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High-quality 2x10W stereo reproduction set

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High-quality 2x10W stereo reproduction set
Michał Gołębiowski, Radioamator i Krótkofalowiec 1970/05
(The description concerns a model made in cooperation with the editorial office of the magazine and practically tested by the designer)

  Despite the rapid progress of transistor technology, electroacoustic devices equipped with electron tubes are still quite often made by radio amateurs. This is justified due to the still high cost of semiconductor elements and the fact that starting transistor devices is generally more difficult and troublesome than their tube counterparts and requires good knowledge of the issue and extensive practical experience.

  This description is addressed to radio amateurs who already have some achievements in the construction of electroacoustic devices and would like to obtain a higher quality reproduction set at a relatively cheap cost.

Technical data of the stereo amplifier

  • Maximum output power with non-linear distortions in the band 40Hz ÷ 16000Hz and Rload = 7.5Ω less than 1% (sinusoidal signal): 2x10W.
  • Frequency characteristics (1.5dB band): 30Hz ÷ 20000Hz.
  • Sound tone adjustment in relation to the frequency of 100Hz:
    at 60Hz: + 6dB ÷ -12dB
    at 12kHz: + 6dB ÷ -15dB
  • Crosstalk attenuation between the channels in the 30 ÷ 16000Hz band: ≥ 40dB.
  • Stereo balance control: ± 6dB.
  • Input resistance:
      "magnetic adapter" input - 100kΩ
      "crystal adapter" input - 100kΩ
      "microphone" input - 80kΩ
      "radio" input - 1MΩ
      "additional" input - 0,5MΩ.
  • Input voltage for maximum output power at f = 1000Hz and Rload = 7.5Ω:
      "magnetic adapter" input - 5mV
      "crystal adapter" input - 70mV
      "microphone" input - 4mV
      "radio" input - 330mV
      "additional" input - 150mV.
  • Signal-to-noise ratio at maximum output power: ≥50dB.
  • Change in output voltage when the load is disconnected: ≤1dB.
  • Mains power: 220V, 50Hz, maximum power consumption 55W.
  • The remaining parameters are presented in the form of appropriate characteristics in Figures 5, 6, 7, 8 and 9.

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About the durability of electron tubes

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About the durability of electron tubes
(Radioamator i Krótkofalowiec 1970/02)

   Currently, over three million TV sets are registered in Poland. These are almost exclusively receivers based on electron tubes and it can be assumed that in the near future the electron tubes will not be completely replaced by transistors. Taking an average of 15 electron tubes in one TV set and adding to them electron tubes operating in radio receivers and tape recorders, you will get approximately 70 million electron tubes systematically used in these devices. Due to their huge number, the "life" time of the electron tube is interesting. The factories usually guarantee the operation time of this type of electron tubes from one thousand to several thousand hours. This does not mean, of course, that an electron tube (operated in proper conditions) cannot "end" before this time elapses, or work for a much larger number of hours.

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Triode Loadline Simulator

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The program allows to determine the optimal operating point of a triode. It does not require installation and can be run by most web browsers. This is the first version of the program, which will be extended with new opportunities.

Using the program is intuitive. Just click at the following link:

RUN

Here is the screenshot of the simulator window.

To change the parameters use triangular indicators that can be dragged using the mouse. There are six indicators, numbered from 1 to 6.

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Tube amplifier "Concertino"

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Photos and description - Marcin Sławicz

The beginnings of the project

  The idea of ​​building my own tube amplifier has been bothering me for the last two years. I am not a maniacal audiophile and using "ordinary" solid state equipment was enough for me (I always preferred to listen to music than equipment). Now, however, my worn-out amplifier is starting to suffer from the ailments of old age, and although I could regenerate it, there is a great opportunity to implement a tube venture.

  At the beginning I was thinking about a design based only on triodes, but rejecting the SE circuits burdened with too many inconveniences. A very interesting description of the push-pull amplifier with direct filament triodes can be found on the Lynn Olson website. It is worth taking a look there because of the extremely interesting solutions used in his projects. The described amplifiers, however, have a major disadvantage - cost (mainly due to the 300B or 2A3 tubes and interstage transformers). So I had to look further.

  My attention was drawn to indirectly heated double 6AS7 power triodes, once used mainly in power supply systems, but also great as electron tubes in the output stage of audio amplifiers. The cost of electron tubes would be much lower, but due to the low voltage gain factor, in this case, expensive and difficult to obtain interstage transformers or two or more triodes in parallel connection would have to be used. Mr. Russ Sadd described on his website a push-pull amplifier with 6AS7 triodes.

  My project took a few more months, during which I slowly became convinced that a successful power amplifier does not have to have triodes in the output stage. I began to consider the use of beam tetrodes working in the gain stage in an ultra-linear configuration. Such a circuit combines the advantages of triode sound (low distortion) with high efficiency and stability of tetrodes and pentodes. I had a choice of 6L6 / 5881, KT66, KT88 / 6550 tubes, commonly used both in guitar amplifiers and in Hi-Fi designs.

  Another period of my project is searching the net in order to select the basic amplifier circuit. The amplifier should not be complicated, because a complex circuit does not guarantee high-quality sound, and with limited measurement possibilities, it can be difficult to start up. Mass-produced devices must ensure the repeatability of production and the relative stability of parameters during subsequent operation. When designing an amplifier for yourself, you can often take shortcuts without worrying about the subsequent service.

  My choice fell on a well-known layout that has been tested in thousands of homes around the world. It will be the next version of the D.T. N. Williamson. Almost every company that used to produce tube amplifiers had a product to a greater or lesser extent based on this famous circuit. You can find hundreds of articles on the Internet describing different varieties of Williamson amplifiers. So let's take advantage of these rich experiences today.

Design assumptions

  In 1947, Mr. Williamson introduced an amplifier circuit that was a real breakthrough in the pursuit of high-quality sound reproduction. The most characteristic elements of this amplifier are the split load phase splitter and the use of a transformer transmitting the signal in the range of 2Hz ÷ 60,000Hz (a necessary condition for achieving the stability of the amplifier with a closed feedback loop).

  All stages of the Williamson amplifier are, in fact, extremely simple, but at the same time they perfectly cooperate with each other, ensuring relatively low signal distortion. Nevertheless, the system has several drawbacks, which efforts were made to improve in the following years. The figure below shows the 1949 version of the amplifier with the component values marked.

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Radio "Латвия" (Riga) M 137

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Grzegorz Makarewicz "gsmok"

  The concept of the radio receiver was developed in 1947. Its mass production, however, began much later, in September 1950. In 1952 it underwent some modifications and was sold under the name "Mir M152". I have no idea what the modifications were. I have not found any reliable information on this. Some detailed information about the radio can be found on the page "M137". According to the content given there:

"One of the features of the Latvian" M 137 "radio receiver is the scale in which the crosshair is connected to the indicator of the range selector. In each of the 5 bands, a bright dot on a red background indicates the tuning frequency of only the selected band. In reception mode, the line goes out. Radio speaker - "10GDP-VEF" (10 W) with a 250mm diffuser. Among the creators of this radio was Gintauts Aboltins-Abolins, later a well-known constructor of the "Orbita" design office, and from 1968 the head of the Department of Electronic Equipment Design and Production Technology of the Radio Engineering Faculty of Riga University of Technology. In the newspaper "Vefietis" you can find articles about console radios based on the receiver "Латвия M137". One carries the inscription "To my dear commander, father and teacher Józef Vissarionovich Stalin on the 30th anniversary of the Komsomol - Komsomol members and youth of Riga." Another one was created on the occasion of the 10th anniversary of the USSR in July 1950."

  The radio is really impressive. When I was in his possession, my reaction was clear.


Photo 1. I tried to whistle, of course, but my mouth was dry with admiration and my eyes went to different parts of the world.

  I showed the radio to the cats, which usually hang around, and unfortunately was a bit disappointed. Their reaction was quite different. They did not even deign to get off the chair.


Photo 2. The cat at the top is "Cypis", the cat at the bottom is "Deedee". Cats are said to sleep 70% of their lives. "Cypis" sleeps at least 95%.

  Let us return, however, to the subject of the description. Here is the radio in all its glory.


Photo. 3.

Basic parameters of the radio:

  • Sensitivity: not less than 50 μV.
  • Output power - 6W in the frequency range 60Hz ÷ 6500Hz.
  • Power drawn from the mains: 190W.
  • Supply voltage: 110V / 125V / 220V.
  • Dimensions: 642mm / 406mm / 292mm.
  • Weight: 30 kg (I found out about it turning the radio while taking the photos below).

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Low frequency voltage amplifiers

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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

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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).

Regeneration:

  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

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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.

AMPLIFIER

  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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Audio Aero Capitole tube amplifier

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Grzegorz Makarewicz ("gsmok"), This email address is being protected from spambots. You need JavaScript enabled to view it.

   At first glance, another stereo tube amplifier in a push-pull circuit. Classic look with three boxes containing a mains transformer and two loudspeaker transformers and a beautifully displayed "battery" of electron tubes. The manufacturer also presented us with a set of electrolytic capacitors - maybe a slightly less common, but also not very innovative design approach. To sum up - the amplifier is nice, but it is boring. The last glance at the vacuum tubes used and suddenly a surprise - the set of tubes a bit strange, not to say crazy. And it is here that the secret of this construction is hidden. But let's start at the beginning.

   The manufacturer of the amplifier, the company "Audio Aero" was founded in 1997 and its formal association concerned the aviation industry more than audio devices. Well, it happened, and an important French player appeared on the market. "Audio Aero" does not specialize in the production of tube amplifiers at present. The presented amplifier called "Audio Aero Capitole PA" is an example of an ephemeris born at the moment of a flash of a creative genius of a designer and forgotten during the struggle with competition on the difficult audiophile market. A small number of these amplifiers remained on the ruins of the lost war, among them the one that ended up in my hands. Here are its basic technical parameters:

  • Output power: 50W (Class A, 8 ohms load),
  • Tubes used: 6SN7 x 4, E34L x 4, KT88 x 4,
  • Frequency response: 7Hz - 35kHz (-1dB),
  • Input impedance: 22K,
  • Input sensitivity: 800mV,
  • Signal / noise ratio: 70dB,
  • Harmonic distortion: 0.2%,
  • Power consumption: 400W,
  • Dimensions: 520mm (width) x 395mm (depth) x 285mm (height),
  • Weight: 38 kg.

   You can say that I was lucky that the amplifier found its way into my hands, because even on the Internet there is very little specific information about this amplifier. The photo below shows the amplifier in all its glory. It is not a copy that I had the opportunity to repair - the photo comes from company materials (unfortunately I do not know its exact source and I cannot provide the author's data). Two photographs taken by me, showing the appearance of the amplifier, are presented in the further part of the description. Why? Well, because they're not very good and you don't see all the details on them.

   Let us return to the issue of the unusual design of the electronic circuit of the amplifier. This unusual feature consists in the use of a parallel connection of triodes and pentodes in the output stage. In fact, only pentodes work here, but in each of the four sets of tubes - one of them (in this case E34L / KT77) is connected to a triode circuit, while the other (KT88 / 6550) works as a pentode in "ultralinear" mode . The previous photo and the two below show the full set of tubes used. The photos taken by me are a bit too dark and not very detailed. Unfortunately, once again, driven by curiosity, after receiving the amplifier, I immediately started to disassemble it and document the interior, and after the repair I forgot to take pictures and took them just before handing the device over. I cannot learn to be systematic and in many reports in the Gallery section I have a problem with showing nice photos. Well, no more self-pity. Coming back to the matter, the photo shows this extremely interesting combination of output tubes in each channel of the amplifier.

Read more: Audio Aero Capitole tube amplifier