Amplifiers, is a tube amplifier or transistor better?

Tube amp or transistor?

The reputation of the tube amplifier is so great that the best compliment possible for a transistor amplification is often to say that it has a "tube sound."

You have surely heard of the tube amplifier for electric guitar, as they are the most widespread. Obviously, tube amplifiers (or tube amps) do not only have advantages: the downside is that they are bulky, heavy, and often very flashy (aesthetically speaking). They are usually more expensive than a transistor amplifier because they require high-quality passive components (capacitors, output transformers, resistors with 1% tolerances, etc ...).

How does a tube amplifier work?

A tube amp works in a completely different way from a transistor amp (also known as solid state).

In fact, tube amplifiers operate at high voltage (several hundred volts compared to the few tens of volts present in transistor ones).

Tubes differ from transistors in their high output impedance, which requires the presence, in tube amps, of output transformers that match the impedance of the speakers.

All this translates into a greater ability to reproduce the peaks of the audio sine wave. In many cases, the perceived sensation when listening is even that the output power is greater than that of 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 consists of electrodes placed inside a vacuum glass bulb (the thermionic valve). The number of electrodes inside the tube defines its type: two electrodes for the diode, three electrodes for the triode, five electrodes for the pentode, etc.
The thermionic valves have the same function as transistors (as is known, introduced later), but operate differently, particularly regarding the level of impedance and voltage.

The operation of audio tubes is based on the generation of an electron flow, obtained through the heating of a filament (so-called thermionic effect). This heating, therefore, determines the release of electrons by the cathode. Usually, the cathode is separated from the filament, and this precaution prolongs its lifespan.

The electrons emitted by the cathode (due to the thermionic effect) are accelerated towards the anode (or "plate"), which is positively polarized compared to the cathode. The cathode is the source of the electrons.

The electron flow is generated at the very moment a voltage is applied to the anode or other electrodes present in the thermionic valves.

For example, a triode tube is composed of three electrodes and a filament for heating. When the cathode is at ground potential, the anode assumes a positive potential. The grid is negatively polarized compared to the cathode. The lower the negative value of the grid, the greater the electron flow generated and thus the current produced. The variation of the grid voltage is weaker than that induced in the anode.

The triode has a limited capacity to amplify the signal and is often used in single-ended configuration (one triode per channel). The power delivered by this type of tube amp is low, and speakers with high efficiency are required.

Despite being not very powerful, these tubes are characterized by a refined sound that makes them truly unique.

Some manufacturers, to achieve greater power, have developed an evolution of these designs: the parallel single-ended (PSE) in which a double number of triodes is 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 evolved version of the triode and is composed of five electrodes and a filament. The grids present are three:

• Control grid;
• Screen grid, with a much higher voltage than the control grid;
• Suppression grid, which helps reduce noise and limit gain loss. This last one is connected to the same potential as the cathode.

Pentodes are audio tubes with excellent amplification capacity, reduced distortion (thanks to the presence of the suppression grid), and a linear frequency response.

These thermionic valves are the most widespread because they allow for very good power delivery, especially in push-pull configuration projects where the valves work in pairs and are responsible for amplifying the positive and negative half-waves separately.

The asymmetric structure of these projects tends to cancel out even-order harmonic distortions, which are instead preserved in a single-ended audio amplifier.

Push-pull tube amplifiers are often economical because they do not require output transformers as refined as those needed for a good single-ended.

However, not all valves have the same characteristics. Depending on the manufacturing plant and construction quality, the sound will also be different.

Valves are complex electronic components made of materials that require mechanical settling over time. This settling leads to a progressive improvement in their initial electrical characteristics.

For this reason, tube amplifiers require a break-in period of many hours (between 50 and 100 hours) to fully appreciate the richness of the "tube sound."

The average life of a valve is estimated to be around 3,000 / 5,000 hours of listening, although preamplifier valves have a significantly longer lifespan (around 12,000 hours of listening).

Maintenance and Tips

The tube amplifier requires some precautions for its correct use. Below, we provide some tips to preserve the life of the valves and the tube amplifier as a whole.

Here are our four tips:

• Is it advisable to always leave tube amplifiers on? 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 and final valves are always under tension and wear out without actual use. In this mode, in fact, only the speakers would remain unused, but all the internal circuits of the tube amplifier would be functioning. Our advice, instead, is to turn on the tube amp and leave it on for 20 minutes before starting your music listening session. In this way, the valves can heat up properly and start working stably, delivering the amount of current necessary for optimal audio signal amplification. An additional tip is to bring the volume to zero and wait one or two minutes before turning off the tube amplifier.
  In this way, the valves will have time to start cooling down without experiencing too violent thermal shocks.
  
  
• Proper ventilation. A very important aspect in the care of tube amplifiers is proper ventilation. Always check that the amplifier is not placed in cabinets or shelves without air or closed. Given the great heat generated by thermionic valves, adequate air circulation is absolutely necessary to prevent heat from stagnating above the valves, causing them to break.
  Ensure that there is at least 30 cm of air between the thermionic valves and the upper shelf to promote circulation. Also, avoid closing the slots on the amplifier panel.
  If there is no adequate circulation, we recommend installing a low-speed fan (therefore silent) near the amplification to extract excess heat and prolong the life of the thermionic valves.
  
  
• Check that the speakers are always connected. Always make sure to securely connect the speakers to the tube amplification. In fact, in the absence of the speaker load, the output transformers of tube amplifiers would be damaged in a short time. Their replacement is particularly expensive, so we recommend paying maximum attention to this situation to prolong the life of your tube amplifier.
  
  
• Beware of liquids! Like all electronic equipment, tube amplifiers must also be protected from accidental liquid spills (water or substances), which could cause immediate short circuits if they end up inside. Humidity is also an enemy of tube amplifiers. Check with a hygrometer that the relative humidity level of the environment where the tube amplifier is placed is adequate. Basements and taverns often have too high humidity levels, so make the appropriate checks before choosing these environments.

Is it worth buying used?

In this case, the answer is: it depends. Especially for amplifiers of this type, when looking for a good used one, you need to pay close attention to the general state of preservation and where you buy it (for example, if you search on eBay, always check the seller's feedback).

In particular, check that the valve contacts are not oxidized or, worse, rusted. Ideally, you should also be able to test the residual emission level of the valves, but this is very difficult since tube testers, in addition to being rare devices, are also very expensive.

It is therefore advisable to rely on your own ears and, after a listening test of at least thirty minutes (to give the valves time to warm up and reach the optimal operating temperature), make your own evaluations.

With the advent of Chinese tube amplifiers, which are extremely affordable and have excellent construction and sonic qualities, it has now become less convenient to purchase a used tube amplifier.

Building a tube amplifier: how is it done?

If you are a fan of audio construction, you might consider assembling a tube amplifier yourself. There are numerous kits on the market for building inexpensive tube stereo amplifiers, such as those from Nuova Elettronica. However, you should know that it will take a lot of time to assemble, as with all DIY projects. Assembling an inexpensive tube amplifier still requires a lot of expertise and, above all, a great deal of attention. The voltages used are indeed very high and it can be very dangerous to build an amplifier on your own if you are not truly experienced.

Our advice, therefore, is to avoid building these kits on your own and opt for a ready-to-use and safe solution, such as a Chinese tube amplifier.

Hybrid amplifiers

A hybrid amplification circuit offers the same typical qualities of a pure tube design, but without the disadvantages. The results obtainable from 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 preamplification stage, followed by a transistor output section. The advantage of this design scheme is that it provides a sound that is “typically” tube-like, but with the power and impulsive force that are characteristic 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 better match with the tube component present upstream of the amplification circuit.

These are the main differences between a hybrid and a tube amplifier.

Tube amplifiers

A tube or thermionic tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of an audio signal. Tubes are voltage amplifiers, while transistors are current amplification devices.
It follows that tubes are a more linear amplification technology, requiring less global feedback and thus making the circuit linear.