Low frequency voltage amplifiers

Low frequency voltage amplifiers
R.Jachimiak, Radioamator 12/1954

   Low-frequency amplifiers play an important role in amateur radio practice; they are found almost everywhere. However, the design and manufacture of the amplifier itself is not very easy. The biggest problem is choosing the right capacitors and resistors so that the system works flawlessly and shows the smallest possible percentage of distortions with the appropriate amplification. The simplest and, at the same time, the cheapest one is an amplifier with resistance-capacitive coupling. For the use of radio amateurs, a number of tables are provided, which are not difficult to use. Having the tube we want to use in the low-frequency voltage gain stage, we search the tables (for this type of tube) for the values of the remaining elements of the amplifier. It also lists the circuit gain K, the percentage of distortion at the Z output, and the voltage that is needed to design the next voltage gain or power gain stage.

   The tables presented are prepared for most of the tubes for triodes and pentodes, respectively. It should be remembered that the given sizes of coupling capacitors Cs and blocking Ck, Ce are the smallest values that can be used. Their values can only be rounded towards the higher values. If the values of cathode capacitors are not given, a capacity of several to several dozen microfarads should be used. All symbols given in the attached tables have been marked on the general diagrams so that they do not need to be discussed in a special way.

In the event that the tube stated is not a single triode but incorporates two systems, such as a double triode, a triode with a diode etc., only the single triode system should be considered. The other systems can be used independently of each other. This also applies to pentodes.


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Regeneration of radio tubes

Regeneration of radio tubes
RADIO Monthly magazine for Technicians and Amateurs, 1st Year, May 1946, No. 3
(Trioda website is not responsible for the content of the article)

  Difficulties in finding older types of electron tubes on the market and their high cost force us to consider the issue of restoring electrical properties to electron tubes which, due to long-term operation or short-term overload, have lost their emission capacity and are not suitable for use in radio receivers.

  The subject of the article will be to provide an experienced radio amateur with a description of electrical methods for regenerating radio tubes. Of course, there can be no question of restoring the emission properties of electron tubes with defects of a mechanical nature, such as a burnt cathode, a short circuit between the electrodes or a bad vacuum. Only tubes with too low emission current can be considered.

  The process of regenerating the cathodes of the receiving tubes is nothing more than an attempt to re-form the cathode, which consists in carrying out thermochemical processes on the cathode surface. As a result of thermal treatment, the so-called an active layer of a metal (eg thorium, calcium, barium, etc.) emitting electrons at a relatively low temperature (about 1000 ° K). This layer may be exhausted by temporary overload or as a result of long-term work. If there is a sufficient reserve of metal used to emit electrons inside the cathode, the electron tube can be reactivated. By analogy with the forming process, regeneration is carried out by heating the cathode to a temperature well above the normal operating temperature, generally distinguishing between two types of regeneration:

  1. overheating of the cathode without drawing emission current,
  2. overheating of the cathode while switching on the voltages of other electrodes.

  The result of the regeneration process depends on the knowledge of the data on the method of forming the cathode of the reactivated vacuum tube. These data for various types of tubes and cathodes are different and mostly by companies producing radio tubes are protected as factory secrets. In addition to the cathode formation data, it is important to determine the degree of cathode wear. The state of wear can be determined by carrying out microchemical tests, during which destruction of the tube of the tube is unavoidable. Therefore, it is impossible to provide exact formulas regulating the reactivation processes of radio tubes. In any case of regeneration we are dealing with randomness. If the electron tube has a supply of electron-emitting metal in the cathode fiber, the regeneration process may be positive. Otherwise, the tube should be treated as useless..

  After these preliminary remarks, we will discuss the appropriate methods of regenerating radio receiver tubes. Depending on the type of cathode structure, various regeneration methods are used.

1. Directly heated cathodes.

A) Thrusted cathodes.

  This type of tubes can be recognized by a bright mirror covering part of the inside of the glass envelope (e.g. Telefunken tubes type RE 054, 064, 154 and others).


  The cathode is heated with the filament voltage, gradually increasing over the course of 10 minutes from the nominal value to twice the value. We do not charge the emission current. Measurement of the anode current increase is a test of success of the regeneration attempt. In case of a negative result, we use the second method of regeneration. The electron tubes, with all nominal voltages connected, are heated with a filament voltage of 1.2 times the nominal voltage. When controlling the anode current, we make sure that the power dissipated at the anode does not exceed the allowable power. If the anode current does not increase, we lower the filament voltage to the nominal value, turn off the voltages of other electrodes and heat the electron tube for a few minutes under these conditions. Then we turn on the anode voltage and observe the anode current with the filament voltage gradually increased by 20%. Such attempts, if we especially care about a given electron tube, can be repeated several times until the desired effect is obtained.

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Stereo audio system for playing records

Radioamator i Krótkofalowiec polski, Rok 14, Maj 1964 rok, Numer 5.
(Radio amateur and amateur radio operator, Year 14, May 1964, Number 5.)

mgr Zdzisław Krzystek.

  The apparatus consists of a "Ziphon" turntable, a 2x4W wideband amplifier and two loudspeakers in closed housings with an opening. It provides sufficient volume for the reproduction of music in the living room.


  The schematic diagram of the amplifier is shown in Figure 1.

Figure 1. Schematic diagram of the amplifier.

    There is a "Mono-Stereo" switch at the amplifier's input, which should be closed when reproducing monophonic recordings. This slightly reduces the noise from the turntable, as the transducer is then insensitive to the penetrating vibrations of the needle (note: this switch is not present in the photos).

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Simple measurement of the number of turns in a transformer

Simple measurement of the number of turns in a transformer.
RADIO Miesięcznik dla Techników i Amatorów, Rok IV, Styczeń-Luty 1949r., Nr 1/2
(RADIO Monthly magazine for Technicians and Amateurs, Year IV, January-February 1949, No. 1/2)
(Trioda website is not responsible for the content of the article)

  We often have difficulties with determining the number of turns in a transformer. In many cases, unwinding the transformer and recalculating the turns in this way is pointless, especially when we want to use one of the factory windings in an undamaged transformer, and the other, based on the calculation, to be wound up.

Fig. 1.

  Fig. 1 shows a system with which we can easily determine the number of turns in a transformer winding, without the need to unwind it with sufficient accuracy for practice.

  On the core of the transformer, one winding of which we want to examine, we wind one coil of thick (about 1mm) insulated wire. This one turn is connected through an adjustable resistor Rr and through an ammeter [A] for alternating current, with the heating winding of some other transformer Tr1 (eg with a voltage of 4V), which we will use as the current source in our measurement. The winding of the transformer Tr2, on which the measurement is performed, is connected through the switch [W] with a sensitive milliammeter [mA] for alternating current. First, leave the tested winding circuit open (the switch [W] is open).

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Acoustic tube amplifiers

Acoustic tube amplifiers
Radioamator, Rok XI, Luty 1961, Nr 2 (Radioamateur, Year XI, February 1961, No. 2)

Simple 2-tube amplifier

  The output power of this amplifier is 3W with a harmonic distortion coefficient of 2.5%; the sensitivity of the amplifier is 150mV. In order to minimize mains hum, the cathode of the first tube is grounded (Fig. 1), and the negative voltage is obtained due to the voltage drop caused by the grid current in such a system is very small, so the input resistance of the tube is approximately half the leakage resistance.

  The output stage is conventional with negative feedback for adjusting the frequency response. In the left position of the potentiometer in the negative feedback circuit, the frequency response is raised for the lowest and highest frequencies of the acoustic band. In the right position of the potentiometer slider there is a significant weakening of higher frequencies, starting from 1000Hz. Any rectifier with a voltage of about 240V and a current of up to 40mA can be used to power the amplifier. The rectifier should have a ripple smoothing filter. The output transformer of the amplifier can be made on a core with a cross-section of 16x16mm, the primary winding should have 3500 wire turns 0.15 in diameter, and the secondary winding - 165 wire turns 0.65 in diameter (for a loudspeaker with a resistance of 4 ohms).

Fig. 1.

Amplifier with an output power of 3W

  This amplifier has better quality paremeters than those described previously, and besides, a separate adjustment of the frequency response in the range of low and high frequencies of the acoustic band. The output power of the amplifier is 3W with a harmonic distortion not exceeding 1.5%. The frequency response is adjustable within ± 16dB at 100Hz and within ± 14dB at 10kHz. Amplifier sensitivity - 100mV.

  The schematic diagram of the amplifier is shown in Fig. 2. The negative feedback loop contains RC elements selected in such a way that the strongest negative feedback falls on the middle part of the amplifier's passband. As a result, the gain in the 400-2000Hz range is lower by about 16dB than for low and high frequencies of the acoustic band. To adjust the frequency characteristics of the amplifier, two potentiometers at its input are used. With the help of a potentiometer with a resistance of 1Mom, the characteristic can be adjusted in the high frequency range. Similarly, the 4.7Mom potentiometer controls the characteristics in the low frequency range.

Fig. 2.

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Speaker amplifier "3L"

Speaker amplifier "3L"

eng. Czesław Klimczewski
Radioamator, Rok III, Styczeń 1953r., Nr 1 (Radioamateur, Year III, January 1953, No. 1)

  In issue 11 of our magazine there is a description of a loudspeaker pickup (amplifier) with two electron tubes operating in a push-pull arrangement using a transformer that controls these tubes. As it is not always possible to purchase such a transformer on the market, winding it presents a certain difficulty - a description of the amplifier is now given, in which the function of the so-called the "phase inverter" is not accomplished via a transformer but via a tube. The amplifier also does not have a low-frequency choke for filtering the rectified pulsed current, but a resistor with a resistance of 3000 ohms and a load capacity of about 5 watts. Such a resistor is easy to buy, or you can wind it yourself on a porcelain or glass tube.

  Due to the non-use of a transformer and choke, the cost of the described amplifier is relatively low and its assembly is easier. This amplifier, made strictly according to the given diagrams, will work perfectly and can power one or more speakers with a total power of about 25 watts.

  Fig. 1 shows a schematic diagram of the amplifier. This amplifier is adapted to work with a radio receiver with sockets for connecting an additional loudspeaker. These sockets must be connected to the amplifier in such a way that one of them, marked with a "plus" sign, should be connected to the socket marked with a "b" sign. If there is no marking on the receiver housing, then the sockets of the additional loudspeaker are connected in it in such a way that the amplifier can be connected freely without paying attention to the compatibility of connections. This attachment can also be combined with a low-frequency amplifier, the so-called "preamplifier" to obtain more power for driving the loudspeakers. For this purpose, the described amplifier uses a W1 switch that turns off the resistance R1 = 5000 ohms, so that it can be connected to a radio apparatus or an amplifier having a small "output" resistance - of the order of 500 ohms or a large one - of the order of 5000 ÷ 6000 ohms. Radio apparatuses have the output resistance of the additional loudspeaker, usually high - in the order of 6000 ohms.


Fig. 1.

  The voltages received from the spool of the potentiometer P1 are sent to the control grid of the first triode of the tube (6SN7) through the resistor Rs = 50,000 ohms (50K), which is a eliminator for possible parasitic currents. This grid is also connected to the 10.000 pF capacitor, which is connected via P2 = 100K potentiometer to the grounded base of the device ("ground"). This potentiometer is used to adjust the "timbre" of the amplified programs.

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Universal 15VA HI-FI amplifier

Universal 15VA HI-FI amplifier.
Radioamator, Rok X, kwiecień 1960, Nr 4 (Radioamateur, Year X, April 1960, No. 4)

Editorial office: The following description concerns a system, the model of which was built at our request and practically tested by the designer.

The amplifier described here is perfect as a final power amplifier for playing music from records or tapes in the apartment. It is also suitable for broadcasting dance music in medium-sized halls and clubs.

Construction is easy; it can be performed by any radioamateur with basic theoretical and practical knowledge.

Output stage.

Due to the need to obtain a power equal to at least 10VA with very low nonlinear distortions, I decided to use the output stage amplifier in a push-pull configuration with negative feedback. This is called "ultralinear" system. The feedback voltage in this system is obtained from taps, or from separate windings on the output transformer. By changing the ratio of the alternating voltage of the screen grid to the anode alternating voltage, we change the working conditions of the end tubes. With this ratio equal to one, the tubes work as triodes, because the shielding grids are connected with the anodes, while with the ratio equal to zero, they act as pentodes (Fig. 1).

Fig. 1. "Ultralinear" system.

For the ratios with intermediate values, the circuit has a number of advantages over push-pull circuits, both with triodes and pentodes, and above all, less nonlinear distortions at large and small signals, at the cost of low power loss. Moreover, such feedback significantly reduces the internal resistance of the system.

For EL34 tubes, the 20% tap is the most advantageous (Usz / Ua = 0.2, of course, from the power supply side).

The "ultralinear" circuit with EL84 tubes gives about twice less nonlinear distortions compared to the conventional push-pull circuit with 10% lower power. The "ultralinear" circuit requires a special design of the output transformer.

Output transformer.

Both halves of the anode windings of the transformer should be placed symmetrically by dividing the window into two sections, each for one half of the anode winding. The unbalance has a significant impact on the phase shifts and amplitude differences between the anode current and the screen voltage, so that even harmonics of higher frequencies may not reduce each other.

Fig. 2 shows a diagram of the windings of such a transformer, and Fig. 3 shows the arrangement of the windings.

Fig. 2. Diagram of the windings of the output transformer.
Primary windings: 0.18mm copper wire in enamel.
Secondary windings: 0.75mm copper wire in enamel.

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Portable universal amplifier

Portable universal amplifier.
Radioamator, Rok X, wrzesień 1960, Nr 9 (Radioamateur, Year X, September 1960, No.)

This description concerns the model awarded with the consolation prize in the Great Radio Amateur Model Contest.

From the editorial office:

The amplifier described below is an example of a well-thought-out design for a specific purpose. A simple but full-fledged system, proper use of power thanks to the use of efficient domestic speakers, good adaptability to portability, cheap and effective finish - these are the main features of the device. We can recommend the construction of this device to anyone who wants to have a good portable amplifier for playing recordings from discs, enhancing solo performances and increasing the acoustic power of the receiver when using dance music..

The AMPLIFIER was designed and made for a stage singer producing with an electric guitar. On this assumption, the device should meet the following conditions:

  • small dimensions and weight,
  • high quality playback,
  • high (10W) output power,
  • easy to carry and easy to use.

It would be very difficult to meet the above conditions to the full extent, therefore a compromise solution was necessarily applied in the model made. First of all, a system with a mains transformer was adopted (Fig. 1), although in the universal version the weight of the apparatus would be absolutely lower.

Fig. 1. Schematic diagram of the amplifier.

Nevertheless, it was found necessary to galvanically separate the amplifier, microphone and guitar circuits from the network. In contrast, the reduction in dimensions and weight was achieved by a different route. The output power of the amplifier was limited to 5W, filling its final stage with a single EL84 pentode. The reduced output power, in turn, was used as rationally as possible, feeding two lightweight speaker sets with relatively high efficiency.

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New types of tubes for push-pull and stereo amplifiers

New types of tubes for push-pull and stereo amplifiers.
Radioamator i Krótkofalowiec, Rok XI, Marzec 1961, Nr 3

Among the many new types of receiver tubes that have recently been introduced to the market by Western European manufacturers, the ELL80 and PLL80 dual speaker pentodes deserve special attention. The idea of placing two tube systems in one bulb has been known since the birth of the now obsolete ECL11 tube, which at the time was a kind of revelation; it was followed by further, more modern types, such as ECL82 ... 86 and their equivalents in the U and P series. Placing two end pentodes in one balloon is, however, something completely new, resulting from the current needs of the electronics industry, in particular from the needs of modern stereo technology.

The final stages of the stereo amplifiers were initially filled with the standard ECC83 + 2xEL84 set, consisting of a total of three tubes. The use of ECL82 tubes made it possible to reduce the number of  tubes to two, and - with the same number of stages - had an impact on the cost of the device. However, equipping the same circuit with the ECC83 - ELL80 set is more rational at the same cost, as it allows for a favorable and transparent assembly, allowing you to easily avoid undesirable microphonics, various types of couplings, etc. As you know, two stages with a very strong overall amplification (in the case of an ECL tube) is quite critical and requires careful elaboration of both electrical and mechanical systems in terms of the stability of the system. The popular ECL11 tube was particularly capricious in this respect.

Fig. 1. Schematic diagram of the 2 x 3W amplifier with ECC83 and ELL80 tubes and its technical parameters.

Fig. 2. Schematic diagram of the 2 x 2.6W amplifier with ECC83 and PLL80 tubes and its technical parameters.

Fig. 3. Schematic diagram of the push-pull amplifier with ECC83 and ELL80 tubes and its technical parameters.

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Design of output transformers


Radio amateur and amateur radio operator of Poland, Year 24, December 1974, No. 12.

Low-frequency tube amplifiers, especially with higher power, are still built by radio amateurs. The output transformer is the most difficult element to design and manufacture. This is evidenced by inquiries and requests for help in the calculations sent to the editorial office. The basic principles of designing transformers intended to be made in amateur conditions presented here briefly should satisfy the wishes of interested readers.

The principles of designing low-frequency transformers in amateur conditions are slightly different from those used in industry. First of all, it is determined approximately what core is needed for the designed amplifier. Then a more or less suitable core is searched for, and after it is acquired, further winding calculations are made. After establishing approximate data as to the necessary winding wires, wires with diameters similar to the selected ones are purchased and only then the number of turns of individual windings is finally determined.

The basic relationships linking the phenomena in a transformer result from the following formula:

Etr = 6,28⋅f⋅n⋅Q⋅B⋅10-4        (1)


  • Etr - the amplitude of the reverse-electromotive force induced in the primary winding, approximately equal to the amplitude of the supplied voltage [V],
  • f - frequency [Hz],
  • Q - core cross-section [cm2],
  • n - number of turns of the winding,
  • B - the highest induction value in the core [T].

The value of the reverse electromotive force is related to the alternating voltage of the final amplifier stage and results from the power and operating resistance. The highest and the lowest frequency of the passband results from the assumptions. The highest allowable induction value in the core should not exceed 0.6T. For Hi-Fi amplifier transformers, it is recommended to take 0.4T. Two unknowns remained in the formula given: the core cross-section (Q) and the number of turns (n). We determine the core cross-section approximately from the formula:


  • Pwy - the output power of the amplifier.

As far as possible, we aim to build a transformer with a large core cross-section, which will allow to reduce the number of turns in the windings. This is important both due to the undesirable leakage inductance of the transformer and the degree of difficulty of its manufacture. In transformers composed of sheets with holes for fastening bolts, it is necessary to check that the core cross-section near the bolts is not smaller than that of the main column.

The simplified transformer substitute circuits are shown in Fig. 1. At the lowest frequency, the influence of the inductance of the transformer primary winding, which is connected in parallel to the appropriate amplifier load, should be taken into account. In most cases, it is the necessity to obtain a sufficiently large value of this inductance that determines the number of turns of the primary winding. At medium frequencies (1000Hz is assumed), only the winding resistances play an important role. At high frequencies, the influence of the leakage inductance is noticeable, the value of which depends on the number of turns, the transformer winding scheme and its quality. This inductance, in combination with the inter-winding capacitances, creates a low-pass filter limiting the transformer bandwidth..

Fig. 1. Simplified equivalent transformer diagrams.
a - equivalent circuit for the lowest frequencies,
b - equivalent circuit for medium frequencies,
c - equivalent circuit for great frequencies (treble and ultrasound).

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