Design features and design of tube ultrasonic frequencies. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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The fundamental differences between tube ultrasonic frequencies (especially powerful ones) and similar transistor ones entail noticeable differences in the requirements for their design.

Let's list these differences:

1. The input circuits of all stages of a tube amplifier have an order of magnitude greater open input resistance than similar transistor circuits, and therefore, they are also an order of magnitude more susceptible to external electric fields (interference).
2. In lamp ultrasonic frequencies, without exception, all lamps, including the most low-power ones, continuously radiate heat during operation. As for powerful terminal lamps operating in class A mode, the temperature of their glass cylinders can reach 90 ... 100 ° C, and the heat generated, if not constantly removed, creates an environment inside the amplifier case that is unacceptable for some other parts (for example , rectifier filter capacitors).
3. All lamp UZCH have a transformer output speaker system, and since the useful output power, as a rule, exceeds 20 watts, reaching sometimes up to 100 watts, output transformers become sources of significant magnetic fields in a wide range of audio frequencies. This creates significant magnetic pickups in almost all parts of the circuit and, as a result, unpredictable positive and negative, but always spurious feedbacks that make the amplifier unstable.
4. A particular nuisance in tube amplifiers is delivered by the power supply circuits for incandescent lamps, which is a priori excluded in transistor amplifiers. At the same time, both the lamps themselves and the connecting wires of the filament circuits serve as a source of additional background and interference, through which a current flows, often reaching up to 10 A, as a result of which fields with a frequency of 50 Hz appear around these wires, which also cause significant interference.

Even the above is enough to understand that the design of a powerful tube ultrasonic frequency converter must be fundamentally different from the designs of transistor amplifiers.

The fundamental principles in determining the design and layout of the tube ultrasonic frequency units should be:

1. The most thorough shielding of all circuits and nodes, both subject to pickups and those that create these pickups. At the same time, shielding technology has its own specifics, which we will pay the most serious attention to further.
2. Rational mutual arrangement of nodes and lamps (especially powerful ones), as well as a well-thought-out system of convection cooling or forced ventilation, providing a reasonably acceptable temperature regime inside the amplifier case.
3. Special circuit solutions that minimize parasitic electrical and magnetic pickups (resonance tuning of rectifier filter chokes, direct current supply of lamp filaments, and a number of others).
4. Unusual for radio amateurs placement of power transformers at a certain, experimentally selected angle, providing a minimum effect of its magnetic fields on output transformers.

In addition to these concerns, the creator of a modern tube amplifier will have many other equally important ones. For example, how to arrange the power supply and output stages with their inherent bulky output transformers so that the center of gravity of the amplifier coincides with the geometric center of the structure. Or how to arrange the operating controls so that, on the one hand, they are convenient to use, and on the other, so that the connecting wires between them and the input lamps are as short as possible. And there are many such problems. In the future, when describing specific structures, we will consider and solve these problems as comprehensively as possible.

Now about design. It just so happened that absolutely all companies producing modern tube amplifiers, as if by agreement (or maybe it was so?), They abandoned modern design styles, and at the same time modern construction materials. All modern ultrasonic frequencies known to the author are decorated in the style of the 50s according to the American model, i.e. have instrumental style. Most often it is a rectangular metal box, sometimes with two side wooden walls, painted black or dark brown (and in some models even dark gray hammer enamel). The proportions of the case are very diverse: with the largest front wall; with a depth greater than the width and height, with a ratio of width to depth and height as 5:4:2.

All controls, except for the mains fuse, are displayed in one row on the front panel. The network switch is made in the form of a conventional instrument toggle switch. Volume and tone control knobs - the simplest cylindrical shape, black with "knurling" and screw fastening.

The top metal cover, rear wall and bottom of the case have numerous perforations or elongated ventilation slots above the terminal lamps, kenotrons and power transformer.

It seems that Western designers and designers have set themselves the goal of emphasizing that the modern tube amplifier, due to its perfection, is closer to special precision equipment than to ordinary household radio equipment, which should look like consumer goods next to such an amplifier.

We do not set such a task, but nevertheless we will adhere to the maximum simplicity in design and ergonomics of our designs, since they are designed for the individual user, are not afraid of competition from other companies and do not need advertising external effects.

However, this does not at all exclude the possibility that everyone who will build the proposed amplifiers will be able to design them in his own taste, using the most modern materials, but not to the detriment of the basic requirements, and first of all, ensuring the proper temperature regime.

Method of adjustment and measurement of parameters

Despite the fact that this book is intended for experienced, qualified radio amateurs who have sufficient practice in adjusting and establishing various designs, the author will allow himself to express several considerations that have appeared in his forty years of experience.

So, first about the terms. What is checking, adjusting, tuning, adjusting, launching, revitalizing, measuring, testing? Can you clearly define these concepts and say how they differ? I think no. In that case, let's start by checking.

Any (we emphasize - any) newly assembled device, whether it be an industrial TV or an amateur tape recorder, should never, under any circumstances, be connected to the network in the hope that it will work right away. And not because it most likely will not work, but because after turning it on, you may not have time to blink an eye, as you will lose this eye forever. This can happen if the rectifier filter capacitor you supplied without first checking is broken or with unacceptable leakage and explodes at the very moment when you lean over the chassis.

Now the questions are: what to check, how to check, with what and in what order? Nothing new and original can be invented here, since this process has long been thoroughly worked out. The first immutable rule: it takes 10 ... 20 times more time to search for one faulty resistor or capacitor in an assembled structure than to carefully pre-check all the parts used together. From this rule, in turn, the law follows: in the process of mounting the amplifier on the table next to the soldering iron, there must be a tester or probes from a lamp multi-scale ohmmeter, and each part, before soldering it or inserting it into the printed circuit board, must be checked by the device for the absence of a break, short circuit, leakage and compliance with the specified rating. With sufficient skill, it takes no more than 20 ... 30 s to check a resistor and a conventional capacitor, and 1,5 ... 2 minutes for a filter capacitor and a potentiometer. But, we repeat, these spent seconds and minutes will more than pay off when setting up the amplifier.

So, we checked all the details during the installation process, defective ones are obviously excluded. Now it's time to check the circuits. In production conditions, for this purpose, special "resistance maps" have been developed for each product, on which, for a number of key points of the circuit, the resistance values ​​\uXNUMXb\uXNUMXbof these points are indicated both relative to the chassis and relative to the "hot" wire of the power source (this can be both plus and minus) .

In amateur practice, drawing up such a map does not make sense, since the product will almost always be created in a single copy, however, the actual resistance value check itself can and should be carried out. It should start first of all with those circuits that absolutely definitely should not be grounded and closed to each other.

Attention! Prior to the start of the test, all potentiometers, without exception, both operational and installation (mode), must be set to the middle position.

Such non-grounded points of the circuit primarily include the "hot" terminals of all rectifiers (pluses or minuses), anodes that shield and control grids of all lamps, plus (or minus) terminals of all oxide capacitors and other similar points and circuits that should not be grounded. Following this, all points of the circuit are checked, which, on the contrary, must be grounded or connected directly to the "hot" points of the power supplies. An experienced radio amateur knows all these points and circuits well (for example, these are the protective covers of all operational potentiometers, which are not on any circuit diagram). Having completed all the operations of checking the circuits and eliminating the identified defects and errors, you can proceed to the next operation - starting the amplifier.

We remind you that you can turn on the amplifier for the first time only with the lamps removed (with the exception of the kenotron). If the radio amateur has an adjustable autotransformer or a transition transformer from 220 to 127 V, we strongly recommend that the first switch-on be carried out at a reduced (half) mains voltage. Before pressing the power button or toggle switch, double-check that the fuse socket is actually a 0,5 or 1 A fuse, and not a 20-amp bug or nail. In addition, do not forget to connect a DC voltmeter with the appropriate limit (250, 350 or 500 V) to the first filter capacitor and carefully follow the indication of the arrow from the moment you turn it on.

If after 20 ... 30 s (the warm-up time of the kenotron glow) the voltage does not appear at this point, immediately turn off the amplifier, then find and eliminate the cause.

If the voltage appears (and it is approximately half the nominal value indicated on the diagram), it is useful to check with a voltmeter the presence of supply voltages on all electrodes of all lamps. In the absence of the lamps themselves in the panels, these voltages, as a rule, are either equal to or very close to the voltage at the output of the rectifier filter, since there is no current consumption and, as a result, a voltage drop across the load resistors.

After making sure that there are no short circuits in the circuit and that there are constant voltages on all lamp electrodes (where it should be), turn off the amplifier and prepare it for switching on to full mains voltage.

Warning. Since the next switching on is also carried out with all the lamps removed (except for the kenotron) and, therefore, there is no consumption, at certain points in the circuit the supply voltage may exceed the allowable one and lead to the failure of some parts. Let us explain what has been said in Fig. 4. Here, the first two lamps are powered through four consecutive links of filters, the voltage on each of which decreases (if there is a load) and corresponds to the values ​​\u180b\u200bspecified in the diagram. At point A, for example, on the oxide capacitor, during normal operation of the amplifier, there should be a voltage of +260 V. But if a capacitor with an operating voltage of XNUMX V is installed at this place (which is quite acceptable), then when the amplifier is turned on without lamps, it may have full voltage rectifier idle (say, XNUMX V) and the capacitor will be broken. To prevent this possibility, such circuits should be temporarily disconnected from the rectifier or loaded with equivalent resistive loads.

Now turn on the amplifier (without lamps and taking into account these recommendations) at the rated mains voltage (220 V) with inserted kenotrons and leave it on for 10 ... wires and especially traces of smoke. If this time everything is in order, you can proceed to the next step.

In principle, it is completely indifferent in what sequence to carry out this process, but for some reason it is traditionally customary to start it from the final stage. We will do the same. Since all the terminal stages are push-pull, let's start with one of the arms (it doesn't matter which one).

First of all, look at what is in the cathode circuit of this lamp: if there is a variable adjusting resistor, then be sure to set it to the maximum resistance position and check with a tester that this is indeed the case. Unsolder the wire going to the anode terminal on the lamp socket and turn on the DC milliammeter with a scale of at least 100 and not more than 250 mA (minus to the anode, plus to the transformer) in the resulting gap.

Now you can insert one terminal lamp, all kenotrons (if there are several) and turn on the amplifier. In this case, one should observe the appearance of incandescence of the terminal lamp, and if it is absent for several seconds, the amplifier must be immediately turned off in order to avoid destruction of the cathode. The reason for the lack of heat may be incorrect wiring of the filament wires on the socket or on the power transformer, or a malfunction of the lamp. If there is heat, observe the reading of the device.

Warning. If the rectifier circuit provides for an anode power-on delay circuit, then the anode current will appear after the set “jump” relay operation time. If there is no such circuit, the current will increase smoothly as both the lamp itself and the kenotrons warm up.

When the current stops increasing and settles at a certain value, check the table. 1 is the maximum allowable anode current for this type of lamp. By decreasing the resistance of the resistor in the cathode of the lamp, set the current value equal to half the maximum allowable. If the terminal lamp is a triode, then the preliminary setting of the mode can be considered complete. If, however, a pentode or a beam tetrode is used in the final stage, then after setting the rated anode current, you should make sure that the current of the screening grid and the power dissipated on it do not go beyond the limits indicated in the same table (P-g2 = I-g2 x U- g2).

Having finished setting the static mode of one terminal lamp, do the same with the other, and in the absence of complications, proceed to setting the phase inverter mode. Here it is very important to first set the adjusting potentiometer in the grid circuit of the right triode to the minimum position (the grid is grounded) and only after that insert the lamp into the socket.

If the voltages on the anodes and cathodes of both triodes after the lamp has warmed up correspond to those indicated in the diagram (within a 10% deviation), you can consider the preliminary static adjustment of one of the stereo channels to be completed and proceed to a similar check and adjustment of the second stereo channel. If the modes are noticeably different from those indicated in the diagram, you should first of all try another lamp, and if this does not help measure the anode current with the device and check the resistor values ​​​​in the anode and cathode circuits again (especially if this was not done before installation).

When, finally, the voltages and currents of all lamps in the rest mode correspond to the recommended ones, you can proceed to the most difficult and critical part of the work - setting the dynamic mode. Dynamic (in the presence of a useful signal) adjustment of the UZCH, in contrast to the static one, is more expedient to conduct cascading from input to output and start from the input stage. However, in our case, we are not yet considering the entire amplifier, but only its terminal block, which begins with the first of the two triodes of the phase inverter.

Before applying a useful signal to the grid of this triode, it is necessary to bring the measuring equipment into combat readiness. This is, first of all, a sound generator with a frequency range of not more than 20 Hz ... 20 kHz and its own clear factor of less than 1%, and secondly, a tube or transistor millivoltmeter with a wide range of measurement limits (for example, LV-9 or MVL), it is necessary - an oscilloscope and preferably a harmonic distortion meter or harmonic analyzer.

Considering that most radio amateurs will not have a non-linear distortion meter (and without it it makes no sense to talk about a really high quality amplifier), we suggest using another, albeit more time-consuming, but still quite reliable method for assessing non-linear distortions. This method is graphoanalytical and consists in the following.

Before starting the dynamic adjustment of the cascade, you need to prepare a form for plotting a graphical dependence of the output voltage of the cascade on the signal level on the grid in coordinates

X-Uin[MB]; Y-Uout[MB]

To do this, it is best to use a sheet of notebook "in the box", which will ensure sufficient accuracy of the constructed graph. Better yet, use graph paper.

The process of plotting is reduced to a discrete change in voltage with a frequency of 1000 Hz from a sound generator on the lamp grid (for example, after 5 or 10 mV) and an accurate measurement of the corresponding signal values ​​at the output of the stage. These values ​​must be plotted on the graph with a sharpened pencil so that the diameter of the dot is minimal.

In the absence of non-linear distortions, the dependence graph is a straight line emanating from the origin of coordinates and inclined to the X axis at an angle characterizing the gain of the cascade.

If the operating point of the lamp (offset on its grid) is chosen optimally, the straight line will be almost absolutely linear up to a certain level of output voltage, after which its slope will gradually decrease, tending to a horizontal line in the limit.

Having built such a graph, you need to take an absolutely even, preferably steel ruler and apply it from left to right along the marked points of the graph, starting from zero. In the place where there is the most insignificant deviation of the points to the right of the ruler, you need to put a mark-point and lower the perpendicular from it to the X axis. The intersection of this perpendicular with the X axis will determine the limiting level of the input signal, at which non-linear distortions are already unacceptable. The level of acceptable distortion will be determined by the maximum range of the input signal 10...15% less than this value.

Once you have determined this range, compare it to the lamp's quiescent bias voltage. In all circumstances, the signal swing must be less than the bias voltage. At the same time, using the constructed graph, you can determine the real value of the Gain of the cascade by dividing any of the values ​​​​of the output voltage (within the linear part of the characteristic) by the corresponding input voltage. Compare it with the nameplate value for this lamp (see Table 1). Usually the real amplification of the cascade is about 50...70% indicated in the table.

If the linear part of the characteristic turned out to be too small, then this most likely indicates an incorrectly selected lamp operating point. In this case, you will have to take several dynamic characteristics at different values ​​of the automatic bias resistor and select the mode that corresponds to the largest length of the linear part of the characteristic. We remind you that this operation can be done only if there is firm confidence in the serviceability of the lamp itself. Otherwise, you should start by checking the lamp or replacing it with another one.

Having finished the dynamic adjustment of one cascade, all other cascades are adjusted in the same way, including the final one, if it is also assembled on a triode.

For the final stage, made on a pentode or a beam tetrode according to an ultralinear scheme, adjustment and measurement are performed several times for various options for connecting the shielding mesh to the taps of the primary winding of the output transformer and, necessarily, with a load dummy connected to the secondary winding (wire resistor 4 ... 8 Ohm power of at least 30 W). This also applies to the final stage on triodes. Please note that it can reach temperatures over 100°C.

From several options for connecting the screening mesh, choose the one that corresponds to the most linear dynamic response. Be sure to connect the screening mesh to the same outlet in the other push-pull arm.

Having carried out the dynamic adjustment of all stages in turn, you can proceed to the dynamic adjustment of the entire amplifier as a whole. Recall that it must be performed at a frequency of 1000 Hz with all operational controls (volume, tone, balance) set to the middle position.

And a little more theory. The word "amplifier" reflects the main essence of its purpose - to amplify the electrical signal. However, an UZCH is not just an amplifier, but a device designed for a very specific and very narrow purpose - to turn weak changes in electric current into powerful mechanical vibrations of loudspeaker cones. Thus, UZCH is just an intermediate link between a purely electric source of alternating current and an electro-acoustic transducer.

Neither the signal source nor the electro-acoustic transducer are within our control: their characteristics are predetermined and cannot be changed. For example, we cannot voluntarily set the input sensitivity of the amplifier to 10 mV or, conversely, 10 V, because all low-frequency signal sources (except for a microphone) in accordance with existing standards have an output voltage in the range of 50 ... 250 mV.

In the same way, the parameters of the output signal of our UZCH are predetermined. If it is designed to work with a 20-watt speaker system with an impedance of 4 ohms, then the nominal signal voltage at the output of the amplifier should be

U = SQRT(PR) = SQRT(20x 4) = 9V, while providing voltage Iload=U/R=9/4=2,25A.

So, the input voltage is 100 ... 150 mV with an internal source resistance of the order of hundreds of kilo-ohms and the output voltage is 9 V at a current of up to 2,5 A. There is no escape from this. But between these borders, we are given freedom. However, not so complete. To ensure the parameters of the output signal, the power supplied by the lamps of the final stage is used. And they, in turn, require for this on their grids a well-defined buildup voltage, determined solely by the design of the terminal lamp. The value of this voltage can be found in the reference book.

And further. We want to have good, deep tone control, say ±14dB swing (i.e. 25 times the voltage). This means that the level of the useful signal will be lost exactly that many times, and it will have to be compensated by preliminary amplification. And we will lose on negative feedback. And also - on subtlety. And yet ... etc. As a result, a rather large signal loss occurs, which can only be compensated by preliminary amplification.

Knowing this value, select the appropriate types of lamps and the number of stages for preamplification. And here no one orders us, since this problem can be solved in many ways. However, enough theory. Let's get back to the dynamic adjustment of the entire AF pass-through from the input jacks to the speaker connector.

So, we have already understood that at the input of the amplifier there will be a signal with a level of 100 ... 150 mV. This means that we should also receive this signal from the sound generator (at a frequency of 1000 Hz - remember?) and bring it to the input connector of one of the stereo channels. Of course, only the standard shielded hose from the instrument should be used as a connector. The volume control must be set to the maximum gear position (clockwise all the way), and the channel switch, if it is in the amplifier, set to the desired position.

Using a tube millivoltmeter, check for a signal directly on the grid of the first lamp, connect the oscilloscope directly to the anode of this lamp (if the oscilloscope has an unprotected input, then through a 0,1 uF capacitor for a voltage of at least 250 V) and turn on the amplifier.

After warming up the lamp, check for the slightest distortion of the sine wave on the oscilloscope. If distortion is clearly observed, compare the actual buildup voltage on the grid with the maximum allowable signal level that you determined for this lamp from the characteristic taken during the dynamic adjustment of the cascade. If the level of the applied signal turns out to be higher than the permissible one (which is unlikely), you will have to install an elementary divider of two resistors at the input of the amplifier (right at the input jacks), the total resistance of which should be within 0,5 ... 1 MΩ. If there is no distortion on the oscilloscope (which is normal), start gradually increasing the signal from the sound generator until visible distortion appears on the oscilloscope screen, then measure the corresponding level of the generator output signal. It should be no less than 500 mV (better if it is closer to 1000 mV).

After adjusting the first stage, again set the output of the generator to 100 ... 150 mV and transfer the oscilloscope probe to the anode of the lamp of the second stage. Its adjustment and signal level measurement, with one exception, are no different from those described. It consists in the fact that usually a negative feedback voltage is applied to the cathode of the lamp from the secondary winding of the output transformer. To set the feedback depth, there is a special setting potentiometer, which must first be set to the zero level position (the engine is grounded).

Setting this potentiometer to the desired position is done last, when absolutely all other adjustments have already been made. This finally sets the input sensitivity.

The adjustment of the dynamic mode of the phase inverter, in principle, is also no different from that described, except for the sequence. First, the first (direct) triode is regulated, and then, using the potentiometer slider in the grid circuit of the second (inverse) triode, exactly the same signal is set on the anode of the second triode as on the anode of the first triode. Signal Divergence onboth anodes should not exceed 0,5, maximum 1%. To achieve this result, the position of the adjustment potentiometer will have to be clarified several times.

The principle of adjusting the final stage has already been discussed in detail earlier. We just have to make sure that when the signal level at the UZCH input is 100 ... 150 mV, the voltage on the grids of the lamps of the final stage is the one required to obtain the maximum undistorted output power. No more, but no less. The required voltage is set using specially provided adjusting resistors connected between the output of the driver and the input of the terminal stages.

This is a technique for adjusting a high-quality UZCH. However, it is equally applicable to the adjustment and adjustment of almost any radio equipment.

These issues are covered in more detail and in detail in the sections on adjusting specific amplifiers described in this book.

Literature

1. High quality tube ultrasonic frequencies

Author: tolik777 (aka Viper); Publication: cxem.net

See other articles Section Tube Power Amplifiers.

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