Tube preamplifier with transformer output. DIY pre-tube tube circuits. DIY tube amplifier: step-by-step instructions, diagrams, materials. Lamp power rectifier circuit
In the wake of great interest in tube technology, I want to describe the design of a tube preamplifier “for the little ones.” Or for those who are not very young, but do not have the time to seriously delve into tube circuitry, but want to try the “tube sound” and look at the pleasant warm glow of the tubes in the dark. Definitely - the characteristics of this design are more than modest, but at the same time it is very functional and - most importantly - does not require special skills for assembly and does not contain expensive and rare elements.
The design is based on a common Soviet radio tube 6ZH1P- "high-frequency pentode with a short response." Its detailed characteristics and application features can be easily found on the Internet, in particular, on the site that I myself use - Magic of Lamps. Its main feature, thanks to which we choose it, is its ability to work with low voltage. Yes, if you are interested in tube designs, you should definitely know that the anode voltage in most of them is hundreds of volts, which means you need an anode transformer, expensive high-voltage capacitors, an output (essentially step-down) transformer and, in the end, precautions and skills during assembly. The second - no less important - is the unique low cost and availability. All other parts are standard passive elements. You will have to order separately, perhaps only a 6V linear stabilizer LM7806 (about it separately), but even then it can be replaced with an adjustable stabilizer LM317 or even with a design with a transistor and a zener diode.
So, in order.
This device is considered a pre-amplifier very conditionally due to the rather low (unity) gain, which depends on the supply voltage. The main function of the device is to match the level and output impedance of the signal source with the load, and, of course, introduce a small level of specific distortion typical of tube technology into the signal.
Source stereo The signal for it can be a player, a digital-to-analog converter (possibly as part of a sound card) or an electronic musical instrument (including one with a high output impedance). The output from the device is fed directly to the final amplifier, or any device with a line input.
Scheme
If you have all the parts at hand, you can actually assemble this device in one evening, taking into account the cabinet work (even such as drilling large holes for lamp sockets). By the way, I strongly recommend taking a metal case. The electronics work will take barely an hour.
Indeed, for one cascade ( There are two of them in the design - for the right and left channels) there are only a lamp (V1/V2), a resistor in the anode circuit (R3/R5) and a decoupling capacitor at the output (C3/C4). In addition, there is a potentiometer (R2/R4) for adjusting the input signal level (I recommend a linear potentiometer with a resistance of approximately 50 kOhm - 100 kOhm), a decoupling capacitor for the input - optional (I personally did not install it).
The rest of the circuit is the power circuit. C1, R1 and C2 - power filter and linear stabilizer DA1. It’s worth dwelling a little on the DA1 chip. It is needed to ensure that no more than the required 6.3V is supplied to the filament of the radio tubes. In this design, I used the closest voltage LM7806, which produces 6V. As I wrote above, you can replace it with other solutions ( If there is a need, I will tell you about them separately). It was also possible, of course, to make a separate filament supply and a separate anode supply. This would give us a few more options, but - at the same time - would significantly complicate the design. But with this connection, the entire circuit can be powered from a standard 12-18V adapter.
Now a few very important words about the power source. As I wrote above, the circuit gain and dynamic range are higher, the higher the supply voltage. However, there are limitations here. We will not take into account the maximum anode voltage of the lamps - it is quite high; we will focus on the weak link of the circuit - the stabilizer. The maximum voltage that can be applied to its input is 35V, maximum current - 1A. The filaments of two lamps in total consume about 300mA. It would seem that the supply is quite decent. However, in practice - the greater the current consumption and input voltage - the more heat the stabilizer generates. The exact thermal characteristics and tolerances are given in the datasheets. Therefore, the maximum permissible supply voltage will be partly determined by the heat sink (radiator) on which the stabilizer will be installed.
In my design, for example, the metal body of the device is used as a dissipating surface - the microcircuit is screwed to the wall through thermal paste. By the way, the insulating gasket not required if, as in most classical solutions, you connect case with minus power supply(in our design, the power supply is unipolar and the “minus” will be “ground” and, accordingly, shield the circuit). The case does not dissipate heat very well (in an hour of operation it does not heat up much, but noticeably), so I limited the supply voltage to 12V. If you install the stabilizer on a fairly massive radiator ( just please don't overdo it! the main idea of the design is compactness!!! ), then the voltage can be increased to 18-20V. Achieve limit value 35V I categorically do not recommend it, since they significantly reduce the service life of the element and soon it may fail due to overheating!
The green numbers on the diagram next to the lamp terminals are the electrode numbers. The location of the electrodes on a standard seven-pin panel is shown below.
Just in case, here is the purpose of the contacts of the linear stabilizer.
And finally, the design itself.
Any metal case the size of a pack of cigarettes will do. In my case, it was once D-Link Media Converter. Using a cone drill, I made two large holes with a diameter of 22mm sockets. It was decided to do the installation mounted. For such a design, a printed circuit board is completely unnecessary. With so many radio elements, only two contact blocks of 10 contacts were enough, and they were not fully used.
Don't forget about star connection- all taps going according to the circuit to ground must be connected at one point with power and housing. True, again, for such a simple circuit with a low anode voltage, this principle is not critical, although it is worth accustoming yourself to observe it everywhere. Experienced electronics engineers will probably point out to me that the wires inside are not laid out the way they are in complex and expensive amplifiers. Of course, it’s worth striving for this, but it’s not for nothing that I wrote in the title - “...in one evening.” With such conditions there is no time for perfectionism, but - on the other hand - I think this is a good demonstration that even the most novice radio amateur can cope with assembling the device.
That's all. A correctly assembled design works immediately. Personally, I'm quite pleased with the sound - at least it matches the level. You can power it from an ordinary adapter, as already written above, with a voltage of 12-18V, but preferably a stabilized one. In this case, the likelihood of power interference will be reduced. I listened through Soundtech Series A on a Quested S6, the signal was supplied from E-mu Tracker.
This module preamp with an input switch was developed by the Frenchman JL. Vandersleyen to work with audiophile power amplifiers of any level. It is implemented on pentode 6Zh32P(analogous to EF86), allows you to connect up to four signal sources and provides amplification of 16 dB. A small switchable low-frequency correction allows you to compensate for the influence of the listening room.
The appearance of the structure is shown in the figure:
(Click to enlarge)
Amplifier specifications:
Frequency band (with 1dB ripple) 10 Hz - 100 kHz
Frequency band (at 0.1 dB ripple) 20 Hz - 50 kHz
Active equalization (see description) + 3 dB at 50 Hz
Rise time<2 мксек
Distortions<0,1% при амплитуде сигнала 1 В в полосе 100 Гц — 10 кГц (на частоте 1 кГц типичное значение 0,03%)
Maximum output signal ~30 V at distortion up to 2% (THD)
Feedback depth - 18 dB
Signal to noise ratio >90dB
Input impedance 50 kOhm
The output impedance of the amplifier itself is 5 kOhm
Circuit Output Impedance - 100K Logarithmic Potentiometer
Channel separation > 50 dB
Inputs - RCA
Power: 6V - 400 mA / 320 V DC - 7 mA
Dimensions 135 x 100 x 30 mm
Due to its fairly compact dimensions, the unit can be built into the chassis of a finished amplifier or used as a stand-alone device (with an external power supply).
Figure 1 shows the operating principle of the gain stage.
Part of the output signal is fed back to the input, in antiphase, to tightly control the circuit gain. Thus, 18 dB of negative feedback reduces overall gain from +34 dB to +16 dB while reducing the stage's inherent distortion.
Due to the reduced influence of the RC feedback circuit (C11, R31) at low frequencies, the circuit gain increases in this range. At the specified values of 220 kOhm and 3.3 nF, a gain increase of 3 dB is provided for frequencies below 100 Hz (see below in the text)
The preamplifier is implemented on pentode 6Zh32P, which was developed specifically for use in the input stages of tape recorders and is characterized by a low microphone effect and high linearity.
The lamp characteristic has excellent linearity at a bias voltage of -3 V, and an anode voltage of 50 V DC, the voltage on the second grid is 180 V, on the third - 0 V (characteristic highlighted in red):
(Click to enlarge)
Schematic diagram
The pre-amplifier circuit is shown in the figure:
(Click to enlarge)
One of the four inputs is selected by slide switch S1. The diagram does not indicate the values of resistors R1, R5, R9, R13; they are selected based on the required input sensitivity.
The input impedance of the amplifier is 50 kOhm. The relatively low input impedance of the lamp is reduced even further due to negative feedback. Therefore, the input resistance of the circuit is determined mainly by the value of resistor R19.
The lamp's own gain is 50, due to feedback it is reduced to 6.5.
The inherent distortion of the lamp due to the negative feedback decreased to 0.03% with a signal amplitude of 1V at the output.
note that lamp's own noise, due to feedback does not decrease, but in the selected modes they turn out to be very low: the signal to noise ratio exceeds 90 dB.
An RC circuit is added to the feedback circuit to compensate for the loss of low-frequency gain that typically occurs due to insufficient listening room volume. As stated at the beginning of the article, the rise is 3 dB for frequencies below 100 Hz.
If you do not need such a function, elements C11-C12, D1, K1-K2 can not be installed, and resistors R31-R32 can be replaced with jumpers.
Setting the Volume Control at the output of the preamplifier is optimal for minimizing
signal/noise ratio. In this case, the risk of putting the cascade into limiting mode is eliminated, since to obtain a maximum output signal amplitude of 30 V, an input signal with an amplitude of 4.6 V is needed! (a rare source is able to give it away)
Pre-amplifier power supply.
The filament voltage of the lamps is supplied to the contacts on the printed circuit board. Thanks to this, it is possible to connect the filaments in parallel, then a voltage of 6-6.3 V will be required with a current consumption of 400 mA. Or you can connect the filaments of both lamps in series, then you will need a voltage of 12V with a current of 200mA...
Based on the anode voltage, the amplifier consumes 7 mA. If you recalculate the value of resistor R33, you can power the amplifier with a voltage from 300 to 320 V DC.
To enable low frequency equalization, +24 VDC is required to drive two 12 V relays.
Pre-amplifier design
Printed circuit boards
All elements of the circuit, including input connectors, relays, flip switch, and volume control are mounted on printed circuit boards. (Fig. 5). All connections are made on connectors, with the exception of filament circuits, which are soldered directly into the board.
Main board
The circuit board has no special features; all circuit elements are mounted on it. First, 7 1.3 mm contacts are soldered (see photo of the design), then thirteen jumpers. The remaining remaining elements are installed in circuit number order, with the potentiometer and roller switch installed last.
The common wire (ground) is connected between the two dual RCA input jacks.
View of the board from the conductor side:
(Click to enlarge)
Arrangement of elements on the board:
(Click to enlarge)
Lamp board
The board is soldered into the main board of the amplifier using 5 mm contacts at an angle of 90 degrees.
The board drawing is shown in the figure below:
The arrangement of elements on the lamp board is shown in the figure:
Inclusion
To test the amplifier, you will need a power supply of 6 or 12 V for the filament circuits and 320 V for the anode voltage.
When first turned on, it is advisable to supply high voltage from a regulated source.
The control voltage values are indicated in the diagram.
When a signal with an amplitude of 300 mV is applied to the input, the output should have a signal with an amplitude of about 2 V.
To check the low frequency correction you will need a +24V source.
When the correction is turned on, the signal rises frequency 60Hz should be 3 dB.
Measurement results
The measurement results are presented in the oscillograms below.
The response of the amplifier to a pulse signal shows its good stability and short rise time of the edges:
(Click to enlarge)
The cutoff frequency is approximately 140 kHz with a roll-off of -1 dB.
The distortion level at a signal level of 1 V is less than 0.03%.
The spectral distribution of harmonics and noise is presented in the spectrograms:
(Click to enlarge)
Note that the spectrum is dominated by second harmonic. However, its level is lower -70 d B, which eliminates the “velvety” color (typical of tube amplifiers, the so-called warm sound) of the signal.
The task of any amplifier is to amplify the signal without making any changes to it.
This amp does it great!
The overall noise level of the amplifier before the volume control is -90 dB.
The graph shows the frequency response when the low-frequency correction circuit is turned on:
(Click to enlarge)
Please note the low impact of correction on the amplifier's frequency response and phase response. The Baxandel tone block (a fairly classic design) has a much greater influence on the output signal.
Construction details.
Resistors:
R1, R2, R5, R6, R9. R10, R13, R14: selected according to the required sensitivity of the inputs (or jumpers)
R3, R4, R7, R8, R11, R12, R15, R16, R17, R18: 470 kOhm / 0.5 W / 1%
R19, R20: 47kOhm/1/0.5W/1%
R21, R22: 150 kOhm / 2 W / 5%
R23, R24: 100 kOhm/2 W/5%
R25, R26: 47 kOhm/2 W/5%
R27, R28: 1.2kOhm/1/0.5W/1%
R29, R30: 360 kOhm /0.5W/ 1%
R31, R32: 220 kOhm / 0.5 W / 1%
R33 1 kOhm/2 W/5%
Capacitors
C1, C2: 1uF/50V/5mm,
C3, C4: 1uF / 250V / 5mm,
C5, C6: 0.1uF/50V/5mm
C7, C8: 100uF/6.3V/3.5mm,
C9, C10: 470 nF / 400 V / 15 mm C11,
C12: 3.3nF/100V/5mm
C13: 10uF/400V/5mm
Miscellaneous:
Lamp: V1, V2 - 6Zh32P (EF86)
Diodes: D1-1N4007
Variable resistor: P1- 100 kOhm (Log/ALPS)
Relay: K1, K2 - SIL / Meder SIL12-1A72-71L
Galette switch: S1 - 5P/2C /Lorlin PT6422
Toggle switch: S2 - NKK B12AH
Connectors: RCA (dual) - 2 pcs., RCA (single) - 1 pc.
Conclusion
The pre-amplifier on the 6Zh32P tube turned out to be absolutely transparent for sound, without introducing tube “warmth” and “velvety”, with a stable gain and low noise level.
A small low-frequency correction allows you to compensate for the weakening of the signal in the low-frequency region by the listening room, and the compact dimensions of the design allow it to be built into a ready-made amplifier.
The article was prepared based on materials from the Electronique Pratique magazine.
Happy creativity!
Briefly, mostly photos (re-uploaded in good quality). I’ll say right away that I had little experience and knowledge in radio engineering and made a lot of mistakes. Not being a fanatical lover of warm tube sound, the assembly process itself was interesting to me.
The hardest part is finding the output transformers. I bought myself ready-made ones from the TU-100M amplifier (I didn’t choose for a long time, I took what they had). The frame was made from an aluminum profile and the strength margin was a little overdone.
The upper part of the body was made of 3mm steel. Holes for transformers and lamps were laser cut. The bottom was also cut from 2mm steel with ventilation holes:
Front panel made from a piece of aluminum:
Scheme
The final amplifier is assembled using a push-pull circuit using two G-807 lamps. The preamplifier contains two amplification stages assembled on a 6N9S double triode (foreign analogue of 6SL7).Advantages of 6N9S:
1) The lamp was originally designed for audio applications;
2) Two triodes in a cylinder;
3) High linearity;
4) Wide distribution, low price.
Disadvantages of 6N9S:
1) High internal resistance.
The pre-terminal amplifier (an intermediate link between single-ended and push-pull amplifiers) is assembled using a phase-inverted circuit on a 6N9S double triode; its main purpose is to form two mutually antiphase, equal in amplitude signals from the input signal. In the TU-100M circuit, the lamp amplifies the input signal and the voltage amplified by it is supplied to the lamp grid of the first arm of the push-pull amplifier.
Part of the output voltage of the first lamp of the phase-inverted amplifier is supplied to the input of the second lamp of this amplifier. The voltage amplified by the second lamp of the phase-inverted amplifier is supplied to the grid of the lamp of the second arm of the push-pull
amplifier Thus, for the first arm of a push-pull amplifier the signal passes through one tube, and for the second through two.
It would be better if the voltage applied to the input of the first arm was equal to the voltage at the input of the second arm. I made a slightly different circuit, with a modified phase inversion stage.
Advantages:
1) Reduced requirements for supply voltage filtering;
2) Extremely low noise level;
3) Equal output voltages of the shoulders.
I found another option on the forums:
Sockets for lamps 6N9S:
The amplifier housing contains a DAC with the ability to connect to a computer via USB:
Adjustment option:
Transformer screens, first sketches on paper:
Cut from 2mm steel: 
After filing and sanding:
Some more photos:
Cleaned it up a bit:
Price: unreasonably expensive.
It’s easier to buy ready-made for 4-5 thousand rubles. But if anyone needs it, I can send you files for cutting and for printed circuit boards.
— most connoisseurs of high-quality music, who know how to handle soldering equipment and have some experience in repairing radio equipment, can try to assemble a high-class tube amplifier, which is usually called Hi-End, on their own. Tube devices of this type belong in all respects to a special class of household radio-electronic equipment. Basically, they have an attractive design, with nothing covered by a casing - everything is in plain sight.
After all, it is clear that the more visible the electronic components installed on the chassis are, the greater the authority of the device. Naturally, the parametric values of a tube amplifier are significantly superior to models made with integrated or transistor elements. In addition to this, when analyzing the sound of a tube device, all attention is paid to the personal assessment of the sound rather than to the image on the oscilloscope screen. In addition, it has a small number of used parts.
How to choose a tube amplifier circuit
If there are no particular problems when choosing a pre-amplifier circuit, then when choosing a suitable final stage circuit, difficulties may arise. Tube audio power amplifier may have several versions. For example, there are single-cycle and push-pull devices, and also have different operating modes of the output path, in particular “A” or “AB”. The output stage of single-ended amplification is, by and large, a sample, because it is in mode “A”.
This operating mode is characterized by the lowest nonlinear distortion values, but its efficiency is not high. Also, the output power of such a stage is not very large. Consequently, if it is necessary to sound an internal space of medium size, a push-pull amplifier with the “AB” operating mode will be required. But when a single-cycle device can be made with only two stages, one of which is preliminary and the other amplifying, then a driver is needed for the push-pull circuit and its correct operation
But if single-cycle tube audio power amplifier may consist of only two stages - a pre-amplifier and a power amplifier, then a push-pull circuit for normal operation requires a driver or cascade that forms two voltages of identical amplitude, shifted in phase by 180. Output stages, regardless of whether it is single-ended or push-pull, require the presence of output transformer. Which acts as a matching device for the interelectrode resistance of a radio tube with low acoustic resistance.
True admirers of “tube” sound argue that the amplifier circuit should not have any semiconductor devices. Therefore, the power supply rectifier must be implemented using a vacuum diode, which is specially designed for high-voltage rectifiers. If you intend to repeat a working, proven tube amplifier circuit, then you do not need to immediately assemble a complicated push-pull device. To provide sound in a small room and obtain an ideal sound picture, a single-ended tube amplifier is fully sufficient. In addition, it is easier to manufacture and configure.
The principle of assembly of tube amplifiers
There are certain rules for installing radio-electronic structures, in our case these are tube audio power amplifier. Therefore, before starting the manufacture of the device, it would be advisable to thoroughly study the primary principles of assembling such systems. The main rule when assembling structures using vacuum tubes is to route the connecting conductors along the shortest possible path. The most effective method is to refrain from using wires in places where you can do without them. Fixed resistors and capacitors must be installed directly on the lamp panels. In this case, special “petals” must be used as auxiliary points. This method of assembling a radio-electronic device is called “mounted mounting”.
In practice, printed circuit boards are not used when creating tube amplifiers. Also, one of the rules says - avoid laying conductors parallel to each other. However, such a seemingly chaotic layout is considered the norm and is completely justified. In many cases, when the amplifier is already assembled, a low-frequency hum is heard in the speakers; it must be removed. The primary task is performed by the correct choice of the ground point. There are two ways to organize grounding:
- The connection of all wires going to the “ground” at one point is called an “asterisk”
- Install an energy-efficient electrical copper bus around the perimeter of the board, and solder conductors to it.
The location for the grounding point must be verified by experiment, listening for the presence of background. To determine where the low-frequency hum comes from, you need to do this: Using a sequential experiment, starting with the double triode of the pre-amplifier, you need to short-circuit the lamp grids to ground. If the background decreases noticeably, it will become clear which lamp circuit is causing the background noise. And then, also experimentally, you need to try to eliminate this problem. There are auxiliary methods that are required to be used:
Pre-stage tubes
- Electrovacuum lamps of the preliminary stage must be covered with caps, and they, in turn, must be grounded
- Housings of trimming resistors are also subject to grounding
- Lamp filament wires need to be twisted
Tube audio power amplifier, or rather, the filament circuit of the pre-amplifier lamp can be powered with direct current. But in this case, you will have to add another rectifier assembled using diodes to the power supply. And the use of rectifier diodes in itself is undesirable, since it breaks the design principle of manufacturing a Hi-End tube amplifier without the use of semiconductors.
The paired placement of the output and mains transformers in a lamp device is quite an important point. These components must be installed strictly vertically, thereby reducing the background level from the network. One of the effective ways to install transformers is to place them in a casing made of metal and grounded. The magnetic cores of transformers also need to be grounded.
Retro components
Radio tubes are devices from ancient times, but they have become fashionable again. Therefore it is necessary to complete tube audio power amplifier with the same retro elements that were installed in the original lamp designs. If this concerns permanent resistors, then you can use carbon resistors that have high stability of parameters or wire resistors. However, these elements have a large scatter - up to 10%. Therefore, for a tube amplifier, the best choice would be to use small-sized precision resistors with a metal-dielectric conductive layer - C2-14 or C2-29. But the price of such elements is significantly high, so instead of them, MLTs are quite suitable.
Particularly zealous adherents of the retro style get an “audiophile’s dream” for their projects. These are carbon resistors BC, developed in the Soviet Union specifically for use in tube amplifiers. 
If desired, they can be found in tube radios from the 50s and 60s. If according to the circuit the resistor must have a power of more than 5 W, then PEV wire resistors coated with glassy heat-resistant enamel are suitable.
Capacitors used in tube amplifiers are generally not critical to a particular dielectric, as well as to the design of the element itself. Any type of capacitor can be used in the tone control paths. Also, in the rectifier circuits of the power supply, you can install any type of capacitors as a filter. When designing high-quality low-frequency amplifiers, the coupling capacitors installed in the circuit are of great importance.
They have a special influence on the reproduction of a natural, undistorted sound signal. Actually, thanks to them we get exceptional “tube sound”. When choosing coupling capacitors that will be installed in tube audio power amplifier, special attention must be paid to ensure that the leakage current is as small as possible. Because the correct operation of the lamp, in particular its operating point, directly depends on this parameter.
In addition, we must not forget that the separating capacitor is connected to the anode circuit of the lamp, which means that it is under high voltage. So, such capacitors must have an operating voltage of at least 400v. One of the best capacitors working as a transition capacitor are those from JENSEN. It is these capacities that are used in top-end HI-END class amplifiers. But their price is very high, reaching up to 7,500 rubles for one capacitor. If you use domestic components, then the most suitable ones would be, for example: K73-16 or K40U-9, but in terms of quality they are significantly inferior to branded ones.
Single-ended tube audio power amplifier
The presented tube amplifier circuit consists of three separate modules:
- Pre-amplifier with tone control
- The output stage, that is, the power amplifier itself
- Power supply
The preamplifier is manufactured using a simple circuit with the ability to adjust the signal gain. It also has a pair of separate tone controls for low and high frequencies. To increase the efficiency of the device, you can add an equalizer for several bands to the design of the preamplifier.
Electronic components of the preamplifier
The pre-amplifier circuit presented here is made on one half of a 6N3P double triode. Structurally, the preamplifier can be manufactured on a common frame with an output stage. In the case of a stereo version, two identical channels are naturally formed, therefore, the triode will be fully involved. Practice shows that when starting to create any design, it is best to first use a circuit board. And after setting it up, assemble it in the main building. Provided that it is assembled correctly, the preamplifier begins to operate synchronously with the supply voltage without any problems. However, at the setup stage you need to set the anode voltage of the radio tube.
The capacitor in the output circuit C7 can be used K73-16 with a rated voltage of 400v, but preferably from JENSEN, which will provide better sound quality. Tube audio power amplifier not particularly critical of electrolytic capacitors, so any type can be used, but with a voltage margin. At the setup stage, we connect a low-frequency generator to the input circuit of the pre-amplifier and apply a signal. An oscilloscope must be connected to the output.
Initially, we set the input signal range to within 10 mv. Then we determine the output voltage value and calculate the amplification factor. Using an audio signal in the range of 20 Hz - 20000 Hz at the input, you can calculate the throughput of the amplification path and display its frequency response. By selecting the capacitance value of the capacitors, it is possible to determine the acceptable proportion of high and low frequencies.
Setting up a tube amplifier
Tube audio power amplifier implemented on two octal radio tubes. A double triode with separate cathodes 6N9S connected in a parallel circuit is installed in the input circuit, and the final stage is made on a fairly powerful output beam tetrode 6P13S connected as a triode. Actually, it is the triode installed in the final path that creates exceptional sound quality.
To perform a simple adjustment of the amplifier, an ordinary multimeter will be enough, but to make precise and correct adjustments you need to have an oscilloscope and an audio frequency generator. You need to start by setting the voltage at the cathodes of the 6N9S double triode, which should be within 1.3v - 1.5v. This voltage is set by selecting a constant resistor R3. The current at the output of the 6P13S beam tetrode should be in the range from 60 to 65 mA. If a powerful constant resistor 500 Ohm - 4 W (R8) is not available, then it can be assembled from a pair of two-watt MLTs with a nominal value of 1 kOhm and connected in parallel. All other resistors indicated in the diagram can be installed of any type, but preference is still given to C2-14.
Just like in the preamplifier, the important component is the decoupling capacitor C3. As mentioned above, the ideal option would be to install this element from JENSEN. Again, if you don’t have them at hand, you can also use Soviet film capacitors K73-16 or K40U-9, although they are worse than overseas ones. For correct operation of the circuit, these components are selected with the lowest leakage current. If it is impossible to carry out such a selection, it is still advisable to buy elements from foreign manufacturers.
Amplifier power supply
The power supply is assembled using a 5Ts3S direct-heated kenotron, which provides AC rectification that fully complies with the design standards for HI-END class tube power amplifiers. If it is not possible to purchase such a kenotron, then you can install two rectifier diodes instead.
The power supply installed in the amplifier does not require any adjustment - everything is turned on. The topology of the circuit makes it possible to use any chokes with an inductance of at least 5 H. As an option: using such devices from outdated TVs. The power transformer can also be borrowed from old Soviet-made lamp equipment. If you have the skills, you can make it yourself. The transformer must consist of two windings with a voltage of 6.3v each, providing power to the amplifier radio tubes. Another winding should have an operating voltage of 5v, which is supplied to the kenotron filament circuit and the secondary one, which has a midpoint. This winding guarantees two voltages of 300v and a current of 200 mA.
Power amplifier assembly sequence
The procedure for assembling a tube audio amplifier is as follows: first, the power supply and the power amplifier itself are made. After the settings have been made and the necessary parameters have been installed, the preamplifier is connected. All parametric measurements with measuring instruments should be done not on a “live” acoustic system, but on its equivalent. This is in order to avoid the possibility of expensive acoustics being decommissioned. The load equivalent can be made of powerful resistors or thick nichrome wire.
Next you need to work on the housing for the tube audio amplifier. You can develop the design yourself, or borrow it from someone. The most affordable material for making the body is multilayer plywood. The output and preliminary stage lamps and transformers are installed on the upper part of the housing. On the front panel there are tone and sound control devices and a power supply indicator. You may end up with devices like the models shown here.
Good afternoon.
I would like to continue the story about a tube preamp for a hybrid amplifier.
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Complete preamp circuit:
The scheme is very simple. We didn't invent anything. The basis chosen last time is a resistive cascade. There is nothing unusual about it.
Active filters on transistors VT1 and VT2 were added to the circuit. They provide additional nutritional cleansing. Since the main filtration will be performed by an external source, the filter circuits were simplified - they were made single-stage.
We plan to power the filament from an external stabilized source. Using powerful filtering of all voltages will ensure that there is no background.
It's time to collect
With the prototype board, everything is as usual: we draw, print, translate, etch, drill and clean it with fine sandpaper... After that, put a respirator on your face, a can of black heat-resistant paint in your hands... paint the board black. This way it will not be visible in the body of the assembled amplifier.
Set the board aside and let it dry. It's time to shake out the boxes and pick up the parts. Some of the components are new, others are soldered from early prototypes (well, good, almost new components shouldn’t go to waste?!).
Everything is ready for assembly, it's time to turn on the soldering iron.
The soldering iron is hot - solder:
Note: It is more convenient to solder, starting with the lowest profile components and moving to higher ones. Those. First we solder diodes, zener diodes, then resistors, a socket for a lamp, capacitors, etc... We, of course, broke this sequence and soldered as necessary :)
Capacitors installed. This project uses domestic K73-16. Good capacitors. We carried out a series of measurements of their nonlinearity spectra in different modes. The results were encouraging. We will definitely write about this someday.
We solder resistors and other small things
We install the socket and electrolytic capacitors.
Note: When soldering a lamp socket, you must insert a lamp into it. If this is not done, then after assembly there may be problems with installing the lamp. In some (the most “severe” cases) you can even damage the lamp base.
All the details are in place. The preamp is ready.
Checking
The scheme is simple and the likelihood of error is minimal. But we need to check. Connect the amplifier to the power source and turn on:
10 seconds - normal flight... 20... 30... everything is fine: nothing exploded or started smoking. The glow glows quietly, the test power supply protections do not operate. You can exhale with relief and check the modes: all deviations are within acceptable limits for an unheated lamp.
After a 10-minute warm-up, all parameters were established and reached the calculated values. The operating point is set.
Since everything is good, we can continue. We connect a test signal source to the input. At the output there is a resistor simulating the input resistance of a power amplifier. We turn on and measure all the main parameters of the cascade.
Everything is within normal limits. The distortion and gain coincided with what was obtained in the previous article. There is no background.
So our tube preamplifier is ready. It's time to move on to creating a powerful transistor output buffer for it. It can be used with the same success in a purely tube design. To do this, you will need to make a powerful tube output for it.
Perhaps it makes sense to make a universal tube preamplifier (maybe in the form of a designer) for use in tube and hybrid designs?
Best regards, Konstantin M.