking about how convenient the selected location will be for subsequent internal installation and about the mutual influence of the elements on each other.

We are talking, of course, about the magnetic fields of transformers and their direction.

I suppose there is no need to present a short school physics course .. Just remember this. ;)

First of all, we place the sockets for our lamps and determine the size of the holes for them:

Here we are awaited by another ambush and a dumb question in our eyes: “And how can we drill such HOLES in a sheet of iron ?!” ... In my case, it was exactly like that. And I could not find the answer to this question in the articles of "colleagues" who happily reported to me about how wonderful they assembled tube amplifiers with their own hands.

I had to go to the nearest construction market and retrain from an electronics engineer to a locksmith.

I took the data with an ordinary caliper-compass before going to the market. It turned out that the diameter of the holes for the sockets for the finger lamps is 18 mm, and the diameter of the hole for the socket for the octal lamp (kenotron) is 28 mm!

The study of the issue showed that for drilling holes with a diameter of 18 mm. you can find a classic drill, but for larger holes you will have to use a "crown" from "Bimetal".

This is how it looks:

It is necessary to drill the holes very carefully, and it is necessary to drill on the side of the top panel, which will subsequently be turned into the body. I approve this based on my own experience. Actually, an inquisitive eye will be able to see the consequences of my flaw in the photographs with which I accompany my story ...

Drill speed is the lowest. In this case, if possible, it is worth using the auxiliary handle of the drill in order to stabilize the beating of the crown as much as possible.

Naturally, the edges of the holes obtained must be processed to remove the burrs that will inevitably remain after drilling the holes.

It turns out something like this:

Recently, despite new records in nanoelectronics, there has been a steadily growing interest of radio amateurs in tube amplifier circuits. Some are delighted with these designs, others are not able to treat them seriously, without undue skepticism. As part of this article, we will consider a few simple do-it-yourself tube amplifier designs.

Positive statements boil down to the fact that a single-ended tube amplifier creates a special melodiousness and sensitivity in relation to sound, as well as a unique musicality. Although, in my opinion, all these indicators are subjective. Based on them, it is impossible to draw conclusions about how high-quality the lamp design is.

The position of the opponents is based on the fact that - purely objective factors that characterize the device are taken into account. For example, a fairly weak power, restrictions in the upper and lower frequency range and a high degree of distortion.

Amplifier radio parts list: Resistors: R1 - MLT 0.5 470 kOhm; R2, R3 - MLT 0.5 1.5 kΩ; R4 - MLT 1 20 kOhm; R5 - MLT 0.5 220 kOhm; R6, R10 - MLT 0.5 1.0 kΩ; 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 Capacitors: C1 - 47 μF, 450 V; C3 - 1000 μF, 6ZV; C2 - 0.15 μF, 250V; C4 - 300 pF (K78); C2 (K72 P6, K72 P9); C1, SZ (K50-27, K50-37, K50-42, Rubicon, Nichicon, Jamicon) lamps: V1, V2 - 6H9C; V3, V4 - 6PZS

Power Supply: radio tube VI - 5CZS chokes L1, L2 - 2.5H x 0.14 A Capacities of capacitors: C1, C2, SZ - 220 μF, 450 V; C4 - 47 μF, 100 V; C1, C2, SZ (K50-27, K50-37, K50-42, Rubicon, Nichicon, Jamcon) Resistances: R1 - MLT 1 300kOhm; R2 - MLT 1 - 43kOhm

This self-assembled circuit is designed to work with a pre-amplifier, which already has all the tone and volume controls, even the computer's line-out is suitable.

Output power 20 W
Nonlinear distortion factor not higher than 1.2%
Scheme sensitivity 500 mV
Frequency response unevenness from 30 Hz to 25 kHz does not exceed ± 1 dB

The design has two stages: a phase inverter and an output stage. The phase inverter is built according to a typical self-balancing scheme. The basis of the output stage are four radio tubes of the 6P14P type, operating according to a push-pull circuit in the AB amplification mode. The bias voltage to the grids of all lamps comes from a common cathode resistor R12. Resistors R13 - R16 block self-excitation of the device in the microwave range.

Deep negative feedback is added from the secondary winding of the transformer to the cathode circuit of the first 6N2P phase inverter lamp. The power supply of the tube amplifier follows from the bridge on the diodes D1, D2, D2, D4. The anode voltage is supplied to the bass reflex through a passive decoupling filter R9C2.

The output transformer T1 is assembled on a magnetic core from steel plates of the Ш-30 type with a set thickness of 35 mm. Primary winding - 2, 1200 turns of copper wire PEL 0.31, the secondary one is wound with 88 turns of PEL 1.0 wire

Winding is carried out on a frame with a middle cheek. The sequence of winding the sections and the connection diagram of the windings are shown in the figure below. The entire primary winding is divided into six sections of 300 turns, the secondary winding is divided into four sections of 44 turns. First, sections 1-8-2-7-3 of the transformer are wound, then the frame is removed from the winding machine, turned 180 ° and the remaining sections 4-9-5-10-6 are wound.

Power supply built on a core of steel plates Ш-40 with a package thickness of 50 mm. The mains winding has 430 turns of PEL 0.8 wire. Secondary windings consist of 400 turns of wire PEL 0.31; the filament winding of the kenotron has 11 turns of PEL 1.0 wire, and the filament windings of lamps L4 and L5 have only 13.5 turns of PEL 1.0 copper wire each.

The design consists of only three lamps and has two channels. On the first 6N23P lamp, a pre-amplification stage is built, the signal from which through two K78-2 capacitors goes to two channels. The balance is adjusted using a variable resistance of 1k.

Transformers TN36-127 / 220-50 and TN39-127 / 220-50 are output, they are connected to the anode circuit of 6P43P lamps. A low speaker with an impedance of 8 ohms is connected to their secondary winding.

High sound quality is also provided by a stationary power amplifier, cited by G. Gendin in the book "Homemade ULF", MRB-1964.
By a strange coincidence, the circuit of this amplifier (Fig. 1) is very similar to the standard 10-watt Kinap, which was in every radio center in the 60-70s, except that the lamps were replaced from 6P3S with more modern ones. The circuit of the phase inverter and the output stage is similar to the high-quality UMZCH circuit discussed above, and the preliminary stages on the lamps L1, L2 accelerate the final amplifier to such a power that, in the presence of deep OOS through R26-R34, to provide the rated output power.

Powerful UMZCH 100 W V. Shushurin (MRB-1967) is designed to work with the equipment of an ensemble of electric musical instruments, and can also be used for sounding small halls, club rooms.
The rated output power of the amplifier is 100 W. Harmonic coefficient at a frequency of 1000 Hz is no more than 0.8%, at frequencies of 30 and 18000 Hz - no more than 2%. In the frequency range 30-18000 Hz, the unevenness of the frequency response is +1 dB. The nominal sensitivity is 500 mV, the nominal output voltage at a load of 12.5 Ohm is 35 V. The noise level of the amplifier relative to the nominal output level is about -70 dB. Power consumption from the network is 380 VA.

In the amplifier circuit (Fig. 1) there are only two stages - an input phase inverter on a 6N2P double triode lamp and an output terminal stage on four 6P14P tetrode tubes. All the cathodes of the output lamps L2 ... L5 are connected at one point on the resistor of the cathode self-displacement chain R12-C6, and the DC tetrodes themselves are connected as triodes. This somewhat reduces the slope of the throughput I - V characteristic, but makes it more linear ...

Another circuit of a high-quality terminal UMZCH F. Kuehne at 20 W is shown in Fig. 1. Basically, this amplifier repeats the previously discussed circuit solutions, which provide high-quality sound reproduction, but as a power amplifier it does not contain volume and tone controls, and it also provides the ability to connect loudspeakers with different ratings of load impedances. In the switch position, as shown in the diagram, is 16 ohms.

Single-channel schemes UMZCH

Complex circuits of tube amplifiers, unlike those already considered simple ones, include such UMZCHs, in which at least three of the following five features are present in aggregate: there is a preamplifier, the output stage is assembled according to a push-pull circuit, the amplification frequency band is divided into two or more channels, the output power exceeds 2 W, the total number of lamps in one amplification channel is more than three. However, multichannel schemes are not so often found in radio amateur creativity, although more often than our domestic industry did in the past. But even without this sign, all the same, the previous scheme of the Bulgarian Kusev was not included in the number of complex ones, because in one channel it has only 2.5 lamps, the circuit is single-channel, and the output amplifier is single-ended.
But at first glance, a simpler circuit of a high-quality UMZCH from the collection of Gendin G.S. (MRB-1965) has enough distinctive features to be classified as complex (Fig. 12). The output power of the amplifier, assembled on two 6FZP tubes of the triode-pentode type, exceeds 4 W, and the sound quality is beyond praise. The amplifier is designed to reproduce a gramophone, therefore, its input signal is 250 mV, the reproducible frequency band is 50 ... 14000 Hz with an uneven frequency response of 1%, the nonlinear distortion does not exceed 2% at rated power.

Figure 12 Schematic diagram of a G.S. tube amplifier. Gendina

The greatest difficulty in setting up tube power amplifiers with a push-pull output is to ensure the symmetry of both amplification arms of the stage. The designer is faced with several tasks that are difficult in themselves, and in the aggregate they give a strong headache, because if they are left unresolved, then the advantages of a two-stroke cascade turn into their opposite. Let me remind you of the advantages of a push-pull circuit. This is the absence of even harmonics in the load, which reduces the total harmonic distortion, and the absence of odd harmonics in the power supply circuit, which facilitates the requirements for blocking capacitors in the power supply filter and provides an additional stability margin for the amplifier. A decrease in the output capacitance of the lamps also works on stability, which significantly affects the operation of the UMZCH at high frequencies. And, finally, with a push-pull connection of the lamps, the output resistance of the cascade increases, and this makes it possible to raise the quality factor of the circuit formed by the primary winding of the output transformer and a capacitor parallel to it, and to improve the filtering ability of the load in relation to the higher harmonics of the useful signal.
Let us consider the solution to the problem of realizing the advantages of a push-pull amplifier circuit using the example of this UMZCH. First, it is necessary to select lamps L1 and L2, or rather their pentode parts, so that they have the same characteristics, in particular, the input and output resistance and permeability, the equality of which allows us to hope for the coincidence of the static I – V characteristics of both lamps. Secondly, it is necessary to provide a symmetric DC mode, that is, the same anode power and bias, and if it was not possible to select completely identical lamps, and this is guaranteed in most cases, then the mode must be selected so as to bring the characteristics of the lamps to identity. As can be seen in the diagram (Fig. 12), all operating elements and supply voltages of both arms are the same, but we emphasize again - this is possible only if the characteristics of the lamps are identical. Tuning the modes to full symmetry is independent task everyone who tries to repeat someone else's scheme. Thirdly, it is necessary to ensure the symmetry of the load, which is the primary winding of the output transformer Tr1. To do this, the primary winding is wound with a double wire in the amount of 1500 turns of wire PEV 0.15 on the core Ш20хСО in 5 layers of 500 turns, alternating them with 4 layers of the secondary winding of 24 turns each, 96 turns in total. The middle point of the primary winding, to which the supply voltage is applied, will be the connection of the initial ends of the wire, and the final leads are connected to the anodes of the lamps. Fourthly, the excitation voltage is supplied to the control grids of both lamps of the output stage in antiphase, therefore, from the anode of the L1 triode, most of the signal is fed directly to the grid of the L1 pentode, and part of it is fed from the trimmer resistor R12, which regulates the amplitude of the input signal on the grid of the L2 pentode, is fed to the phase inverter - a triode of the L2 lamp. In addition, the R9-C5 chain has been added to the L2 pentode grid chain to equalize the phase relationships when the input signal passes non-identical circuits. Now you can consider the push-pull stage symmetrical and enjoy the sound quality.
However, this is not all. In order for the UMZCH to work even more stable at such maximum output power values ​​for 6FZP lamps, the entire amplifier is covered by the feedback from the output to the cathode of the input triode L1 through the divider R7-R4, and from it to the grid through the resistor R3. Local EPs are also present in each cascade. The filter in the power circuit С10-Др1-С11, which reduces the coefficient of ripple of the anode voltage to 0.1%, also commands respect.

The next UMZCH for reproducing G. Krylov's recordings is hardly more complicated than the previous one. Its output power is 6 W with a nonlinear distortion of 3%; at an output power of 4 W, the harmonic distortion factor is 1%. Frequency response unevenness in the range from 25 Hz to 16 kHz - 1 dB. Input sensitivity - 170 mV. Background level -55 dB. A feature of the amplifier (Fig. 13), which consists of a pre-amplification stage, a push-pull output stage and a rectifier, is a kind of excitation circuit of the final stage without using a phase inverter.

Figure 13 Schematic diagram of Krylov's tube power amplifier

The signal from the volume control R1 is fed to the control grid of the 6Zh1P type lamp, amplified by it and fed to the control grid of the L2 output lamp of the 6P15P type. The signal voltage from the cathode of the L2 lamp is fed further to the cathode of the LZ lamp.
The signal voltage U supplied to the LZ lamp can be determined from the formula:
U = (I1 - I2) (R7 + R8),
where I1 and 12 are the variable components of the currents L2 and LZ. It is not possible to increase this voltage, since for good use LZ lamp current I should be close to 12, and it is impossible to increase the resistance of the resistor R8 due to a decrease in the anode voltage. Therefore, this circuit is of interest only when using lamps with a high slope, operating at a low excitation voltage. Of the common lamps, the 6P15P pentode meets this requirement.
To reduce harmonic distortion and reduce the output impedance, the amplifier is covered by negative feedback with a depth of 14 dB. The feedback voltage is removed from the secondary winding of the output transformer and is fed through a resistor to the cathode of the L1 lamp.
The power transformer is assembled on a core made of Ш32 plates, the thickness of the set is 32 mm, the window is 16x48 mm. The network winding contains 880, and the anode 890 turns of PEL 0.33 wire, the filament winding consists of 28 turns of PEL 0.8 wire.
The output transformer (Fig. 14) is made on a core made of Sh26 plates, the thickness of the set is 26 mm, the window is 13X39 mm. The primary winding contains 1200X 2 turns of wire PEV-2 0.19, the secondary - 88 x 3 turns of wire PEV-2 0.47. It is necessary to strictly maintain the equality of the number of turns of the sections of the secondary winding and connect the sections in parallel.

Figure 14 Schematic diagram and winding diagram of the output transformer of the tube power amplifier G. Krylov

The amplifier is mounted on a 1.5 mm aluminum chassis with dimensions 240x92X53 mm. The first stage should be as far as possible from the power and output transformers. The body to the potentiometer R1 should be connected to the chassis.
The distance between the power and output transformers must be at least 15 mm. The axes of their coils must be mutually perpendicular.
Adjusting the amplifier is reduced to adjusting the amount of feedback by changing the resistance of the resistor R10. If the amplifier is energized, then the terminals of the secondary winding of the output transformer must be reversed. To avoid self-excitation of the amplifier at ultrasonic frequencies, the feedback depth should not be more than 15 dB.
The bridge rectifier on diodes D209 can be replaced with a selenium rectifier ABC - 120-270. It is advisable to replace capacitors C5, Sat with one capacitor with a capacity of 150 μF for a voltage of 300 V. The loudspeakers of the acoustic unit must have an impedance of 8-10 ohms. The author used two 5GD10 loudspeakers connected in series.

The classical use of the properties of a push-pull circuit can be observed in the "simple * UMZCH K.Kh. Mikhailov (R-8/57). In this 6-watt amplifier (Fig. 15) there is a lamp L1 at the input - a double triode 6N2P, one half of which excites one arm of the final stage LZ and the second half of the same lamp L1, the latter in turn serves as a phase inverter to excite lamp L2.By selecting resistors R6, R11, the mode of providing symmetric excitation of the push-pull circuit is selected.

Figure 15 Schematic diagram of a tube power amplifier K.Kh. Mikhailov

A feature of the circuit is the presence of a separate tone control at the input of the UMZCH, the value of the input voltage at the same time reaches 125 mV. In addition, to ensure the stability of the amplifier in a wide frequency range, a frequency-dependent OOS R5, R11, R15-C9, R16-C10 was introduced. Indicative for such a simple circuit is the use of the filament circuit of the final stage with a symmetrical grounding of the midpoint, and for the input stage, a reduced filament voltage of 5 V is used to reduce the level of internal noise of the L1 lamp. As in the previous circuit, the cathodes of both lamps of the final stage L2 and LZ are connected to the same resistor R12, which provides additional adjustment of the symmetry of the mode.

Figure 16 Schematic diagram of the F. Kuehne tube amplifier

Figure 16 shows a diagram of a relatively simple tube power amplifier with an ultra-linear characteristic of the German specialist F. Kuehne. This device constructively combines an input switch, a preamplifier for an electromagnetic pickup with a low and high frequency filter, tone controls, as well as an output stage and a power supply. In the presence of a high-quality output transformer, the reproducible frequency band (when the tone controls are set to the middle position) has a linear characteristic in the range from 50 to 30,000 Hz. At a frequency of 30 Hz, the output power drops slightly.
Input jacks 1, 2 and 3 are designed to connect program sources that give a signal with a voltage of the order of 500 mV, that is, to supply a signal from the line output of a tape recorder, receiver or from a piezoelectric pickup. Socket 4 is provided for connecting a high quality electromagnetic studio pickup. It is connected to a two-stage pre-amplifier assembled on an L5 lamp. Depending on the position of the P2 switch, the amplifier can pass either the entire frequency band, or when the capacitor C16 is turned on, only the middle and higher frequencies. The lowest frequencies, at which vibrations of the electric motor can occur, which noticeably deteriorate the quality of the reproduction of the gramophone, are cut off.
Capacitor C17 in the grid circuit of the right (according to the diagram) triode of the L5 lamp and resistance R29 serve to raise the lowest sound frequencies. In position 5 of the switch P1, the capacitor C14 is connected in parallel with the capacitor C17, the rise of the lower frequencies decreases slightly. At the first three positions of the switch, the grid of the right (according to the diagram) triode of the L5 lamp is closed to ground, which allows the transmission of a radio program or magnetic recording to suppress interference from the input of the pickup. In position 4, the capacitor C18 cuts off the higher sound frequencies somewhat, in position 5 this effect is enhanced. Section P16 short-circuits the inputs that are this moment are not used. Therefore, when you turn the P1 switch to positions 1-3, the inputs with the same digital designation are alternately turned on, in positions 4 and 5, the fourth input (gramophone).
The tone controls (R2-R4) are placed in front of the L1 lamp, and the R8 volume control is behind it. The right triode of the L2 lamp performs the function of a phase inverter, assembled according to a divided load circuit. The final stage on LZ and L4 lamps is assembled according to an ultralinear scheme, which creates negative feedback in the shielding grid circuit. The second negative feedback circuit goes from the secondary winding of the output transformer through the resistance R20 to the cathode of the lamp L2. The output transformer should be selected based on the loudspeaker available.
Potentiometer R35 in the lamp filament circuit is designed to attenuate the background level. In addition, the resistances R36 and R37 in the filament circuit of the L1 lamp lower the filament voltage to 4.5 V, thereby reducing the level of noise and background. This, according to F. Kuehne, is a somewhat unusual scheme, and for many radio amateurs of the Union, as, for example, for Yu. Mikhailov (Fig. 15) already in 1957 (!), It is quite common, for a number of years it has been successfully applied in filament circuits of the first lamp of various amplifiers, while lowering the filament voltage did not affect the operation of the lamps.

Figure 17 Schematic diagram of the tube amplifier A. Kuzmenko

The circuit of a high-quality low-frequency tube amplifier of 8 W A. Kuzmenko (R-5/57) is similar to the previous one in many respects, even the ratings of individual circuits are the same. The author of this design (Fig. 17) believes that he has achieved an improvement in sound quality by introducing a variety of feedbacks, including OOS to the screen grids through taps 16 and IB of the output transformer Tr1, general OOS through the divider R12-R30, local OOS in the circuits excitation of all cascades.
A significant difference of this circuit from the previous one is the presence of a correcting chain R14-C7 in the anode circuit of the left lamp triode circuit L2. With the help of this chain, a decrease in the frequency response of the amplifier in the high-frequency region is achieved, which arises from the influence of several factors, the main of which can be considered the presence of local OOS, as well as the low quality of the output transformer Tr1.

Figure 18 Schematic diagram of the lamp UMZCH S. Matvienko

The later model of the broadband lamp UMZCH S. Matvienko (Fig. 18) is even more complicated than the previous ones. To achieve high-quality sound in a 10-watt amplifier, in which the output stage operates at the power limit, the author of this design adds his own elements and circuits to the circuit, which help to solve the problem - to achieve a high level of frequency response uniformity (no more than 0.1%) in wide frequency band 20 ... 30,000 kHz.
The amplifier is covered by an OOS loop, which operates in the mid-frequency range - this is the R5-R29-R12-C8 chain. In addition, all the stages are covered by the local OOS, and in this amplifier there is a pre-output stage, which creates a symmetrical antiphase excitation almost "literally" repeats the circuit of the output stage of G. Krylov (Fig. 13). However, already in the final stage, we observe an additional adjustment R27 of the cathode resistance of the LZ, L4 lamps, thanks to which it is possible to balance the modes of both lamps, and here the OOS is carried out on the screen grids from part of the turns of the primary winding of the output transformer Tr1.
The circuit also uses all the existing possibilities for controlling the timbre color of the sound signal. Separate tone control is provided at a level of 12 dB at high frequencies R14-C9, СЮ and 14 dB at low frequencies R15-C14, Др1, as well as a loudness-compensated volume control resistor R3.
For the stable operation of the UMZCH, an anode power supply with a low ripple coefficient is necessary, therefore, at the output of the rectifier, it is necessary to install a U-shaped filter from a choke and two capacitors, as, for example, in the Kusev circuit (Fig. 9) or Gen Din (Fig. 12).

Figure 19 Schematic diagram of the lamp UMZCH F. Kuehne

Next comes a series of developments by the aforementioned F. Kuehne. A schematic of a high quality 10 W amplifier is shown in Figure 19. Tone controls with separate control for high R1-C1, C2 and low frequencies R2, R3, R4 - СЗ, С4 and volume control R5 are placed at the input of the amplifier, the sensitivity of which is about 600 mV.
The pre-amplification stage is assembled on a / 11 tube. The upper (according to the diagram) triode of the L2 lamp operates in the amplification mode. Its control grid is connected directly to the anode of the L1 lamp (there is no coupling capacitor). This eliminates the phase shift element, which, under certain conditions, could cause instability in the negative feedback. Due to the direct connection, the control grid of the L2 lamp is at the same high potential (+70 V) as the anode of the L1 lamp. Therefore, the voltage at the cathode of this lamp has to be increased to 71.5 V. The difference in voltage (1.5 V) is the required grid bias.
The control grid of the upper triode through the resistance R12 is connected by direct current to the lower (according to the diagram) triode of the L2 lamp. As a result of this, and also due to the total resistance in the cathode circuit, the same bias voltage is applied to both triodes. The control grid of the lower triode through a capacitor CU is connected by alternating current with a common minus, that is, the lamp is controlled not by the grid, but by the cathode (similar to the cascode circuit). Since the signal in the control grid of the lower triode is 180 ° out of phase with respect to the control grid of the upper triode, voltages are supplied to the terminal lamps, which are also 180 ° out of phase. This method of phase rotation is characterized by high symmetry, good gain and lack of phase distortion. The final stage circuit is common.
The correcting circuit R6-C5, connected in parallel with the load resistance of the lamp L1, and the filter in the negative feedback circuit consisting of the capacitor C8 and the resistance R10, stabilize the negative feedback in the ultrasonic frequency range.
For the pre-amplification stage, the most low-noise, highly stable resistances are selected. The values ​​of the capacitor C8 and the resistance R10 are selected taking into account the total advantageous resistance of the amplifier from the following table:

The output transformer is wound on an armored core made of transformer iron with a thickness of 0.5 mm without an air gap. The cross section of the middle rod of the core is 28x28 mm. The primary winding consists of four sections, each with 1650 turns of PEL or PEV wire with a diameter of 0.11 mm. Spacers between layers of paper with a thickness of 0.03 mm. The secondary winding consists of two sections of 76 turns each, wound with two layers of wire of the same brand with a diameter of 0.6 mm with paper gaskets 0.1 mm thick.
The winding sequence is as follows. First, one of the sections of the primary winding is wound on the frame, then half of the secondary winding, then two sections of the primary winding, then the other half of the secondary, the last is wound up the fourth section of the primary winding. The two middle sections of the primary winding are connected in parallel and wound in one direction, and the rest in the opposite direction. Both end sections are also connected in parallel. The groups thus constituted are included sequentially. Also, both halves of the secondary winding are connected in series (with a speaker impedance of 16 ohms).

Figure 20 Schematic diagram of another tube UMZCH F. Kuehne

The next 20 W UMZCH F. Kuehne contains a bridge circuit for switching on the load in the final push-pull stage. In it, the constant component (Fig. 20) does not flow through the load, therefore, the anode circuit is powered in addition to the output transformer, and it is a matching autotransformer.
The power transformer has two anode voltage windings (270 V each). The constant voltage across the electrolytic capacitors C9 and CU is 290 V, the voltage in the cathode circuit at idle is 18 V. It is noteworthy that the capacitors in the power supply are not connected to the case.
The bias voltage of the terminal lamps L2 and LZ is removed from the resistances in the cathode circuit R13 and R14. It is advisable to make one of them variable in order to be able to accurately adjust the symmetry in both end lamps. The voltage on the screening grid of the lamp of one arm is supplied from the anode circuit of the lamp of the other arm. In the circuit of the screening grid of the LZ lamp, a variable resistance R17 is included, which serves to suppress the background of the alternating current. In case of strong hum, it is necessary to re-phase one of the windings of the power transformer. Resistances R7, R10 and R12, R15 in the circuits of the control and screening grids of the terminal lamps serve to protect against generation, they are soldered directly to the lamp panels.
The voltage at the cathode of the lamp L1, the upper half of which operates in the amplification mode, and the lower half serves to turn the phase, is 28 V. The lower triode is controlled through the total resistance R5 in the cathode circuit, i.e., similar to the amplifier, the circuit of which is shown in Fig. 19. To obtain the same grid bias for both triodes, it would be possible, as in Fig. 19, to connect the control grid of the lower triode to the junction point of the resistances R1, R2, R5. Instead, in the circuit under consideration, a voltage divider R3, R4, C2 is used for the lower triode, which supplies a given voltage to the control grid and simultaneously closes it to the chassis through the capacitor C2. The capacitance of the capacitor C2 is chosen large so that OOS occurs at low frequencies and the amplification at a frequency of 50 Hz is suppressed by 10% (the background becomes practically inaudible), and at a frequency of 20 Hz - by 50%. Below 20 Hz, the gain decreases sharply. Such a circuit design sometimes causes some bewilderment if we say that the amplifier should pass the widest possible bandwidth. However, a radio amateur experienced with high quality amplifiers is familiar with their whims. A tone with a frequency of 20 Hz is practically not heard. Moreover, tones of a lower frequency are not audible. If our "too good" amplifier is driven at very low, inaudible frequencies, then as a result of cross modulation with the audible tones, noise can occur, which greatly distorts the sound picture.
The final stage of the amplifier is covered by negative feedback. The optimal load of the final stage is about 800 ohms. However, even with a different load (for example, at 600 or 1600 ohms), the sound power output is 17.5 watts. The quality of the output autotransformer Tr1 is not so demanding as for conventional push-pull stages. Each lamp operates on a whole winding, and since the AC lamps are connected in parallel, the total resistance of the winding is reduced to 25% of the nominal value. In order to obtain complete symmetry and ground the output terminal, the middle tap of the winding is connected to the chassis. This clamp serves at the same time as the neutral wire of the voice coil winding, which is part of the common winding of the autotransformer.

Figure 21 Location of windings on the transformer frame

Fig. 21 shows the location of the windings on the frame of the Tr1 autotransformer. The core consists of transformer iron plates assembled without a gap. The cross-section of the middle rod of the core is different 7.3 cm2. Winding I contains 650 turns of wire PEL 0.35; winding IV - 490 turns of the same wire; winding II contains 119 turns of wire PEL 1.0; winding 111-41 turns of the same wire.

Another diagram of a high-quality terminal lamp UMZCH by F. Kuehne for 20 W is shown in Fig. 22. Basically, this amplifier repeats the previously discussed circuit solutions, which provide high-quality sound reproduction, but as a power amplifier it does not contain volume and tone controls, and it also provides the ability to connect loudspeakers with different ratings of load impedances. In the position of the switch, as shown in the diagram, the impedance of the drivers is 16 ohms. Below, under the diagram, are the switch positions for 8 ohms (left) and 4 ohms.

Figure 22 Schematic diagram of a 22 W amplifier by F. Kuehne

In all the listed Kuehne schemes, foreign-made lamps are used, the procedure for replacing them with domestic ones is given at the end of the book in a special table.
To provide increased power of the output amplifier while maintaining high-quality sound, parallel connection of the output stage lamps in each arm of the push-pull circuit is often used, as is done in the 20-watt terminal UMZCH V. Bolshoy (R-7/60).

In the amplifier circuit (Fig. 23) there are only two stages - an input phase inverter on a 6N2P double triode lamp and an output terminal stage on four 6P14P tetrode tubes. All the cathodes of the output lamps L2 ... L5 are connected at one point on the resistor of the cathode self-displacement chain R12-C6, and the DC tetrodes themselves are connected as triodes. This somewhat reduces the slope of the throughput I - V characteristic, but makes it more linear.

Figure 23

In the anode supply circuit, instead of the L6 kenotron, it is better to put a bridge of semiconductor diodes for a reverse voltage of 400 V and a forward current in the open state of 0.5 A, and also add a U-shaped smoothing filter. By the way, the filter choke is best performed on a toroidal core and covered with a grounded shield. The power transformer Tr2 is standard for a power of 200 W.

V. Shushurina (MRB-1967), similar in circuit design, but more powerful 100 W UMZCH, is designed to work with the equipment of an ensemble of electric musical instruments, and can also be used for sounding small halls and club rooms.
The rated output power of the amplifier is 100 W. Harmonic coefficient at a frequency of 1000 Hz is no more than 0.8%, at frequencies of 30 and 18000 Hz - no more than 2%. In the frequency range 30-18000 Hz, the unevenness of the frequency response is +1 dB. The nominal sensitivity is 500 mV, the nominal output voltage at a load of 12.5 Ohm is 35 V. The noise level of the amplifier relative to the nominal output level is about -70 dB. Power consumption from the network is 380 VA.

Figure 24 Schematic diagram of a 100 W tube amplifier V. Shushurin

The schematic diagram of the power amplifier is shown in Fig. 24. The first two stages are made on lamps L1 and L2a. The second triode of a 6N6P (L26) type lamp is used in a phase-inverted stage with a shared load (R10 and R12). The final stage of the amplifier is assembled according to a push-pull circuit on lamps LZ, LB, and to provide the necessary power in each arm, two lamps are connected in parallel.
To obtain a uniform frequency response and low harmonic distortion, the last three stages of the amplifier are covered by deep negative voltage feedback. The feedback voltage is removed from the secondary winding of the output transformer Tr2 and is fed through the R19C8 chain to the L2a lamp cathode circuit.
L8-L6 lamps of the final stage operate in AB mode. A negative bias to their control grids is supplied from a separate source - a half-wave rectifier on a diode D7.
The power supply of the anode circuits of the terminal lamps is carried out from a full-wave rectifier on diodes D6-D13, connected in a bridge circuit, and the power supply of the shielding grids of these lamps and the anode circuits of lamps L1 and L2 is from a rectifier on diodes D2-D5. Rectifier filters - capacitive. The capacity of the filter capacitors is chosen so that when the power delivered by the amplifier changes from zero to nominal, the supply voltages change by no more than 10%.
The power amplifier in the form of a separate, fully electrically and structurally complete unit is mounted on a metal chassis with dimensions 490X210X70 mm. All the vacuum tubes, transformers and electrolytic capacitors are mounted on top of the chassis. The rest of the parts are mounted in the basement of the chassis.
The power transformer is made on the Ш32Х80 magnetic tape. window 32X80 mm.
Winding 1-2, designed for a voltage of 220 V, contains 374 turns of wire PEV-1 1.0, winding 5-4-85 turns of wire PEV-1 0.25, winding 5-6-790 turns of wire PEV-1 0 , 55, winding 7-5-550 turns of wire PEV-1 0.41, winding of 9-10-11 turns of wire PEV-1 0.9, windings L-12 and 13-14-11 turns of wire PEV-1 1 ,4. The location of the windings on the power transformer frame is shown in Fig. 25.

Figure 25 Location of the windings on the frame of V. Shushurin's tube amplifier

The output transformer Tr2 is made on the same magnetic tape as the power transformer. The windings are sectioned. The layout of the winding sections on the frame is shown in Fig. 25.6. Primary winding 1-3 consists of four sections of wire PEV-1 0.55, 450 turns in each section. The sections are connected in series, and a branch is made from the middle (pin 2). The secondary winding 4-5 consists of ten PEV-1 0.55 wire sections connected in parallel, 130 turns in each section.
Subject to correct installation, the use of pre-checked parts and the manufacture of the output transformer according to the recommended scheme, the establishment of the power amplifier is reduced to setting the required bias voltage of the lamps of the output stage (-35 V) with the trimming resistor R41 and balancing the lamp arms of this stage with the resistor R14. It must be remembered that it is impossible to turn on the power amplifier without load, as this can cause an electrical breakdown between the windings of the output transformer "

High sound quality is also provided by a stationary power amplifier, cited by G. Gendin in the book "Homemade ULF", MRB-1964. By a strange coincidence, the circuit of this amplifier (Fig. 26) is very similar to the standard 10-watt of the Kinap company, which was in every radio center in the 60-70s, except that the lamps were replaced from 6PZS with more modern ones. The circuit of the phase inverter and the output stage is similar to that discussed above (Fig. 12), and the preliminary stages on lamps L1, / 12 accelerate the final amplifier to such a power that, in the presence of deep OOS through R26-R34, to provide the rated output power.

Figure 26 Genedin's tube power amplifier

This amplifier is distinguished by its complete functionality, it has all the necessary adjustments, at the input you can connect any sound source, be it a microphone, pickup, tape recorder, radio receiver, TV or radio transmission line. At the output, you can connect any of the available types of dynamic heads, for which a P2 switch is provided in the secondary winding of the Tr2 output transformer.
The anode circuits are powered at a low ripple level due to the presence of the C12-Dr1-C13 filter, all the midpoints of the filament windings through the trimming resistors R19, R23, and they are still fed with a bias of 27 V through the divider R16-R17. In the V1 rectifier, you can use diodes of the D226 or D7Zh type.

The high-quality UMZCH N. Zykova (R-4/66) uses together the tone controls of the lower and higher frequencies and tone controls for three fixed mid frequencies (each of which differs from the previous one by about an octave f = 2f2 = 4f3), which allows you to get almost any the frequency response of the sound reproduction channel, and also significantly increases the possible degree of correction of the amplifier characteristics at higher and lower frequencies (up to 30-40 dB). In addition, the use of midrange controls greatly simplifies the design and construction of loudspeaker systems for high-quality sound reproduction.
The nominal output power of the amplifier is 8 W. The maximum sensitivity from the pickup jacks is 100-200 mV, from the line output -0.5 V, from the transmission line -10 V. The amplifier reproduces the audio frequency band from 40 Hz to 15 kHz with unevenness at the edges of the range of 1.5 dB (without controls timbre).

Figure 27 Schematic diagram of a tube power amplifier 8 W N. Zykov

Figure 28 Scheme and variant of winding the output transformer for N. Zykov's tube amplifier

Nonlinear distortion factor at a frequency of 1 kHz at a rated output power - 0.5%; with an output power of 6W - 0.2%. The active load resistance of the amplifier is 4 Ohm, the noise level is 60 dB. The output impedance of the amplifier is 0.3 ... 0.5 Ohm. The amplifier can be powered from 110, 127 and 220 V AC mains, power consumption from the mains is 120 W.
A switching device is connected to the input of the amplifier (see Fig. 27), with the help of which a receiver P (100 mV), a TV T (100 mV), a pickup, a line output of a tape recorder M (0.5 V), a transmission line can be connected to it. L (10 ... 30 V), as well as the tape recorder input (to the linear output of the L V amplifier).
The first stage of the amplifier is assembled on a lamp L1a, it is used to amplify the signals coming from the sockets of the pickup, receiver P or TV T. In the next two stages, assembled on the lamp L2, typical tone controls for low and high frequencies of type II are included (potentiometers R7 and R10) and a midrange tone control (potentiometers R22, R23 and R 24).
To reduce the noise level, the incandescent circuits of lamps L1 and L2 connected in series are powered by a low-voltage rectifier.
An amplifier of the pre-terminal stage and a phase inverter are mounted on the LZ lamp. Good symmetry with minimal distortion in the case of large control signals is achieved by using a relatively low-resistance anode and cathodic load with the inverter phase.
The final stage of the amplifier is push-pull, it is assembled according to an ultra-linear circuit. The last three stages of the amplifier are covered by deep negative feedback, the voltage of which is removed from the secondary winding of the output transformer and fed into the cathode circuit of the LZ lamp.
The power transformer Tr1 is assembled on a core made of Sh20 plates, the thickness of the set is 45 mm. The network winding contains 2x (50 + 315) turns of PEL 0.38 wire, increasing - 700 turns of PEL 0.29 wire. The winding of the low-voltage rectifier consists of 45 turns of the same wire, and the filament winding of the lamps consists of 17 + 4 turns of PEL 1.0 wire.
The filter choke Dr1 with an inductance of 4 H is wound on a core of USh16 plates, the thickness of the set is 15 mm, its winding contains 2300 turns of PEL 0.25 wire. Coil L1 = 6.5 - wound on a core of USh12 plates, set thickness 18 mm, its winding consists of 3100 turns of PEL 0.14 wire. The L2 and L3 coils are made on armored cores of the SB-4a type. The coils are wound in bulk on cylindrical frames made of ebonite or textolite and contain 2200 turns of wire PEV-2 0.1 (inductance 0.35 ... 0.4 H).
The output transformer Tr2 is assembled on a core made of Sh19 plates with a set of 45 mm thick. Fig. 28 shows a diagram and a variant of the arrangement of its windings. The primary winding 1-6 is wound with a wire PEV-2 0.18 and contains 3000 turns, the secondary winding 7-12 - with a wire PEV-2 0.57, 180 turns. The pins are located so as to make the short jumpers of pins 3-4, 7-9-11, 8-10-12. You need to put tubes on the terminals and unsolder them on the mounting blocks installed on the transformer.

The advantage of A. Baev's low-frequency power amplifier (MRB-1967) is that it is assembled from widespread radio components, its electrical circuit is well developed and, when repeated, can be easily adjusted using one voltammeter. The amplifier develops a maximum output power of 30 or 60 watts, depending on how many lamps operate in the output stage (two or four).
The band of reproducible frequencies is 30 ... 18000 Hz; nonlinearity of the frequency response no more than 3 dB. The sensitivity in the "Microphone" operating mode is about 5 mV, and in the "Pickup" mode - 150 mV. The amplifier is powered by a 220 V network; power consumption 80-160 W depending on the output power.

Figure 29 Diagram of the tube amplifier A. Baev

Less powerful, but better quality is the circuit of the portable audio frequency amplifier B. Morozov (MRB-1965). The described amplifier (Fig. 31) can find the widest application in the radioification of rural clubs and houses of culture, schools and other audiences.

Figure 31 Scheme of a tube power amplifier B. Morozov

The nominal output power of the amplifier is 35 W, and the maximum is 45. It reproduces the frequency band in the range from 20 Hz to 20 kHz. The frequency response of the amplifier has a 3 dB cut at a frequency of 20 kHz and a rise at a frequency of 20 Hz +7 dB. The unevenness of the frequency response in the frequency range from 40 Hz to 12 kHz does not exceed +1 dB. Harmonic distortion at power up to 25 W is practically absent, the noise level at maximum gain and short-circuited input is 48 dB. Under the same conditions and the included microphone stage, the noise level is 40 dB. The amplifier output is 24 V, rated for 18 ohms, 12 V for 4.5 ohms, and 3 V for 0.28 ohms.
Each input of the bass amplifier has its own volume control, which allows you to make combined recordings, for example, to record speech against the background of music. The microphone stage of the amplifier is assembled according to the rheostat-capacitive circuit on the left (according to the circuit) triode of the L1 lamp of the 6H9 type. The second stage of the amplifier is assembled on the right triode of the 6H9 lamp; it is a conventional voltage amplifier. Resistor R14 is the ohmic equivalent of a microphone stage. This resistance maintains the preset mode of the L1 lamp when the microphone stage is turned off. The filament of the L1 lamp is powered direct current, which significantly reduces the background level of the entire amplifier, when the microphone stage does not work (the amplifier operates from another signal source), the anode power supply of the microphone stage lamp should be turned off with the Vk2 switch. When operating from the "Sv" pickup and the "L" transmission line, the signal, bypassing the microphone stage, immediately enters the tube grid of the first voltage amplifier. Resistors R15, R16 and R6, R7 form a voltage divider that allows you to receive equal signals from the pickup, transmission line and microphones.
Thanks to such a deep negative feedback (20 dB), the frequency and nonlinear distortions introduced by the final and pre-final stages are sharply reduced, as well as the dependence of the output voltage level on the load resistance decreases. "
For the symmetry of the pre-final stage in the entire frequency range, a balancing capacitor C17 is connected in parallel to the resistance R38 (390 kOhm). By shunting resistance R32, it compensates for the drop in the frequency response at higher sound frequencies. To exclude self-excitation of the amplifier at high frequencies, resistance R32 is included in the grid circuit of the upper (according to the diagram) triode of the 6HV lamp.
The final stage of the amplifier is assembled according to a push-pull circuit on four 6PZ lamps; it works in the class AB1 mode. Each of the 6PZ lamps is loaded onto a separate winding of the output transformer. To combat high-frequency generation, resistances R39, R41, R42, R43, R44, R45, R46, R47 are included in the control and screen grids circuit of each of the lamps.
The negative bias is supplied from a special rectifier, which makes the operation of the final stage more stable and also reduces the distortion introduced by it.
The amplifier is powered by a rectifier assembled in a bridge circuit on 16 diodes of the D7Zh type. Diodes are shunted with 100 kΩ resistors, which protect them from breakdown in the event that the resistance of the diodes to the reverse current will differ sharply from each other (the resistance of the diodes to the reverse current must be at least 200 kΩ),
The power transformer Tr1 is assembled on a core made of Sh-40 plates, the thickness of the set is 60 mm. All transformer windings are wound on a common getinax frame. The mains winding is wound first. It contains 250 turns of PEL 0.93 wire and 190 turns of PEL 0.74 wire. Both sections are included in series. A winding II of the incandescence of 6PZ lamps, connected in series, is wound on the mains winding. It contains 50 turns of PEL 0.8 wire with a tap from the 25th turn, which is grounded. This winding simultaneously shields the mains winding from others. A step-up winding is wound over the filament winding, which consists of 920 turns of PEL 0.35 wire. On this winding, 13 turns of PEL 0.8 wire are wound from one edge to power the incandescence of the L2 and LZ lamps, and then, stepping back 3 mm from the filament winding, in the same row, a winding is wound in two layers to power the displacement rectifier, which contains 160 , turns of wire PEL 0.15. When winding the transformer, waxed paper is laid between the rows, and two layers of varnished cloth between the windings.
The choke is made on the Sh26xZO core by winding 2000 turns of PEL 0.31 wire. For the output transformer, a set of Sh25 plates with a thickness of 60 mm is used. The anode winding consists of four sections of 1350 turns of PEL 0.2 wire. The secondary winding consists of five sections, four contain 80 turns of PEL 0.66 wire and one - 25 turns of PEL 1.5. First, one section I of the secondary winding is wound in one layer. On top of it, two layers of varnished cloth are wound, then - section II of the anode winding in five layers, laying them with a layer of varnished cloth or two layers of thin waxed paper. Over the section of the primary winding, two layers of varnished cloth are wound, then the section of the secondary winding is wound, then again the primary, and so on. The last will be the fifth section of the secondary winding. The order of winding is shown by serial numbers in the diagram.

The high-quality stereo amplifier of I. Stepin (MRB-1967) can work both with a piezoelectric pickup and with a receiver with a VHF range and a special attachment for receiving stereophonic transmissions. The amplifier has high gain and high sensitivity. From the input of the pickup, it is at least 100 mV. The limits of the tone control of the amplifiers are 15-20 dB at the lowest sound frequencies and 12-16 dB at the highest. The volume control range for each channel is 40 dB. The amplifier reproduces the audio frequency band from 50 to 13000 Hz with an uneven frequency response of 6 dB.
The imbalance of volume control, timbre and frequency characteristics of amplifiers for both channels does not exceed 4 dB. Crosstalk attenuation at a frequency of 1000 Hz is about 45 dB, at a frequency of 10000 Hz - 30 dB. Due to the use of separate power supply of the final and preliminary amplification stages, the background level at the amplifier output at a nominal output power of 10 W (for each channel) and an open input is not worse than 50 dB. Nonlinear distortion factor at rated output power no more than 4%. Power consumption 130 watts.

A schematic of one channel of a full stereo tube amplifier with tone control is shown in Figure 33. It can operate from any (including high-impedance) source of audio signals, providing an output voltage of at least 0.25 V. A distinctive feature of the amplifier is the use of high-symmetry pre-amplification stages and the use of cross-feedback, stabilizing the operating modes and parameters of the UMZCH.

Figure 33 Schematic diagram of a tube power amplifier E. Sergievsky

Main technical characteristics: Rated input voltage 0.25V. Input impedance, 1 MΩ. Rated (maximum) output power 18 (25) W. The nominal range of reproducible frequencies is 20 ... 20,000 Hz. Harmonic distortion at an output power of 1 W in the nominal frequency range of 0.05%. Relative noise level (unweighted value) no more - 85 dB. The slew rate of the output voltage is not less than 25 V / μs. The tone control range is -15 ... + 15dB.
The input signal through the R1 stereo balance control and the loudness control on the Cl, C2, СЗ, R2-R4 elements is fed to the input of the first UMZCH stage, assembled on a low-noise 6Zh32P (VL1) pentode. Nuvistor 6S62H with better noise characteristics can also be used in this stage (Fig. 34). It is only important that the voltage gain of this stage is more than 50, which will make it possible to compensate for the signal attenuation at the edges of the reproduced frequency range introduced by the tone control.

Figure 34 Using a Lower Noise Input Stage

Figure 35 Drawing of the printed circuit board of the tube power amplifier E. Sergievsky

The phase-inverted and pre-terminal stages are covered by cross-feedback, which compensates for the effect of mounting capacitance and improves the phase relationships of inverse signals at higher audio frequencies. The chains of this connection are formed by capacitors C13-C16. In addition to cross-feedback, the amplifier covers three main feedback loops. The voltage of the first of them is removed from the secondary winding of the output transformer T1 and through the circuit R34, C 17 is fed to the input (control grid of the VL2.2 lamp) of the phase inverter, the voltage of the second is removed from the anode loads of the lamps of the final stage VL5, VL6 and is fed through the R28C26 and R35C25 circuits to the cathodes of the triodes of the pre-final stage VL4.1 and VL4.2. And finally, the third OOS chain covers only the final stage along the screening grids.
UMZCH is mounted on a printed circuit board made of foil-clad fiberglass with a thickness of 1.5 mm (Fig. 35). For installation, constant MLT resistors, variables SZ-ZOV-B (Rl, R2, R13, R15), SZ-ZOa (R22) and C5-5 (R42), capacitors K50-12 (C19-C22, C27-C29) were used , K73-5 (C23-C26), KT (C13-C16) and KM (the rest).
The output transformer is made on an armored tape radio tape SHL25X40 (tape thickness 0.1 mm). It is also possible to use an Ш-shaped magnetic core made of Ш25 plates and a set thickness of 40 mm. Windings 1-2 and 13-14 each contain 50, and 6-7-8-9 - 15 + 15 + 15 turns of wire PEV-2 1.0, windings 5-4-3 and 10-11-12 consist of 600 +800 turns of wire PEV-2 0.2.
When winding the output transformer, it is necessary to ensure strict symmetry of the halves of its primary winding, dividing the frame into two identical parts with a partition parallel to the side ones. Before setting up the UMZCH, it is necessary to carefully check the correct installation and reliability of the rations. Then, after turning on the power, measure the voltages in the filament circuits of all lamps (they should be in the range of 6.3 ... 6.6 V), on their electrodes and on capacitors C20-C22 and C28, C29 (their permissible deviation from those indicated on schematic diagram should not exceed 5%).
Further, setting the tone controls to the middle position, and the signal level control to the maximum volume position, apply a sinusoidal signal with a frequency of 1 kHz and a level of 0.1 V to the amplifier input. Then, alternately connecting the oscilloscope to the control grids of the VL5 and VL6 lamps, you need to check the shape of the positive and negative half-waves of the signal with a smooth increase in the voltage at the amplifier input (up to saturation). Having finished this operation, it is necessary to achieve complete symmetry and equality of the amplitudes of the monitored signals on the grids of the output lamps with the trimming resistor R22 with an accuracy of 0.05 V.
After that, having connected to the secondary winding of the transformer T1 a load equivalent in the form of a constant resistor with a resistance of 16 Ohms and a power of 20 W and setting a voltage of 0.25 V at the amplifier input, you should check the alternating voltages on the electrodes of all lamps for compliance with those indicated in the schematic diagram.
Further, by monitoring the voltage at the equivalent of the load resistance, by its maximum value, experimentally find the place of the output of the secondary winding of the transformer, to which the OOS R34-C17 circuit should be connected. Then, having measured the nominal (with an input signal of 0.25 V) and maximum (with barely noticeable saturation) voltage at the equivalent load resistance, use a well-known formula to determine the nominal and maximum power of the amplifier.
The schematic diagram shows a variant of connecting a load with a resistance of 16 ohms. To operate an amplifier with an AC resistance of 8 ohms, when adjusting the amplifier, you should connect the corresponding load equivalent to it and, according to the method described above, select a new place for tapping the secondary winding of the output transformer.

Again the construction of the author already known from this book. This is a powerful two-channel UMZCH A. Baeva (MRB-1974). This design cannot be classified as multichannel, because both channels are identical and can be used simultaneously in the "double mono" mode (analog "stereo" for signals with a large stereo base or "quasi-stereo" for large rooms or pads) or "quad" in the presence of two sets of amplifier.
The amplifier has the following data: maximum power per channel 65 W, channel load resistance 14 Ohm, frequency band 20 ... 40,000 Hz with a harmonic distortion factor of 0.6 ... 0.8%, sensitivity from the microphone input. 5 ... 0.6 mV, from input 3-20 mV, from input 4 0.8 V. Separate tone control at frequencies of 40 Hz and 15 kHz within 15 dB.

Figure 36 Schematic diagram of the power amplifier A. Baev

A schematic diagram of one channel is shown in Fig. 36. Microphone amplifiers are assembled on transistors T1 - T4. To obtain a good signal-to-noise ratio and high input impedance, their first stages are assembled on field-effect transistors. The stages are covered with negative current feedback (through resistors R3 and R13), due to which they have a high input impedance throughout the entire operating frequency range. To reduce the output impedance of the first stages, the source current is chosen large enough - about 0.8 mA. Despite this, the noise level at their outputs is very low, since the noise of the field-effect transistors does not depend on the current in the channel.
From the drains of the transistors T1 and T3, the signals go through the separating capacitors C2 and C6 to the second stages of amplifiers, assembled on transistors T2 and T4. Resistors R4, R6, R14 and R16 are feedback elements, and resistors R4 and R14, in addition, serve to select and stabilize the operating mode of transistors.
Variable resistors R7 and R17 are used to adjust the volume of signals sent to the microphone amplifiers.
To eliminate the background of the alternating current, the filaments of the lamps L1 and L2 are powered by direct current supplied from a rectifier assembled on diodes D17, D18 (Fig. 37). For the same purpose, into the heating circuit of the LZ lamp from the R55 divider. R56 is supplied with a positive (with respect to the cathode) voltage of 50 V.

Figure 37 Schematic diagram of the power supply of the tube power amplifier A. Baev

Figure 38 Design of the output transformer of the power amplifier A. Baev

The review of single-channel push-pull amplifiers is completed by the scheme of K. Weisbein's stereophonic bridge UMZCH (Raz / 99), recently published in the Raduamator magazine. The author believes that the output transformer is the most critical component of any high quality audio amplifier, as it creates many types of distortion. The output stage of the proposed amplifier is built according to the scheme of a series-parallel push-pull amplifier (PPP-Push-Pull-Parallel), proposed by the German engineer Futterman in 1953.The cascade is a bridge, two arms of which are formed by the internal resistances of the output tubes, and the other two - by the source resistances anode power supply.
The constant components of the anode currents of the lamps flow through the load in antiphase, therefore, there is no constant magnetization of the output transformer, as in a conventional push-pull amplifier. The variable components of the anode currents of the output lamps flow through the load in phase, since antiphase voltages are applied to the lamp grids.
If in a conventional push-pull amplifier, AC output lamps are connected in series, then in an antiparallel amplifier - in parallel. Therefore, the optimal load resistance for an antiparallel amplifier is 4 times less than for a conventional push-pull amplifier. This means that the inductance of the primary winding of the output transformer in an antiparallel amplifier with the same nonlinear distortions at a given lowest frequency will be 4 times less than in the usual one. The design of the output transformer is greatly simplified. In an antiparallel amplifier, the output transformer can be replaced with a kind of autotransformer with a midpoint, which will lead to a decrease in distortion at higher frequencies due to leakage inductance and distributed capacities between the windings of the output transformer. The schematic diagram of the amplifier is shown in Fig. 39.

Figure 39 Scheme of a tube power amplifier K. Weisbein

The technical characteristics of the UMZCH are as follows. Output power with harmonic distortion less than 1% 20 W. Input sensitivity 250 mV. The sensitivity of the power amplifier is 0.5 V. The frequency range is 10-70,000 Hz. Load resistance 2, 4, 8, 16 Ohm. The tone control range is 10 dB.
The first stage of the amplifier is made on half of the 6N23P tube (6N1P, 6N2P, 6N4P), the second stage is a conventional resistive amplifier. A wide-range tone control is included between the first and second stages. A P2K switch is used as a potentiometer.
The use of a phase-inverter stage, assembled according to a cathodic coupled circuit (VL3), provides high symmetry of the output voltages in a wide frequency range and low harmonic distortion. With the previous stage (VL2), which is a cathode follower, the phase inverter stage is galvanically coupled to reduce phase shift at low frequencies, which improves the stability of the amplifier.
The output stage is assembled according to the PPR scheme on 6P41S lamps, which have sufficient power and a small internal resistance (12 kOhm). Instead of 6P41S, 6PZS, 6P27S, EL34 lamps can be used. The amplifier is covered by negative feedback, the voltage of which is fed through a resistor from the output winding of the autotransformer to the cathode circuit of the first stage of the power amplifier.
The amplifier is powered from two identical half-wave rectifiers on D237B diodes. The power transformer has 4 anode voltage windings of 240 V each. It is noteworthy that the capacitors in the power supply are not connected to the case.
The power transformer is wound on a toroidal core. It is better if each channel of the stereo amplifier has a separate power transformer. The amplifier provides for separate switching on of the filament and anode voltages, which makes it possible to increase the resource of the output lamps.
The amplifier is mounted on a metal chassis using a surface-mounted mounting method using circuit boards, as well as lamp panel petals, which reduces pickup and mounting capacity.
Establishment comes down to checking the correct installation. The voltage drop between the cathode of the cathode follower and the cathodes of the phase inverter lamp should be 2 V. When the amplifier is correctly assembled between the terminals 10 and 13 of the output transformer, the voltage should be zero. If the background appears, it is necessary to re-phase one of the anode windings of the power transformer.

Figure 40 Location of the windings of the output transformer of K. Weisbein's amplifier

The design of the output transformer (Fig. 40) should be discussed in more detail. The transformer is wound with PEV-2 wire on a toroidal magnetic tape assembled from steel tape 0.35 mm thick and 50 mm wide. The outer diameter of the torus is 80 mm, the inner diameter is 50 mm. Steel grade EZZO. The winding is divided into sections to reduce leakage inductance and obtain high symmetry of the two winding halves. The winding data of the transformer are given in the table. The output transformer can also be made on an E-shaped core with a cross section of 7-8 cm, the windings of which are divided into sections. The sections are connected in series with each other.

We have long been accustomed to the fact that we are everywhere surrounded by microelectronics, transistor technology. In TVs, players, receivers, tape recorders everywhere we hear the sound in the speakers, amplified by special microcircuits, which 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. Proudly worn by mods receivers that were powered by special batteries- anode batteries and batteries for heating lamps, it was then just a miracle that you could receive and hear a radio on the go.
Lamps were very widespread. In cinemas, there were powerful tube amplifiers at the output of which usually two lamps G-807, 6R3S were used, less often GU-80.
And the famous mobile cinema installations "KINAP" of Odessa production for an alternating voltage of 110v, which were powered from a standard network through an autotransformer, at the output of the amplifier were the famous 6P3S lamps - lamps that were used in homemade transmitters at medium wavelengths and it was a couple of trifles to make it, having still a tube receiver , a microphone and a wire antenna stretched in the yard, through which one could communicate over the air with a friend from the next 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 power output stages of transmitters, radar technology, but nevertheless, the technical process is moving forward.
What attracts a tube amplifier?
The first and most important thing is high-quality reproducible sound. The amplifier has primarily low distortion and high 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 hear it,” that is, you hear music, in the smallest subtleties each instrument is music around you, you merged with it and nothing else exists, you are nervous.

Leg amplifier circuits

Construction scheme
According to the construction scheme, amplifiers can be divided:
1. First of all, single-ended or push-pull - one lamp or two lamps in the so-called push-pull connection are used in the ULF output stage. In the push-pull version, it is possible to obtain high power at the output, with a 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 on the corresponding speaker system - speakers.
An amplifier consists of several sequential stages, as a rule:

• a pre-amplifier, sometimes called a microphone amplifier;
• amplification stage;
• repeater;
• bass reflex (with push-pull design);
• driver (for driving powerful output stages);
• output stage with a transformer in the load;
• load - acoustic system, speakers, headphones;
• power supply unit for different voltages: incandescence 6.3 (12.6), anode voltage 250V (300V and higher, depending on the lamps used in the output stage);
• case (metal chassis), since the transformer is heavy, and there are at least two of them in the circuit - power and output.

The diagram of the tube amplifier is shown. Pentode input amplifier, ECF80 lamp (6BL8, 6F1P, 7199), 6AN8A triode, output stage based on KT88 or KT90 or EL156 beam tetrode, as a 5U4G kenotron rectifier. Output transformer for the Tanso XE205 single-ended tube amplifier. The power transformer in the anode winding has taps that switch depending on the used output lamp.
The main specifications tube ULF, an example is shown in brackets - the parameters of the amplifier on the famous 300B tube.
Power - W, into ohms. (twenty)
Frequency response band - Hz, kHz (5 -80,000)
Load resistance - Ohm (4-8)
Input sensitivity, mV (775)
Signal to noise ratio (no noise) dB (90)
Total harmonic distortion, 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
dimensions, mm
Price

Components for manufacturing

Tube Amplifier Accessories
Output transformer... One of the most important elements of a high-quality audio circuitry is the output transformer used. Applied high quality audio output transformers for Hashimoto, Tamura, Elektra-Print, Tribute, James Audio, Lundahl, Hirata Tango, AUDIO NOTE, etc.
Capacitors... To create the desired 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 between the amplifier stages, then the outer plate is connected to a lower impedance, i.e. to the driver, if as a blocking, then the outer plate is to the ground, in the picture the outer cover is marked with a stripe.

In the photo there are capacitors for amplifiers of low-frequency sound Jensen audio capacitors, aluminum, copper, silver are used as foil, respectively, the price varies widely. Audio line capacitor manufacturers: Audio Note, TFTF, Mundorf, Jensen, Duelund CAST and others. Frequency response varies depending on the version: paper case - copper foil, copper case and copper plates, staniol - mylar in oil, aluminum foil in an aluminum case and silver-plated leads, so fans of high-quality sound take various measurements of the characteristics of parts to determine the best price ratio - quality. Electrolytic capacitors have a wide range of choices: Black Gate and others. For cathode circuits, Caddock is preferred.
Switches
Resistors. Various resistors are used for manufacturing: Audio Note tantalum resistors, Beyschlag metal-film resistors, Allen-Bradley, etc.
Lamps... Since we are talking about lovers of tube sound, then one of the main elements for construction is a lamp. Domestic lamps 6n2p, 6n8s, 6p3s, 6p14p, 6s33s, 6r3s. Fascinated by perfect sound, true lovers of tube sound prefer only NOS lamps - these are completely new lamps that have been released a long time ago, an example is the 6AC5GT, 45 lamps (the lamp 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 being applied. But many people prefer vintage lamps.
Electronic tube manufacturers.
German - Telefunken, Valvo, Siemens, Lorenz. Europe - Amperex, Philips, Mazda. England - Mullard, Genalex, Brimar. America - RCA, Raytheon, General Electrics, Sylvania and others. Amplifier tubes 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.
Many high-quality amplifiers are produced (produced) in the world.
Audio amplifiers are loaded on the speaker system, but there are also quite a few of those who sometimes want to listen to music on headphones, for example, MrSpeakers Alpha Dog.

On the picture. Stereo amplifier MB520 20W, price £ 950 or more, bandwidth 15Hz ~ 35kHz, S / N ratio 82dB, load impedance 8/16 Ohm, size 412x185x415 mm. A preamplifier on EF86, a 12AU7 tube is used as a phase inverter, a rectifier for each channel is on 5AR4, output tubes are EL34. Stainless steel is used. Motor driven attenuator controlled by remote control, position is indicated by green LED.
The MB805 is a monoblock amplifier, priced at £ 5,999. Power per channel (load 8 Ohm) 50W, signal-to-noise level is -90db.
MB81. Mono amplifier on GU-81, cost £ 12,500. The signal-to-noise ratio is -100dB, flatness in the frequency band 20 Hz - 20 kHz - 1dB, load 4Ω - 16Ω. Input sensitivity 600mV, input impedance 100k. Power consumption from the network 220/240/115 volts average 450watts, 750w max. To a load of 8 ohms, the output is 200 watts. Input amplifier on a 6SL7, 6SN7 tube, drivers on two EL34s.
SE (single-end) - single-ended output, meaning the signal is amplified unchanged.

Videos for tube sound lovers

Eimac 250TH Audio Amplifier

Video of the work of the tube amplifier with a demonstration of the reproduction of musical reproduction.

We continue our review of Chinese tube audio equipment.
Consider in the review a preamplifier-buffer based on 6N3 (6N3P) tubes.

Why do you need audio preamplifiers?

1. Amplify the signal (voltage) to play louder. To the voltage level of the signal that the power amplifier can drive.
2. Amplify the current signal (the signal voltage does not change much, it may even be less than the original signal). Used for "pumping" low-power sources such as DAC (DAC), zv. cards, cell phones, etc. to power amplifiers or high impedance headphones. The result of the work of the amplifier-buffer is that the signal will sound in more detail.
3. Reduction of distortion. It seems odd that adding another amplification stage can reduce distortion. Otherwise, all amplifier circuits would consist of one transistor (lamp, microcircuit). It all depends on the resistance of the signal source and input. resistance of the signal receiver (note, at sound frequencies). The ideal ratio is low (preferably near zero ohms) resistance at the output of the signal source and high (several times or orders of magnitude) at the ULF input gives the best signal transmission quality without distortion. In real life, this is not always the case. The mismatch of the input-output impedances leads to an increase in distortion. To solve this problem, another cascade buffer appears. This amplification stage has a very low output impedance. Its task is matching: a source-amplifier of a signal.
4. Commutation, tone control, sound enhancers, sound. processors and so on are usually built into preamplifiers.

In our case, a simple tube preamp. P 4. is missing. P 1 works only in the case of a load with a resistance greater than 200 ohms. P2 and P3 work fine.

Let's move on to the review of this device.

I asked for this amp for review back in May 2016. How much the amp cost then - I don't remember. I put up the current price at the moment in this store. We agreed to send it in November. They sent it on December 15, 2016. And the parcel arrived on January 13. 2017 Nov.

The parcel arrived in a serious package - a bubble wrap, lamps, a power transformer are separately packed and so on.




Lamps. Asterisk on double triodes 6N3. Perhaps military (Chinese acceptance)?

Board dimensions (with lamps installed):

Power transformer:


The "ears" of the trance attachment were somehow not bent. Aligned with pliers and a screwdriver.

I have only one primary winding for 220 V (red). They promised for another 110 V. Well, okay. Not relevant yet.
Secondary blue - 170 V, white - 6.3 V. You can check all three windings with a tester. The winding with the highest resistance is the primary (220 V), the second (170 V) is the anode voltage, with the lowest resistance is the incandescence of the lamps. To be sure, I connected the trance to a 220 V network (through a 1 A fuse) and checked the voltage on the secondary windings with a tester.

Having disconnected from the network, we insert two lamps into the panels and connect the amplifier to the power transformer. Everything is indicated on the amp. The terminals on the board are excellent. Everything is pretty isolated. But with an anode voltage under 200 V, it is better not to go into a working amplifier once again with your finger.

There is no stupid illumination of electronic lamps with LEDs in my copy (but there is a place for soldering :-). Backlight - only natural :-)

We use it as a preamplifier-buffer
We connect to a signal source and a power amplifier. On the upper side of the amplifier, everything is indicated where to connect.

I have it connected like this: PC (Flac) -> DAC Constantine + (Philips TDA 1545A + Analog Devices 826 opamp) connected via USB -> before the subject -> Pioneer A-777 amplifier -> Mission M51 bookshelf speakers. A pair of amps + speakers give a neutral sound.

Based on the results of the hearing. Listen only after the lamps have warmed up. 20 minutes after switching on. Otherwise, the "sand" from the speakers plays. The volume control is normal. Those. does not crack, at the minimum volume no sound is heard, with the balance everything is ok, there is no crackling when turning the knob. Surprisingly, the device is definitely a plus. Usually, the Chinese varnishes in the regulation of the sound are kosyachnye.

Adding the device from the review to the sound path - the sound became more saturated, the high frequencies began to sound clearer, cymbals and brushes on the drums sound clear. There is no lamp heat. There is no bubnezh. There are no background noises, pickups and other bad sounds. A trance of this design, too, does not interfere with the amplifier and does not "hum". In 20 minutes after turning on the trance heats up to 30 degrees. This is a warm one and it works. The sound has become a little "softer". The bass has become more pronounced and, how to say, velvety :-). Guitar solos - everything is ok. This is the first Chinese tube device, after turning on which you can normally listen to heavy music (and everything else). I listened to my standard test discs - Gamma Ray (Land of the Free II) and Blackmore's Night (Under a Violet Moon). Everything plays great. More interesting than without this thing.

At a low volume level (at night we listen to music through the speakers) the preamplifier also gives an excellent result.

Then I listened to a harder one - Amon Amarth (Jomsviking) - the same, everything is fine.

I also listened to Vera Brezhnev from the browser about knowing the password - also ok ;-)

These are the conclusions from listening.

Use as a headphone amplifier
Headphones must have an impedance of 200 ohms. Otherwise, there will be no sound volume amplification. The higher the headphone impedance, the greater the sound amplification. I have a Beyerdynamic DT 990 Pro 250 Ohm monitor. Path - computer (Flac) -> DAC Constantine + (Philips TDA 1545A + Analog Devices 826 opamp) via USB connected -> pre subject -> Beyerdynamic DT 990 Pro. There is almost no gain in volume. It is comfortable to listen at half of the volume control of the subject. The sound results are the same as when connecting a large amp. Only the sound is a bit "gesture" (the peculiarity of the headphones is that they are "hard").


conclusions
I really liked the sound of this amp. I would like to use a pre or headphone amp. You will need it in the norms. place the body.

This concludes the usual part of the review.

Technical part of the review

Photo of the board




Anode voltage supply filter capacitor:


Top cover (aka screen):




Measurement results
We supply a signal to the amplifier - sine 1 kHz 0.3 V (like the output from the headphone jack of a cell phone)


Preamp volume control to maximum.
Amplifier load - 50 ohms. As you can see from the oscilloscope readings, the amplifier does not amplify the signal with such an output load, but on the contrary decreases:


Amplifier load - 150 ohms. As you can see from the oscilloscope readings, the amplifier does not amplify the signal with such an output load, but on the contrary does not reduce it much:


Amplifier load - 300 ohms. As you can see from the oscilloscope readings, the amplifier amplifies the signal with such an output load:


Without load. More precisely, the load is a 50 kOhm volume control installed at the amplifier output:


We supply a signal to the amplifier - a rectangle of 1 kHz 0.3 V. At the output:


We supply a signal to the amplifier - a triangle of 1 kHz 0.3 V. At the output:

RMAA 6.4.1:

Amplifier circuit:

The product is provided for writing a review by the store. The review is published in accordance with clause 18 of the Site Rules.

In the article, I tried to convey some experience of building a universal pre-amplifier using tubes.
Why on lamps?
Because this design initially assumed joint work as part of a tube audio complex, which, in addition to it, should include two tube monoblocks (single-cycle, 6E5P + GU-50).
The monoblocks are not yet ready, but during the test listening, an integral stereo amplifier of a similar composition was used, which together showed good results.
Ultimately, it all comes down to the speakers. The higher their quality, the less it is necessary to interfere with the sound path.
A well-tuned and properly made amplifier (not necessarily a tube amplifier), together with good acoustics, does not require the use of various kinds of "enhancers" and "optimizers" (IMHO). This is the ideal.
Well, what to do with our small-sized rooms, limited by financial means (for the majority, I think), when you still want not only to listen to your favorite music, but also to get a certain dose of adrenaline and feel the drive?

When my good friend, a wonderful athlete, music lover and life-lover asked me to build him a preamplifier for a home stereo complex, the TOR (technical task, you know) sounded like this:
- that it is obligatory to be lamp;
- so that there is loudness, but in moderation;
- so that the bass and treble could be turned "to the fullest";
-to get at least 4 signal sources on it;
-to adjust the volume separately by channel;
- so that you can "play around" with the interconnect cable from the pre to the UMZCH;
- well, and so that the design is a "brick" (such, you know, a high-tech computer), well, hide the lamps, otherwise there is a lot of dust all around.
These are the initial parameters. Twist, Shura, twist!: dance:

It was decided to put this volume control on a resistor with one tap at the input after the switch. In fact, the load is also "not sugar", you need to check how it will behave after all the already tested signal sources. It turned out that it was pretty decent, so we finish the prototyping, draw the diagram -

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The power transformer TAN-1 127 / 220-50 is used. The anode voltage is obtained by the doubling circuit and has no characteristic features.
The glow voltage is stabilized, the reed switches and indication circuits of the switched on signal source, as well as the high voltage turn-on delay circuit (for about 40 seconds), assembled on an adjustable Zener diode SR1, transistor T2, relay RL1 (RES-48 passport RS 4.590. 204, 6 Volts, 42 ohms) and timing elements R5C9. For normal operation of the delay circuit, the capacitor C9 must be with a low leakage current, here it is composed of two tantalum electrolytes of Soviet production connected in parallel. Diode D13 - discharge, allows you to quickly restore the delay circuit after power off.
The input selector switch can be used almost any, well, for example, a biscuit. It does not affect the passage of the sound signal, but only switches the corresponding reed switch coils and the indication circuit of the selected source. I had at hand some kind of 2-section imported biscuit maker, for 5 positions, tk. the number of inputs is four, the 5th pairs of contacts are not used. Indicator LEDs are selected in blue, with a diameter of 3 mm and fit well into the "interior". In their place, almost any LEDs can work, including incandescent light bulbs - whoever likes what.
Instead of the commonly used method of supplying a "raising" potential to the incandescent circuit of lamps from a resistive voltage divider (to protect against breakdown of the filament-cathode and eliminate the background), here is used the method of connecting the glow bus after the stabilizer to the ground, often used abroad, through the high-voltage capacitor C11.
The power transformer windings are wired appropriately to obtain the desired voltages and currents. In a standard TAN-1, two 6.3-volt filament windings are connected in parallel (which is a little small, but what can you do), to obtain the operating filament voltage (6.1 V), a transistor (KT819) with a low collector-emitter saturation voltage and current gain is used h21э is about 80. On the board, it is installed on a small heatsink, for which there are mounting holes.

installed vertically on 2 x 10 mm racks -

The variable resistors themselves are of a printed wiring design -

They are installed on one side of the printed circuit board, all other elements on the other.
Reed switch board:

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located on the rear wall, near the input RCA connectors, also on 2 x 10 mm stands used for mounting printed circuit boards.
The board is double-sided, on the one hand there are printed tracks, on the other there is a screen with holes countersunk under the legs.
The reed switches themselves - with two groups of contacts, of Chinese origin (how can we do without them),
as already mentioned type TRR-2A-05-D-00 in DIP package.
Silicon low-power self-induction damping diodes are soldered directly to the corresponding legs of the reed switches. Both boards are closed on top with a screen also made of foil fiberglass. All of them are connected to the zero bus.
The power supply is also assembled on a printed circuit board

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All the insides are visible in the photo:

I think no special comments are needed here. The neutral core is made of 1.5 mm copper wire and is connected to the body at one point on the back wall of the block. The power filter capacitors C3, C4, C13 and C14 are mounted directly on the petals of the mounting panels near the lamps.
All controls are brought out, at the very bottom there is a power switch, a little higher is a 4-position switch for controlling the switching of reed switches and an input selector indicator, 4 blue indicator LEDs, then separate volume controls and controls for bass and treble.
The design does not contain any super audiophile details, all resistors are MLT type, designed for the corresponding power, film capacitors, types K73-9, K73-11, K73-17, also for the corresponding voltages. Electrolytes made in Taiwan are similar to our K50-35 for 400 V.
Almost any transistors in the power supply can be used, suitable for the parameters indicated in the diagram, their choice is not critical. Diodes in the anode power supply - any 600 V fast and a current of at least 1 A, and in a filament rectifier - you can use any diode assembly for a current of at least 3 A and a voltage of 50 V. The 0.5 mH choke is from an old German phone, you can put any other one or replace it with a resistor of one hundred ohms (1 watt).
The power transformer TAN-1 was also chosen because it ended up in old storage facilities. It is secured to the middle shelf via a rubber mat and operates quietly without hum.
To eliminate unnecessary vibrations and microphonic effects, four rubber feet from an old phone are glued to the bottom of the unit using double-sided car tape of the 3M brand.

For homemade amateurs, skilled hands of craftsmen and professionals who have the necessary park of machines, here are the drawings of the body structure elements. Maybe someone will come in handy.
▼ 🕗 28/01/11 ⚖️ 244.91 Kb ⇣ 212 Hello reader! My name is Igor, I'm 45, I'm a Siberian and an avid amateur electronics engineer. I have invented, created and maintain this wonderful site since 2006.
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