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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING On the manufacture of output transformers for lamp UMZCH. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers An interesting trend is observed: the further we move away from the "tube" era, the more myths and fog are created around the output transformer of a tube amplifier. And not only in matters of calculation, but also in its manufacture. Manufacturers can be understood, praising their products is the law of advertising, but in many articles by independent authors, the process of winding a transformer looks like a description of a secret rite. Let's see how difficult it is and how much time it takes. The conversation will focus on output transformers for single-ended stages, as well as other transformers where high symmetry of half-windings and strict requirements for operating conditions are not required. It is assumed that you have a sufficient section of the magnetic wire, winding wires and at least a primitive device for winding coils, equipped with a coil counter. This refers to any design - from an electric or hand drill, clamped in a vice, to a bent threaded stud, reinforced in two wooden bars.
Making a coil is laborious, but not difficult. A drawing of the details of the frame of the prefabricated coil made of getinax or textolite with latches is shown in the figure. In the drawing in position 1 - cheeks; 2, 3 - plates. The dimensions h, b, y, y1 and the thickness of the frame parts are related to the dimensions and shape of the magnetic circuit. The best material for its manufacture can be considered fiberglass (without foil) with a thickness of 1,5 ... 2 mm. When manufacturing parts, leave an allowance for final fine-tuning during assembly. If you try to immediately cut the part to size, then there is a high probability that nothing will snap into place, and the coil will fall apart. At the assembled reel, file sharp corners with a needle file and wrap with one or two layers of paper 0,1 ... 0,15 mm thick. It will take two to three hours to make a coil. We will not touch upon the manufacturing technology of a biscuit design transformer at all, since with a relatively small number of biscuits it loses to a classical design with a shallow sectioning both in terms of the duty cycle and in terms of leakage inductance. Then the more interesting begins - winding. Most amateurs use ordinary winding, that is, the wire is wound coil to coil, and a gasket is laid through each layer. Winding in this way without a machine with a stacker of 3000-4000 turns with a thin wire is a titanic work. The question arises: why not wind up in bulk? If we discard the noble indignation of true audiophiles and turn to the primary sources [1, 2], it turns out that the fill factor for a thin wire (0,15-0,4 mm) is not so bad: G. Tsykin gives values of 0,7 .. .0,75, I got 0,5 ... 0,53, which is quite acceptable for single instances of a transformer with sectioned windings. The leakage inductance is practically independent of the winding method and density. The self-capacitance of the winding (when winding in bulk) is 5 ... 10% less. The main problem seems to be reduced dielectric strength. By the way, high values of the fill factor make it possible to make the transformer smaller or to obtain a large magnetization inductance in the same dimensions. This is important, since for high-quality devices one should strive to implement a transformer with minimal dimensions for a given primary winding inductance. The smaller the size of the transformer magnetic core, the better - the lower the leakage inductance for a given sectioning. Let's get back to ensuring electrical strength. Everything in the books is correct, but most of the recommendations relate to the serial production of transformers and their compliance with certain standards. It is unrealistic to make a transformer in accordance with them at home: there are neither appropriate materials nor technologies. Therefore, we will proceed from two criteria: the first is real operating conditions, the second is unacceptable in production, it is quite suitable for self-production of single samples. So what voltage can be on the primary winding of the transformer? Suppose the output power P of the amplifier is 5 W (this is a lot for a single-cycle cascade on common lamps), the load resistance R reduced to the primary winding is 2 kOhm, the supply voltage Ua is 300 V and the efficiency of the transformer is 0,85. To obtain such power, the effective voltage on the primary winding must be equal to: Urms= √PR/Efficiency= 117V. Accordingly, its amplitude will be equal to: Urms= √2 Urms = 166 V. Taking into account the supply voltage, the maximum voltage on the primary winding relative to the amplifier case will be equal to: Uw - U + Ua - 466 V. This determines the requirements for interwinding insulation (as a rule, one end of the secondary winding is grounded) and the insulating properties of the frame. Two layers of cable paper with a thickness of 0,12 mm are enough, you can use condenser paper in 4-5 layers or a combination of a layer of sanitary fluoroplastic tape and a layer of writing paper. The fiberglass frame more than provides the necessary electrical strength. High-quality output transformers are always made sectioned, otherwise it is not possible to obtain acceptable values of leakage inductance. In the simplest case, the primary winding is divided into two parts, but better - into three, between which the secondary winding is placed. Deeper sectioning is also possible, but at the same time, the fill factor of the magnetic circuit window is significantly reduced and the capacitance between the windings increases. Due to the complexity of winding, deep sectioning is rarely used. Let us dwell on the three sections of the primary winding. The minimum leakage inductance is achieved with an uneven division of the number of turns - in the extreme sections, their number is two times less than in the middle. If we neglect the active resistance of the winding, then in the absence of a signal, all turns of the primary winding are equipotential; at maximum power, the voltage across the parts of the winding will be proportional to their inductance. Therefore, the maximum alternating voltage occurs on the middle section of the winding; its amplitude is 83 V. The breakdown voltage of the insulation of a winding wire with a diameter of more than 0,15 mm (PETV, PEV, PVTL, etc.) is not less than 600 V, and the number of microdefects is permissible no more than 5-7 per 15 m. For a wire with a diameter of more 0,35 mm microdefects are generally unacceptable. Therefore, the winding can be wound in bulk without any gaskets at all; the probability of occurrence of short-circuited turns is very small. For better laying of turns and increasing the reliability of the transformer, it is advisable to lay a gasket of 300 mm thick capacitor paper in two layers every 500-0,022 turns of the winding (such paper tape can be obtained from old paper capacitors - for example, the KBG group). Therefore, the main task when winding a transformer is to prevent the turns from falling. Interwinding insulation is achieved in a standard way - the gasket is made wider than the frame by 4-5 mm and a notch is cut along its edges. This can be done quickly by rolling the gasket into a tube: its edge is bitten along the contour with sharp wire cutters. Since in this case thicker and stiffer insulation is used (both for dielectric strength reasons and for the possibility of normal laying of the next winding), the coils will not stick if you are careful enough. It is desirable to exclude the falling of the coils when laying the interlayer insulation. Here difficulties arise. Since the surface of the winding has irregularities, even if there is a notch on the edges of the gasket, it is not possible to exclude the sinking of the turns - the wire pulls it together. This issue is resolved as follows. A bandage is applied to the edges of the gasket from a narrow strip of thin sticky paper (you can use "painting tape") with a notch along the edge, it keeps the gasket from slipping (or closes the turns from which the gasket has already slipped). So, the order of winding the transformer is as follows - the primary winding sections are wound in bulk with interlayer spacers every 300-500 turns, the secondary winding sections - turn to turn without spacers (with a wire diameter of more than 0,6 mm, this process does not cause difficulties). I remind you once again that the interwinding insulation must be sufficiently rigid - the turns of the secondary winding must lie flat. When winding the sections of the primary winding, it is necessary to ensure sufficient tension on the wire and try to keep the surface of the winding as even as possible. By the way, when winding, it is advisable not to touch the wire with your hands, but to hold it with a piece of thin felt or soft suede. Winding is carried out from edge to edge of the coil. The winding leads are made directly with a winding wire with a fluoroplastic tube put on it (a thin tube stretches perfectly; stretching a millimeter tube, you can get a tube of a smaller diameter). If the wire is too thin, then to increase the mechanical strength of the output, the wire is folded three to four times and tightly twisted. This pigtail is used as the output of the winding, of course, its beginning must be insulated and securely fixed to the winding. Conclusions from colored wires, of course, are more beautiful, but this option is more practical. The final insulation of the windings is made of two layers of cable paper (you can also use writing paper). The filling factor of the window of the magnetic circuit with two sections of the primary winding is about 0,45, with three sections of the primary winding - about 0,4. These are averaged data on the results of winding several dozen transformers of different capacities. It is quite possible to cope with such work, depending on the experience, in a couple of evenings. Why is a transformer coil impregnated? The main goal is to increase the electrical strength under adverse external conditions; impregnation also improves heat removal from the inner layers of the coil and increases its mechanical strength. Of course, there is a downside to the coin, any impregnation increases the transformer's own capacitance. In 99,9% of cases, an amateur amplifier is in a place of honor in a room under almost normal conditions. The thermal load on the output transformer of a high-quality amplifier is also not great. Firstly, such transformers are designed according to slightly different criteria than network ones, and secondly, when listening to music, even if the amplifier has a significant output power, the average output power is only a few watts. Therefore, I do not recommend using any impregnation and thereby worsen, even slightly, the electrical parameters of the transformer. Of course, if you intend to listen to music in a tropical climate, plan to install an amplifier in a car or offer it to a rock band, then you need to think about the impregnation composition and impregnation method. Another thing is the magnetic circuit of the transformer. In amateur practice, twisted magnetic cores from serial transformers are often used, which tend to delaminate when disassembled. This is not dangerous, but peeled records will create overtones. If possible, they should be glued, but this will not do much. An effective way to calm the transformer (you still need to glue it) is to dip the horseshoes of the magnetic circuit in oil varnish before final assembly. It is also advisable to paint over the laminated magnetic circuit with varnish. During the final assembly of the transformer, the gasket that forms the non-magnetic gap is coated with the same varnish (for SHL and PL, there are three and two of them, respectively), the thickness of which is specified in the calculation. It can be made from a thin sheet of electric cardboard, textolite, getinaks or other hard heat-resistant material. It is very important to ensure that the gap in the magnetic conductor is fixed with a reliable tie: the stability of the gap helps to minimize the non-linear distortion of the transformer itself at low frequencies. A transformer manufactured in this way will have electrical parameters that are no worse, and possibly better, than those made in the factory workshop. In conditions close to normal, such transformers work flawlessly. So, the complexity of self-manufacturing of the output transformer is greatly exaggerated. The main troubles are connected with the search for a magnetic circuit, winding wires and related materials, and not with winding. The key to good results is the usual accuracy and attentiveness. Even without experience, it is quite possible to make a set of output transformers for a stereo amplifier in a week. Of course, not everything can work out right away, but water does not flow under a lying stone, so feel free to get to work and assemble your best tube amplifier. I note that now there are many modern insulating materials, so it is not necessary to use paper at all. The use of polyethylene terephthalate, lavsan film, reinforced fluoroplastic, fiberglass is welcome; use whatever is easier to get. Power amplifiers may experience a significant voltage drop across the output transformer when the load is suddenly shed. If, during comparative listening of equipment, you prefer to switch the load on the go, then you should not increase the electrical strength of the transformer, it is easier to bypass its primary winding with a suitable varistor or 1 kV arrester. Naturally, the quality of the transformer also depends on the magnetic circuit used, but this should not be raised to an absolute. Electrical steel 3411 was most commonly used in power transformers for household appliances. It is inferior in its magnetic properties to modern steels (manufacturers often use steel 3408), but these differences are not so great that they cannot be partially compensated for at the transformer design stage. On a twisted magnetic circuit from a network transformer, you can make an excellent output transformer. In general, there is an interesting paradox. Many manufacturers offer high-quality output transformers, but limit themselves to bringing only their main parameters - pure "pig in a poke". And transformers with magnetic cores made of steel 3408 and an amorphous alloy are "two big differences"! Literature: 1. Tsykin G.S. Transformers of low frequency. - M.: Svyazizdat, 1955.
Author: E. Karpov, Odessa, Ukraine; Publication: radioradar.net
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