Lamp UMZCH with transformers from the TV. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Lamp UMZCH with transformers from the TV. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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In the manufacture of structures with transformers by radio amateurs, one of the problems is their winding. In this article, the author recommends using ready-made transformers from old TVs, from which capacitors, a choke, and dynamic heads can also be removed for the design described here. These details will take on a second life.

For radio amateurs who wish to assemble a tube UMZCH, a problem arises, often insurmountable for beginners, the need to manufacture an output transformer. The great laboriousness of this work, the lack of the necessary winding wires or the required magnetic circuit often scare away radio amateurs. Meanwhile, some commercially manufactured transformers can be successfully used as an output transformer. For example, the network transformer TS-200-2 from the black-and-white TV "Seagull" or "Temp-209" can also be used as an output, and without any alterations in it.

The only thing that is required is to solder the flexible leads of the secondary winding (these are leads 13, 14 and 13', 14') and solder them according to the diagram in fig. 1.

Lamp UMZCH with transformers from the TV

The sound quality of the UMZCH, assembled according to the "classic" scheme with a push-pull end stage on 6P14P lamps and such a transformer, exceeded all expectations. The nominal output power of this tube amplifier at a load of 4 ohms reaches 6 watts per channel. This, of course, is not much, but more than enough for residential premises. In addition, the main thing is not power, but the naturalness of the sound.

A diagram of such a UMZCH is shown in Fig. 2. The device has some features: the absence of a common negative feedback covering the entire amplifier, increased resistance of grid resistors R7, R13-R15, which reduces the load on the previous stage and reduces non-linear distortion.

(click to enlarge)

The signal is input through the treble tone control R1C1 and the volume control R2. The triode of the VL1.1 lamp has a divided load of resistors R4 + R5 and R6, from which the voltages opposite in phase are removed, which are necessary for the operation of the push-pull cascade. The right lamp triode VL1.2 is used in the first stage of the second channel of the stereo amplifier. A pair of VL2 triodes forms a preamplifier to obtain the necessary signal amplitude for "building" a powerful cascade. The small non-linearity of the amplification of this para-phase cascade is successfully compensated for by even harmonics when the signal is summed in the UMZCH output transformer.

The output stage of the UMZCH operates in an ultralinear mode: screen grids of powerful pentodes are connected to the taps of the primary winding of the output transformer to form a local OOS. This feedback, along with the reduction of non-linear and frequency distortion, also reduces the output impedance of the stage, improving the damping of the loudspeaker. The trimming resistor R20 in the cathode circuit is used to equalize the currents of the VL3, VL4 lamps. Minimization of distortion is achieved by adjusting the balance with resistor R5 and selecting pairs of lamps.

The quality of sound reproduction with such an output transformer is rated as very good both at medium and high frequencies. The author tested the broadband UMZCH with a rectangular signal. The characteristic for tube amplifiers, the blockage of the fronts of the pulse signal was observed only at the highest (15 ... 20 kHz) frequencies and was insignificant, which indicates a good frequency response of this UMZCH.

A few words about the design and adjustment of the amplifier. It is assembled on a panel of foil fiberglass with a thickness of 3 mm. The foil is used only as a screen and is connected to a common wire near the UMZCH input. Mounting the mounted amplifier, on single-pin mounting racks. Unlike printed wiring, it often allows for lower parasitic capacitances and reduces the chance of unwanted capacitive coupling between stages. Wiring diagram not shown; hinged mounting is simpler, and any radio amateur can develop it himself, depending on the parts available.

As mounting racks, you can use pieces of foil textolite up to 10x10 mm in size, glued to the carrier panel, and strips of this material as a common wire and power bus. Lamp wires must be twisted.

It is desirable to select lamps VL1 and VL2 according to the minimum noise level, and lamps VL3 and VL4 should be close in their parameters. If you take new lamps from the same batch, this is often enough, and matching is not required. Isolating capacitors C2 - low-voltage film (not ceramic), C5, C7, C8, C10 - with an organic dielectric, for example, paper K40U-9, polypropylene K78-2 or polyethylene terephthalate K73-9 for an operating voltage of at least 400 V. Oxide capacitors - K50-32 or imported (Jamicon and similar). Resistors - C2-23 or similar. Resistors and capacitors, standing in different arms of the UMZCH and performing the same functions, it is advisable to select in pairs with a spread of no more than 2%.

As a mains transformer T2 and inductor L1 of the power supply of the amplifier, the corresponding products from almost any tube TV are suitable.

Establishing UMZCH comes down to checking lamp modes (see table).

Lamp UMZCH with transformers from the TV

and balancing the final stage. By setting the volume control slider in the lower position according to the diagram, a DC voltmeter is turned on between the anodes of the VL3 and VL4 lamps and by adjusting the mode with the resistor R20, the voltage is zero.

Then, a load or its equivalent (a powerful 4 ohm resistor) and an oscilloscope are connected to the UMZCH output. By applying a signal with a frequency of 1000 Hz from the 3H generator to the input, resistor R5 achieves a symmetrical limitation of the output signal with a slight overload. It should be remembered that without a connected load, it is impossible to apply a signal to the input of the amplifier.

The output power of such an amplifier can be increased by using more powerful lamps in the output stage (for example, 6PZS, GU-50) with a corresponding change in modes.

Author: A.Dmitriev, Podolsk, Moscow region

See other articles Section Tube Power Amplifiers.

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