In this article, the vacuum tube amplifier circuit will be studied in detail. Of course, this technique has long been outdated, but to this day you can meet fans of "retro". Someone simply prefers tube sound to digital, and someone is engaged in giving a second life to equipment that has fallen into disrepair, restoring it bit by bit. Many radio amateurs who operate on the air use tubes to build some circuit cascades. For example, UHF is easier to build on high-power lamps, since they will be too complicated on transistors.
Amp block diagram
The block diagram looks like this:
- Signal source (output of microphone, phone, computer, etc.).
- Volume control - potentiometer (variable resistor).
- A pre-amplifier built on a tube (usually a triode) or a transistor.
- The tone control circuit is connected to the anode circuit of the preamp tube.
- Terminal amplifier. Usually performed on a pentode, for example, 6P14S.
- A matching device that allows you to dock the output of the amplifier and the speaker system. As a rule, this role is played by a step-down transformer (220/12 Volt).
- Power supply that generates two voltages: DC 250-300V and AC 6.3V (12.6V if needed).
According to the block diagram, the principal is built. It is necessary to thoroughly study each node of the system so that the manufacture of the amplifier does not cause problems.
Power woofer
As mentioned above, the power supply must produce two different voltages by value. To do this, you need to use a specially designed transformer. It should have three windings - network, secondary and tertiary. The last two generate an alternating voltage of 250-300 V and 6.3 V, respectively. 6.3 V is the supply voltage for the filaments of radio tubes. And if it, as a rule, does not need any processing, for example, filtering and rectification, then the 250 Volt variable needs to be changed a little. This is required by the scheme for connecting the amplifier to the power source.
For this, a rectifier unit is used, which consists of four semiconductor diodes, and filters - electrolytic capacitors. Diodes allow you to rectify an alternating current and make it a direct current. And capacitors have an interesting feature. If you look at the equivalent circuit for capacitors for AC and DC (according to Kirchhoff's law), you can see one feature. Atwork in DC circuits, the capacitor is replaced by resistance.
But when working in an alternating current circuit, it is replaced by a piece of conductor. In other words, when you install capacitors in the power supply, you will get a pure DC voltage, the entire AC component will disappear due to short-circuited outputs in the equivalent circuit.
Transformer requirements
An important condition is the presence of the required number of windings to power the anodes and filaments of the lamps. Depending on which power amplifier circuit is used, a different voltage supply to the filaments is required. The standard value is 6.3 V. But some lamps, such as G-807, GU-50, need a voltage of 12.6 V. This complicates the design and forces the use of a large transformer.
But if you plan to assemble an amplifier exclusively on finger lamps (6N2P, 6P14P, etc.), then there is no need for such an incandescent supply voltage. Pay attention to the dimensions - if you need to assemble a small amplifier, then use single-coil transformers. They have one drawback - it is impossible to get high power. If there is a question of power, then it is better to use transformers such as TS-180, TS-270.
Device case
For low-frequency amplifiers, it is best to use a case made of aluminum or galvanized, mounting radioelements to produce a hinged method. The disadvantage of assembling the device on a printed circuit board is that due to heating, the legs of the sockets for the lamps begin to peel off fromtracks, soldering is destroyed. The contact disappears, and the work of the ULF becomes unstable, extraneous sounds appear.
If a transistor amplifier circuit is used in the preliminary stage, then it is more reasonable to make it on a small piece of textolite - it will be more reliable. But the use of a hybrid scheme imposes its own nutritional requirements. For a ULF guitar, you can arrange it in a wooden case. But inside you need to install a metal chassis on which the entire device will be assembled. It is advisable to use a metal case, as it allows you to easily shield the cascades from each other, which eliminates the possibility of self-excitation and other interference.
Volume and tone control
A simple amplifier circuit can be supplemented with two controls - volume and tone. The first regulator is installed directly at the ULF input, it allows you to change the value of the incoming signal. You can use variable resistors of any design that will work fine in ULF. There should be no problems with tone control either - a variable resistor is included in the anode circuit of the first stage. You just need to determine in which direction the rotation is made to add high frequencies, and in which direction to build up low frequencies.
It is desirable to do everything the way industrial amplifiers do, otherwise it will be inconvenient to use the design. But this is the simplest tone control scheme, it is wiser to install a small unit that will allow you to change frequencies in a widerange. Tube amplifier circuits may contain small modules on semiconductors - tone blocks, low-pass filters. If there is no desire to make a tone block on your own, then it can be purchased in stores. The cost of such tone blocks is quite low.
Stereo amplifier
But stereo ULF is much more pleasant to listen to than monophonic. And to make it twice as difficult - you need to assemble another ULF with the same parameters. As a result, you will get two inputs and the same number of outputs. Moreover, the circuit of the power amplifier and the preliminary stages must be identical, otherwise the characteristics will differ.
All capacitors and resistors are the same in terms of parameters - in terms of magnitudes and tolerances. A special requirement for variable resistances is that it is necessary to use paired structures both for volume controls and in the tone block. The point is that it is necessary to ensure uniform adjustment of these parameters in both channels.
System 2.1
But to improve the sound quality, you can add a subwoofer that will enhance low frequencies. In this case, the general scheme for connecting the amplifier will not change, only the third block will be added. In fact, you should end up with three completely identical mono amplifiers - one for the left channel, right, subwoofer.
Please note that the volume control in the subwoofer is carried out separately from the ULF. This will allow you to change the gain level later. The cutoff of "extra" frequencies is carried out usinga simple circuit, which includes several capacitors and resistances. But you can use ready-made low-pass filters, which are sold at any radio parts store.
Conclusion
Above, we considered the circuits of tube amplifiers, which are most often repeated by radio amateurs in their designs. It is within the power of a person who knows how to handle a soldering iron and technical literature to make them on their own. But if you do not distinguish a resistor from a capacitor and do not strive to learn anything, but you need an amplifier, then it is better to ask an experienced craftsman to make a ULF.
