Tube amplifier or transistor ?
The reputation of the tube amp is so great, that the best possible compliment for a transistor amp is often to say it has a "tube sound".
You've probably heard about the electric guitar tube amp, they're the most popular ones. Obviously, tube amplifiers (or tube amplifiers) have not only advantages: the counterpart is to be cumbersome, heavy and often very showy (aesthetically speaking). Usually they are more expensive than a transistor amplifier because they need high quality passive components (capacitors, output transformers, resistors with 1% tolerances, etc ...).
How does a tube amplifier works?
 |
A tube amp works in a completely different way from a transistor amp (also called a solid state amp). In fact, tube amps work in high voltage (several hundred volts compared to a few tens of volts in transistor amps). The tubes are different from the transistors because of their high output impedance, which requires the presence, in the tubes, of output transformers that adapt to the impedance of the loudspeakers. This means a greater ability to reproduce the peaks of the audio sine wave. In many cases, the listening experience is that the output power is even greater than with transistor amplifiers of equal power. Tube amplifiers also tend to heat up a lot and dissipate a considerable amount of energy in the form of heat. Many, in fact, operate in Class A. |
The technology of tube amplifiers
 |
A tube is made up of electrodes, placed inside a vacuum glass ampoule (the thermionic tube). The number of electrodes inside the vacuum tube defines the type of electrode: two electrodes for the diode, three electrodes for the triode, five electrodes for the pentode, etc. The operation of audio tubes is based on the generation of an electron flow, obtained by heating a filament (so-called thermionic effect). This heating, therefore, determines the release of electrons from the cathode. Usually, the cathode is separated from the filament and this prolongs its duration over time. The electron beam tends to reach only the cathode. The flow of electrons is generated at the same time that a voltage is applied to the anode or to the other electrodes present in the thermionic tubes. For example, a triode tube consists of three electrodes and a filament for heating. When the cathode is at ground potential, the anode assumes a positive potential. The grid is polarized negatively against the cathode. The lower the negative value of the grid, the greater the flow of electrons generated and therefore of current produced. The variation in grid voltage is weaker than that induced in the anode. The triode has a limited capacity to amplify the signal, and are often used in single-ended configuration (one triode per channel). The power output from this type of tube amp is low and high efficiency drivers are required. While not very powerful, these tubes are characterized by a refined sound that makes them truly unique. Some manufacturers have developed an evolution of these designs to achieve higher power levels: the parallel single ended (PSE) in which double triodes are used, with a double single ended circuit and a doubling of the output power, all without compromising sonic quality. |
The success of pentodes
The pentode is a more advanced version of the triode and consists of five electrodes and a filament. There are three grids:
- Control grid;
- Grid screen, with a voltage much higher than the control voltage;
- Suppression grid, which reduces the noise and limits the loss of gain). The latter is connected to the same potential as the cathode.
Pentodes are audio tubes with excellent amplification capability, reduced distortion (thanks to the presence of the suppression grid) and a linear frequency response.
These thermionic tubes are the most common, because they allow to deliver very good power, especially in push-pull designs where the tubes work in pairs and are responsible for amplifying, in a distinct way, the positive and negative semi-waves.
The asymmetric structure of these designs tends to cancel out even-order harmonic distortions, which are kept intact in a single-ended audio amplifier.
Pushpull tube amplifiers are often cheap, because they don't need as much cared for output transformers as those needed for a good single-ended.
However, not all tubes have the same characteristics. Depending on the manufacturer and the build quality, the sound will also be different.
Tubes are complex electronic components, made of materials that require mechanical adjustment over time. This settling leads to a progressive improvement of their initial electrical characteristics.
Precisely for this reason, tube amplifiers require a break-in period of many hours (between 50 and 100 hours) in order to better appreciate all the richness of the "tube sound".
The average life of a tube is estimated at around 3,000 / 5,000 hours of listening, even if the preamplifier tubes have a much longer life cycle (around 12,000 hours of listening).
Maintenance and tips
The tube amplifier requires some care in order to use it correctly. Here are some tips for preserving the life of the tubes and the tube amplifier as a whole.
Here are our four tips:
- Should I keep the tube amplifiers on at all times? Absolutely not, because in addition to causing excessive current consumption, especially in the presence of Class A tube amplifiers, the main problem is that the preamplifier tubes and the power amps are always live and consume without real use. In this mode, in fact, only the loudspeakers would remain unused, but all the circuits inside the tube amplifier would be working. Our advice is to turn on the tube amp and leave it on 20 minutes before you start your music listening session. In this way, the tubes can be warmed up properly and start working in a stable manner, delivering the amount of current necessary for optimal amplification of the audio signal. A further advice is to set the volume to zero and to wait one or two minutes before turning off the tube amplifier.
In this way, the vacuum tubes will have time to start to cool down, without suffering too violent thermal shocks. - Correct ventilation. A very important aspect in the care of vacuum tube amplifiers is the correct ventilation. Always check that the amplifier is not placed in cabinets or shelves without air or closed. Considering the great heat generated by the thermionic tubes, in fact, it is absolutely necessary to have an adequate air recirculation, so that the heat does not stagnate above the tubes, causing them to break.
Make sure that at least 30 cm of air is present between the thermionic tubes and the upper shelf, in order to facilitate circulation. Also, avoid closing the slots on the amplifier panel.
If there is not an adequate recirculation, we suggest to install near the amplification a fan with a low rotation speed (therefore of silent type), in order to suck the excess heat and to prolong the life of the thermionic vacuum tubes. - Check that the loudspeakers are always connected. Always make sure that the speakers are permanently connected to the vacuum tube amplifier. In fact, without the load of the speakers, the output transformers of the tube amplifiers would be damaged in a short time. Replacing them is particularly expensive, so we recommend that you pay close attention to this situation, so as to extend the life of your tube amplifier.
- Beware of liquids! Like all electronic equipment, tube amplifiers must also be protected against accidental spillage of liquids (water or substances), which could generate immediate short-circuits if they end up inside. Humidity is also an enemy of vacuum tube amplifiers. Check with a hygrometer that the relative humidity level of the room in which the tube amplifier is placed is adequate. Wineries and taverns often have too high a humidity level, so make the necessary checks before choosing these environments.
Should I buy a used one?
In this case the answer is: it depends. Especially for amplifiers of this type, when looking for a good used amplifier, you have to pay a lot of attention to the general state of preservation and where you buy (for example, if you look on ebay, always check the feedback from the seller).
In particular, check that the tube contacts are not oxidized or, worse, rusty. The ideal would also be to be able to test the level of residual emission of the vacuum tubes, but this is very difficult since the tube testers, as well as being rare devices, are also very expensive.
It is therefore better to rely on your own ears and, after a listening test of at least thirty minutes (to give the tubes time to warm up and reach the optimal operating temperature) make their own evaluations.
With the advent of Chinese tube amplifiers, with their extremely cheap price and excellent construction and sonic qualities, it has now become uneconomical to buy a used tube amplifier.
DIY Building a tube amplifier: how do you do it?
If you're an audiophile, you might consider assembling a tube amplifier yourself. There are many kits on the market to build cheap stereo tube amplifiers, such as those of Nuova Elettronica. But you should know that it will take a long time to assemble, as in all DIY jobs. The assembly of a cheap tube amplifier requires a lot of competence and, above all, a lot of attention. The voltages used are in fact very high and it can be very dangerous to build an amplifier on your own if you are not really an expert.
Our advice, therefore, is to avoid building these kits yourself and to opt for a solution ready to use and safe, such as a Chinese tube amplifier.
Hybrid amplifiers
A hybrid amplifier circuit offers the same qualities as a pure tube design, but without the disadvantages. The results of this mixed technology are very valid and worthy of consideration. The circuits used in these particular tube amplifiers are called "hybrids".
Usually, a hybrid amplifier consists of a tube preamp stage, followed by a transistor output section. The advantage of this design scheme is that you can have a sound that is "typically" tube, but with the power and impulse force that are typical of a transistor amplifier. The output stage of a hybrid amplifier is usually driven by "Mosfet" transistors, which are also characterized by a softer sound and which better matches with the tube component that is present upstream of the amplification circuit.
These are the main differences between a hybrid and a tube amplifier.
