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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.

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Xindak MT-3 amplifier

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Grzegorz 'gsmok' Makarewicz

   A tube amplifier with the so-called middle shelf. Traditional circuit solution with non-traditional, rarely used ability to switch between triode and pentode modes. I was encouraged to post a short description and a not-so-short set of photos by the information appearing in many places that it is an automatic polarity amplifier that does not require the adjustment of the quiescent current of the output tubes. Well, it is not true and I warn users of this amplifier against a lighthearted approach to this important issue.

   The first photo shows the amplifier which heats up during the procedure of adjusting the quiescent currents after replacing the electron tubes in the amplifier's power stage.

   Looking for information about the XINDAK MT-3 amplifier, I was surprised to find that despite a large number of sales offers and positive opinions, there is practically no detailed data on its SAFE operation. Replacing the electron tubes in a tube amplifier is not the same as changing a toothbrush. It must be approached in a thoughtful and, above all, safe manner. And here I come back to the introduction. In one of the audiophile magazines (not Polish, fortunately) I found an opinion expressed by an "expert" on the subject that due to the automatic polarity of electron tubes, this amplifier is a particularly good proposition for those who like to experiment with electron tubes. They can replace them at will without the need for any regulations. I have very little hair left on my head, but this remnant was brought to attention when I read about it. As an example of the dangers related to the replacement of electron tubes, let us use the fact that after replacing the tubes in the amplifier, which I present here without any regulation, the measured currents were from 35mA to 80mA for individual tubes.

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Radio dla Techników i Amatorów (Radio for Technicians and Amateurs) 1946/03

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RADIO dla Techników i Amatorów, Rok I, Marzec 1946r., Nr 1.
(RADIO for Technicians and Amateurs, Year I, March 1946, Number 1)

  • From the editorial office (1)
      The new era of nuclear energy that is unfolding before us must now put its tasks on the agenda: Technical culture should become the property not only of individuals, but also of broad strata of our society. Acquiring certain habits, methods and technical knowledge is also a basic condition for creating a sufficiently wide team of people who can actively cooperate in the reconstruction and construction of radio and television in a democratic Poland.
      The tasks facing our radio technicians are enormous, not only because of the necessity to reconstruct what was, but also to create what has not been there so far, i.e. the universality of radio.
      These were the reasons that led us to the issue of the new monthly magazine for technicians and radio amateurs - "RADIO"
  • Review of issues in the construction of receivers (2)
       The transmitter and receiver are important elements of radio signal transmission. Technical improvements are made to the transmitting and receiving devices in order to reach the limits of practically achievable possibilities. On the receiving side it is necessary to adapt to the conditions created by the transmitting side and to the conditions of the propagation environment.
       The conditions on the transmitting side depend primarily on the lack of available wavelengths. In amplitude modulation using both sides of the spectrum to avoid mutual interference, the power of the transmitters and the wavelength spacing are reduced more than otherwise desirable.
    • Influence of progress in the construction of electron tubes.
        The progress in the construction of electron tubes in the last 10 years has been taking leaps and bounds through many implementation stages, differing not only in terms of electrical parameters, but also in terms of external performance, connections to the pedestal and construction of the pedestal. In this way, almost every year, a large number of tubes of various types had to be withdrawn from circulation. The newly constructed vacuum tubes were immediately applied to new types of radio receivers, and rarely any type of radio receiver was manufactured for more than one year.
  • Frequency modulation (6)
       In recent years, frequency modulation has gained a lot of importance and has found numerous applications. Suffice it to say that currently in America, radio listeners are called to buy radio receivers that enable reception of radio waves modulated in this way, because a number of radio stations will soon use frequency modulation.
  • New English radio on three electron tubes (10)
      A new radio receiver from one of the companies in Cambridge has appeared on the English market. Its description is given in "Wireless World". We believe that our readers will be interested in the description of this radio receiver, if only because of the desire to find out what types of radio receivers will have a chance of success in the post-war period.
    • Schematic diagram of the radio receiver.

    •   New English radio receiver on electron tubes (schematic diagram).
    • Some design details.
        The base of the structure can be very easily removed from the box. The loudspeaker is easy to disconnect as it is connected via plugs. The knobs on the front of the radio receiver are designed in such a way that they can be pressed in.

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Simple measurement of the number of turns in a transformer

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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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Radioamator (Radioamateur) 1950/10

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Radioamator, październik 1950r., rok I, numer 10
(Radio amateur, October 1950, volume I, number 10)

Diagrams of receivers types: Nora W.16 Tosca and Nora GW. 16 Tosca (cover page 2)

The diagrams below show connections in receivers manufactured by 'Nora' of the "Tosca" "W16" and "GW16" types. Both receivers are two-tube with a third rectifier, two-range (medium and long waves) and belong to the category of simple devices. They have the same receiving frequency, but they differ in the method of supplying electricity and the types of tubes. The "Tosca" "W16" receiver is powered by alternating current from the lighting network and has the first AF7 type tube that acts as a detector. The second - is a loudspeaker type AL4 The AZ1 rectifying tube works in an anode power supply.

The "Tosca" "GW16" receiver can be supplied with alternating or direct current from the lighting network. It has receiver tubes that correspond to the types of electric tubes in the first camera, i.e. detector - CF7 and speaker - CL4. The power supply uses a CY1 rectifying electron tube and the "Urdox" U920 current regulator.

Both devices have volume control and selectivity control, which is achieved by changing the capacitance of the differential capacitor located in the antenna circuit. They also have built-in eliminators, which allow for clear reception of foreign stations, undisturbed by local radio broadcasts. The timbre of the sound is regulated by switching on and off the appropriate permanent capacitor located in the anode circuit of the loudspeaker tube. Both devices have identical boxes.

Soviet television (1)

In 1922, when a radio broadcasting station in New York had a power of less than 1.5 kW, a 12 kW transmitter was built and put into operation in the Soviet Union. In the same year 1922, the Soviet Union took the first place in the world in terms of the power of transmitting stations, ahead of other countries' radio technology, which often drew on the experience of Soviet engineers. For example, in the words of the Americans themselves, the Soviet system for building super-powerful transmitters was used to build a 500 kW station near Cincinati. A modulation system developed in the USSR was also used in the New York TV transmitter.
Excellent results were achieved in the Soviet Union and in the field of television.
The theoretical foundations of television were prepared in 1888 - 1890 by many Russian scientist, physicist A.G. Stoletov, who studied the effect of light on the electrical conductivity of gases and constructed the world's first photoelement.
...
The rise of the technical and economic power of the Soviet Union, the achievements of Soviet science created the conditions for a jump in the development of Soviet television from the standard of 343 lines to 625 lines, which was ahead of Europe (405) and America (525 lines).
The transition of the Moscow TV station to a new standard was connected not only with increasing the clarity of the image, but also with the expansion and increased power of the devices.
The task of significantly increasing the technical and operational capabilities of television was completed with full success.
Soviet readers were surprised to read in a magazine from an English news outlet in the USSR recently that England still used the pre-war standard on television and that it was considered "completely satisfactory"...

Let's learn radio technology - Cathodes (3)

The cathode of the electron tube, in order to work normally, that is, to emit free electrons to the outside, must be heated to a certain strictly defined temperature. The cathodes of the tubes are heated by electric, direct or alternating current, the "direct filament" tubes are designed rather for direct current operation, while the "indirect filament" tubes may be heated freely, using direct or alternating current. The electric power of the filament current lost as heat in the cathode is calculated by multiplying the filament voltage in volts by the filament current in amperes. For example, if we have a 4 volt electron tube whose filament current is 1 ampere, then the filament power is: 4 x 1 = 4 watts. Filament is needed in order to keep the cathode temperature constant. As the hot cathode radiates heat to the outside and thus cools down, it is still necessary to supplement these deficiencies by supplying electricity from the filament source..
The power needed to heat the cathode in the electron tube depends on the surface of the cathode and the temperature at which it works. The least amount of glow power is required for electron tubes with an oxide cathode because, as we know, the operating temperature of the oxide cathodes is not high. The cathode area determines the amount of electron emission. High-emission electron tubes require large-area cathodes, which entails high filament power. Electron tubes with low emission have a small area and therefore the required filament power is small. Knowing the glow power of the electron tube, we can approximately determine its maximum emission. For one watt of power lost in the cathode, as we know, in the case of the oxide cathode, we can count about 100 mA of emission, so in the case of an electron tube with a filament power of 4 W, the maximum emission current will be in the order of 400 mA.
...
Tubes with the same filament voltage are connected in parallel to the power source, similar to e.g. electric bulbs for the lighting network.
...
Parallel connection of electron tubes is used in alternating current battery and mains devices - generally with low-voltage electron tubes.

On the other hand, in DC or universal apparatus, i.e. DC and AC, all filament filaments are connected in series. Since in this case the same current flows through all the tubes, all tubes used in such a system must be built for the same filament current.
...

However, in individual cases, instead of e.g. one 100 mA tube, two parallel connected 50 milliampere tubes may be connected to the power supply circuit. The power supply of tubes connected in series with each other must be at a nominal current. If the sum of the voltages of all serially connected tubes is lower than the voltage of the supplying source, then we have to connect the resistance in series with the tubes and adjust the current to the nominal value. Instead of a constant resistance of an appropriate value, an iron-hydrogen resistance called "Urdox" is usually used, which works automatically, i.e. it adjusts the filament current to the appropriate nominal value regardless of the voltage supply.

...
Now that we know the properties of the cathodes, let's get acquainted with the leads of the electron tubes and see what terminals the filament filament ends are connected to..

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Acoustic tube amplifiers

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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"

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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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