Assembling the simplest low-frequency tube amplifier (I)

Radioamator i Krótkofalowiec 1961/05. Author: K.W.
(A corner for beginner radio amateurs)

   Despite the constant progress in the production and application of semiconductor elements such as diodes and transistors, the electron tube is still an essential component of most radio engineering devices. As we know, the electron tube, invented about fifty years ago, created great development prospects for radio engineering and became the basis of its extraordinary career. The knowledge of the construction and principles of operation of the vacuum tube is the first step of "initiation" of each radio technician and therefore it is also valid for beginner radio amateurs. We will establish our knowledge of electron tubes in the simplest way, i.e. by hand-assembling and testing a single-tube low-frequency amplifier. This amplifier can be used as a detector receiver and be - despite its simplicity - very useful, for example, if you need to listen to a broadcast using a larger number of headphones (2 - 6 pairs).

   The schematic diagram of the amplifier is presented in Fig. 1 in two variants, which differ in the way of feeding the signal from the detector to the amplifier circuit. In the first case (Fig. 1a), a low-frequency coupling transformer with an appropriately selected ratio is used. This system should be used when the signal obtained from the detector receiver is very weak, and we want to obtain the highest possible gain. No volume (gain) control is provided here. The use of the amplifier in the circuit shown in Fig. 1b is, however, advisable when the receiver plays programs at a relatively high volume; this amplifier is slightly simpler in design, and at the same time allows you to adjust the volume. However, we must remember that the gain provided by this system is lower than the maximum one provided by the same electron tube coupled to the detector by means of a transformer. Of course, in both cases the acoustic signal from the output of the detector receiver is connected to the same electrode, the so-called the "control grid" of the vacuum tube.

Fig. 1. Schematic diagram of the amplifier
a - coupling with transformer, b - galvanic coupling

   The operation of the amplifier circuit with an electron tube is very simple and easy to understand for everyone who has read the description of a low-frequency amplifier with one transistor (Radioamator No. 3/61) and learned about the general principle of operation of this type of circuits described in the article. Here, too, the weak signal from the detector receiver serves only to control the amplifying circuit, while the headphones work at the expense of energy from a local power source - a battery or an AC adapter. Like the transistor, the electron tube is the element by which the current flowing in the circuit of the energy source is regulated.

   For a proper understanding of the operation of the electronic circuit, it is necessary, however, to familiarize the Readers with the structure of the tube. It is not a difficult task, as most of you probably already know a bit about it. Who is able to resist the temptation and does not break even one damaged (worn-out) vacuum tube to see with his own eyes what is inside. An electron tube consists of - in one of the simplest cases - a filament (the so-called "cathode"), a "control grid" and an "anode". Fig. 2 shows us the construction of such a tube, popularly known as a "triode".

Fig. 2. Construction of a triode (old type)

The phenomena inside the tube are by no means so complicated that they cannot be explained in a simple way. In Fig. 3 we can see an ordinary electric bulb with an additional electrode in the form of a metal plate placed inside the bulb. We will call this metal plate the "anode" of the tube. Between the anode and the filament of the vacuum tube, a meter is connected in series with the Ba battery (with a voltage of several dozen volts), which indicates the value of the current flowing in the circuit. The Bż filament cell only serves - as its name suggests - to glow the filament of the tube. Observed by the famous inventor T.A. Edison's phenomenon is very interesting: he stated that a current flows in the anode circuit if two conditions are simultaneously met, namely when:

  • the filament of the tube is incandescent,
  • the positive pole of the Ba battery is connected to the anode.

   If at least one of the above conditions is not met, the anode current - as we will call it now - does not flow in the circuit. This is illustrated in Fig. 3.

Fig. 3. Edison's experiment with a modified light bulb

   Edison could not explain these strange phenomena occurring in the ordinary lighting bulb thus modified. Besides, it was not easy, given the state of knowledge at the time. Today we know that the heated filament, or "cathode" of the electron tube, is the source of free electrons, which make up the so-called emission current of the tube. This current flows through the vacuum in the tube if there is a positive voltage at its anode in relation to the cathode.

   The actual "birth" of the electron tube took place much later, several years before the outbreak of World War I, when an additional, third electrode was placed in a modified Edison lighting bulb. This electrode is called a grid. Fig. 4 illustrates the operation of the grid: if we bring a fairly large negative voltage to it (Fig. 4a), it strongly inhibits (repulsively) the electrons emitted from the cathode, which constitute an elementary negative charge, and the anode current flowing through the electron tube is very small. If the negative grid voltage is small, its braking effect is much weaker, therefore - as we can see in Fig. 4b - the anode current of the electron tube has a much greater value. Fig. 4c shows us another case: a small positive voltage is applied to the grid of the tube. Now the grid not only does not repel the electrons emitted by the cathode, but also helps them travel to the positive anode; therefore, the current flowing through the tube reaches a high value. At the same time, however, a small part of the electrons emitted by the cathode, namely those electrons that hit the sparsely spaced wires of the control grid, form the so-called "grid current". This is perfectly understandable, because in this case the grid, having a small but positive potential, becomes similar to an anode in its operation.

Fig. 4. Operation of the control grid

   The above illustration of the phenomena taking place in the vacuum tube allows us to generally understand its operation in the amplifier circuit (Fig. 5). As we found out, changes in the voltage on the control grid cause changes in the intensity of the current flowing in the anode circuit of the electron tube. In this situation, a fairly significant current flows through the headphones, depending on the voltage at the control electrode, which results in the exact reproduction of the control signal in the headphones.

Fig. 5. Amplifier circuit with a triode

   Having become familiar with the design of the electron tube and the operation of the amplifying circuit, we can now return to our diagram from Fig. 1a and start building the amplifier. Here is a list of the elements needed for this purpose:

  • V - battery-powered electron tube, type 1S5T - 1 pc
  • Tr - input transformer (as described) - 1 pc
  • Ba - anode battery 30÷70V - 1 pc
  • W - 2-pole switch - 1 pc
  • radio sockets with nuts - 8 pcs
  • aluminum or iron sheet
  • small assembly equipment, such as bolts, nuts, assembly cable, etc.

   The Readers will probably have the most trouble getting the right "input" transformer, as it is not very popular at present. It is a transformer with a step-up ratio of at least 1: 4. Transformers removed from old German radio receivers are perfect here. But you can also do it yourself according to the following data:

  • cross-section of the central core column: about 1÷2 cm2,
  • primary winding: 1000 wire turns in enamel wire with a diameter of 0.1 ÷ 0.15 mm,
  • secondary winding: 4,000 turns of enamel wire with a diameter of 0.1 mm.

   You can also use any of the loudspeaker transformers in stock (eg for Pionier radio receivers, Szarotka. Stolica, etc.). Of course, they have to be adjusted accordingly. Since the primary winding of the loudspeaker transformer is about 2000 ÷ 3000 turns, it can be used as a secondary one for the new circuit. All that remains is to remove the unnecessary low-resistance winding (about 100 turns of the wire with a diameter of about 0.5 ÷ 0.8mm) and instead wind the primary winding in the amount of about 800 ÷ 1000 turns. The diameter of the wire can here be any one according to the available space.

   The 1S5T electron tube is used in the set of tubes of the popular Polish radio called "Szarotka". Fig. 6 shows its external appearance and the connections of the individual electrodes of the vacuum tube with the leads outside.

Fig. 6. 1S5T electron tube
a - external appearance, b - terminal layout

As we can see, this electron tube is a multi-electrode, because apart from the already known cathode (pins 1 and 7), the control grid (pin 6) and the anode (pin 5), it has other additional electrodes. We will not deal with them in detail for now, because in our circuit we will use the 1S5T tube as a triode. It will be possible thanks to its proper inclusion in the amplifier circuit. Therefore, we will connect pins 4 and 5 together and use both electrodes attached to them as an anode. It is pictured in Fig. 7, where the 1S5T tube is shown in a triode arrangement. The additional electrode attached to the third leg of the vacuum tube remains not connected anywhere, because we do not need it here.

Fig. 7. 1S5T electron tube converted into a triode

   It is best to start mounting the amplifier from its metal base, the so-called "chassis". Fig. 8 shows an example of such a base; of course, the location of the screws securing the input transformer, or even possibly the dimensions of the entire base, should be adapted to your transformer. A photograph of an amplifier model may also be helpful, although one should rather try to gradually design the construction of simple devices on the basis of a schematic diagram. Mindless copying of the described models is not the best practice, because we must remember that a real radio amateur can correctly construct electronic systems only on the basis of a schematic diagram and possibly additional and general guidelines.

Fig. 8. An example of the metal base of the amplifier
The individual dimensions should be adapted to the dimensions of the transformer used.

   The base of the amplifier is best made of 0.5 mm thick aluminum sheet, which is the easiest to work with, or of iron or zinc sheet.

   The next step is to attach the transformer, electron tube socket and radio sockets. In the case of a chassis made of sheet metal (and possibly also made of other materials), the radio sockets should be secured with appropriate insulating washers so that none of them is in contact with the ground. The preparation of washers and their assembly is shown in Fig. 9.

Fig. 9. Manufacturing of an insulating washer and installation of a radio socket

When using a chassis made of non-conductive material, the sockets will be mounted directly in appropriately fitted holes (diameter approx. 6 mm). Further assembly of the amplifier includes making connections of individual elements in accordance with the schematic diagram (Fig. 1a) and assembly diagram (Fig. 10).

Fig. 10. Amplifier assembly diagram

Connect the connecting wires (with a diameter of 0.5 ÷ 1 mm in insulation) with eyelets for the socket nuts and solder to the appropriate feet of the tube socket. The circuit described here is so simple that it does not require any additional explanations, as its assembly will certainly not cause any trouble to anyone.

   Now connect the power source to the finished (and tested) amplifier circuit. First - we turn on the Bż filament battery, that is a 1.5-volt cell (the so-called "American"). For this purpose, the link should be terminated with wires with plugs - preferably colored, in order to easily distinguish between the poles. Fig. 11 shows such a battery and indicates its positive and negative poles.

Fig. 11. Polarity of 1.5V filament battery

The correct polarity of the filament battery is very important in our system, and a few words of explanation should also be given to this matter. In normal amplifier circuits, a certain negative voltage is applied to the control grids of the individual tubes. The value of this voltage depends on the parameters of the system. This is to ensure the operation of the vacuum tube with as little distortion as possible. Most readers will surely remember that in the previously known amplifier circuits with transistors (Radio Amator No. 3 and 4/61) we used analogously certain "pre-voltages" for the bases of transistors. The circuit of our tube amplifier has been deliberately simplified, therefore it does not contain any additional elements to obtain this so-called "control grid polarity". However, this grid also has some negative potential with respect to the filament simply because it is connected (via the transformer secondary winding) to the negative pole of the filament battery - the mean negative grid pre-voltage to filament is in this case about 0,7 V). However, it should be clearly emphasized that this is a rather unique practice, acceptable in this particular case due to the low anode voltage used and the intended use of the amplifier. As we know, the detector receiver usually provides very low acoustic voltages, so you can not worry about the so-called "clipping". Such distortion, as most of the readers must already guess, results from applying too high voltages of acoustic frequencies to the grid of the amplifying tube, greater than its negative pre-voltage. After this as long as possible, but really necessary explanation, we will remember that the filament battery must absolutely be connected to the system according to the polarity given in the diagrams and drawings. It is advisable to mark the corresponding plugs and sockets with colors or symbols.

   After connecting the filament battery to the tube socket, insert the 1S5T electron tube (carefully so as not to bend the delicate leads). The amplifier is connected to the mains using the W switch and the correct assembly is checked by observing the filament of the tube. In full light it is difficult to see anything, but in a dark room you can easily see the cathode glowing bright orange. It is visible in the form of a thin thread stretched along the vertical axis of the tube.

   The easiest way to set up a Ba anode battery is to use 8 ÷ 12 flat 4.5 V batteries (from a pocket flashlight). Buying large anode batteries is pointless not only because of the high cost, but above all because of the very low power consumption of our system. The individual batteries are connected in series with each other, i.e. the plus of one battery - with the minus of the next, as shown in Fig. 12.

Fig. 12. Serial connection of flat 4,5 V batteries

It should be remembered that the long end of the battery is the negative pole and the short end is the positive pole. We will also terminate this battery with appropriate length cables with banana plugs. The anode battery is connected to the system as indicated in the diagrams (Fig. 1a and 10), i.e. the negative pole to the cathode, and the positive pole to the anode of the electron tube (through headphones). Reverse battery connection will not damage the amplifier, but it will not work then.

   It remains to connect the headphones to the appropriate sockets and the detector receiver to the input of the amplifier.

   The system assembled in the above way is shown in Fig. 13.

Fig. 13. System: detector receiver - amplifier
The connection should be made with wires terminated with banana plugs

The whole thing is so simple that our receiving set should immediately give the most satisfactory results. Of course, the condition for the proper operation of the system is the proper operation of the detector receiver itself, which should be previously checked with headphones. At the same time, by comparing the strength of the broadcast received directly from the detector and from the output of the amplifier, we can roughly estimate the quality of work of our first design with an electron tube.

   As already mentioned in the introduction, more pairs of radio headphones can be connected to the system. The easiest way to do this is as shown in Fig. 14.

Fig. 14. Making a "splitter" for a small number of headphones:
a - external appearance, b - connection diagram

As you can see, a small plastic soap box with an appropriate number of radio sockets was used to build the "splitter". The design of the amplifier with gain control (Fig. 1b) will be discussed in the next issue of the journal.

The content of the article for electron tube enthusiasts was provided by Grzegorz 'gsmok' Makarewicz