We all agree: there is nothing like a tube-amplified sound: preamps, power amps, microphones, effects... It's hard to believe that an obsolete technology like this gives better results acoustically (since in data-processing devices, that is, computers, using tubes means more space and lower performance).
This month we pay tribute to the vacuum tube, the component that marked a before and after in the world of audio amplification, showing its properties and its current situation in the industrial, commercial and musical context.
PRELIMINARIES: COMPONENTS, STRUCTURE AND FUNCTION
What is a tube, and why does it produce a sound that is preferable to that generated by a transistor? We may never have thought about it, but the electronic principle of the tube is as follows: it works by thermionic emission of electrons from a filament or cathode, controlled by a grid, which are then collected on a plate. Following the general criteria, we can distinguish four types of tubes commonly used in audio, differentiated by the number of internal amplifying elements (not counting the filament) that make them up: diodes (two elements), triodes (three elements), tetrodes (four elements) and pentodes (made up of five elements). This characteristic is shown in the first digit of the identification code. The rest show data related to the elements and the voltage at which they operate.
The characteristics of tubes vary widely depending on the model selected. In general, tubes are more fragile amplifying components than transistors, operate at high temperatures (which is why they work inside an evacuated glass envelope, since temperatures exceed 100º C and can even triple that value, depending on type and model) and need a warm-up period (on the order of seconds). Tubes have relatively low gain, high input impedance and low input capacitance. Tubes saturate (producing audible clipping) smoothly and also recover from overload quickly and smoothly.
Tubes are capable of working at higher voltages than any other device, but high-current tubes are rare and expensive. This means that most tube amplifiers use an output transformer. Although it is not a specific characteristic of tubes, output transformers add second-harmonic distortion and show a gradual roll-off in high-frequency response,
which is hard to duplicate with transistor circuits. It is because of this nonlinear and theoretically imperfect behavior of the signal generated by tubes that they produce a warm sound, with color and body, that cannot be reproduced even with the best solid-state circuit.
WHY DO THEY FAIL?
There are basically three reasons why a tube stops working: wear from use beyond its physical limits, short circuit, or air getting in.
The cathode sleeve is coated with a substance that causes electrons to be emitted. When activated, the cathode filament makes it glow white-hot, and over time the heat deteriorates the film formed by that substance. This translates into a progressive reduction in emitted electrons, which ends in total failure once the film is used up.
Short circuits are usually caused by overheating, which makes the filaments and other conductive elements buckle, bend and come into contact. Power tubes are the most likely to "short out," with Chinese tubes being the worst offenders.
As for gas, as mentioned, the envelope enclosing the tube's structure is evacuated at the factory. If air gets in, since it contains oxygen, it will probably cause a combustion reaction inside and make it overheat (if not catch fire), just as would happen in a light bulb. This is usually due to a manufacturing defect in the sealing of the envelope. That is why it is important to buy screened tubes.
As you can see, tubes are delicate, so it is extremely important to take care of them, not only for their function but also for maintaining the physical integrity of the amplifier or device you use them in, since they are in contact.
THE TUBE INDUSTRY
The structure supporting the filaments and the glass envelope of a tube is made of metal and is assembled entirely mechanically, which means manufacturing tolerances are very wide and there is a high probability of defects appearing in new tubes. Such defects usually show up within the first fifteen or twenty hours of operation. For this reason, it is very important to choose tubes screened by companies devoted solely to that, such as TAD (USA), Sovtek (Russia) or Ruby Tubes (USA). These companies buy tubes directly from the manufacturers (today they are only made in the Czech Republic, Russia, the USA, Yugoslavia and China) and subject them to exhaustive individual testing, in some cases having to discard up to 31% of the tubes purchased. If you install untested tubes, there is a significant risk that one of them will "blow" a few hours after it starts operating and, as a result, cause serious damage to the amplifier. Keep this firmly in mind, since it determines the life of your amp.
THE CURRENT MARKET
As expected, Chinese tubes leave quite a lot to be desired in terms of quality. Some of the best I have tried in amplifiers are Yugoslavian (for example, Ei) and Russian (Svetlana, Reflector Corp, Ryazan). Thanks to tube screening companies and the quality of many manufacturers, today we can find excellent tubes, some of which even surpass those of twenty years ago, when practically all the factories in Western Europe closed (those factories produced excellent tubes, which were coveted by a large number of musicians, who rushed to buy up the remaining stock).
Below is a table of tubes from different screening companies and manufacturers that are especially advisable for their quality (all the tubes listed go through rigorous screening).
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We live in a world of mass production, which means that most tube amp manufacturers, even well-known ones (Fender, Marshall, Ibanez…), use Chinese tubes in their products because, although they provide high gain, they are not screened and their sound is poor. This significantly reduces costs, together with low-cost labor, since I know for a fact that amplifiers built in countries such as the USA, England or Germany usually have quality, screened tubes installed (bear in mind that poor-quality tubes in a good amplifier ruin the good sound it can produce, on top of the consequences of rapid deterioration, which can end, as mentioned above, in considerable damage).
On the other hand, keep in mind what many manufacturers mean by a "100% tube amplifier." They tend to consider all-tube amplifiers to be those in which the amplifying structure of the preamp and power amp is built with tubes, but the circuit also includes a rectifier and an effects loop which, in most cases, are built from solid-state diodes, which brings us back to transistor territory (the difference between a tube unit and a solid-state one is more than noticeable). Two very widespread examples of these amplifiers are Mesa Boogie and Soldano, whose production has not a single amplifying stage that is not built from tubes. You all know their famous quality, as high as their prices; well, this is partly the reason (in addition to being built in the USA and with select tubes, not to mention that custom tube amplifiers can be ordered on request, as in the case of Soldano).
TUBES IN PREAMPS
The preamp is responsible for increasing the amplitude of the sinusoidal voltage signal coming from the instrument. The most popular tube model here is the 12AX7 (originally ECC83), originally used by Marshall. The 12AX7 produces the most gain, which largely explains the sound of these amplifiers.
I especially liked the Sovtek (made by New Sensor, Russia), Ruby Tubes (made by Teslovak, Czech Republic), General Electric and Sylvania (the last two, original production), which would take the prize in my book. Highs, lows and mids are all very well defined, which is important in the preamp.
As we have said, most of the tubes used by audio amplifier manufacturers are Chinese, so it would be advisable to replace them.
TUBES IN POWER AMPS
As indicated above, the gain generated by tubes is relatively low. That is why building high-power amplifiers requires a lot of resources and therefore leads to a very high retail price, depending also on the nature of other components of the amplifier (rectifier, transformer…).
The first thing we find in a power amp is the tube that prepares the signal coming from the preamp so it can then be delivered to the power tubes. This tube is known as the phase inverter or driver. Manufacturers such as Fender (the very popular 6L6 tube model was originally used by this company) use the ECC81/12AT7, and others, such as Marshall, have used the ECC83/12AX7, the same one we mentioned earlier.
If the amplifier uses only one power tube (rare nowadays), the first tube in the power stage acts as a phase inverter, matching the high output impedance of the preamp with the low input impedance of the power tube.
However, it is most common to find more than one tube in the power stage, and always two or multiples of two (this is an advantage for recording, since half of the power tubes can be disabled with a switch, which reduces the power without losing gain, something necessary in the studio). Depending on the numerical code and the model of the tubes, the amplifier will have more or less power. In such cases, the power tubes are connected in a push-pull arrangement and the first tube in the stage acts as driver-phase inverter. This tube generates two equal signals that are opposite in phase (180º), which are needed to drive each of the two sides of the push-pull pair.
TUBES IN RECTIFIERS
Here we can consider a series of pros and cons. Using a tube to build a rectifier affects the tone produced by the amp, in that the sound has more body and is more compressed, whereas rectifiers built from solid-state diodes provide a more powerful and much faster signal response, but lose out in tone. You can check this yourself: if we use a guitar amp with a tube rectifier, we feel the need to dig into the strings harder, since its response develops more slowly than in a transistor amplifier (this is where Mesa Boogie's idea of building its Dual Rectifier and Triple Rectifier came from).
TRANSFORMERS IN TUBE AMPLIFIERS
The output transformer is a fundamental and decisive part of any tube amplifier. The final sound of the amplifier depends to a large extent on its quality and construction. Its job is to match the high impedance of the power tubes (between three and five kilohms) to the low impedance of a speaker (between four and 16 ohms).
If you are going to use an amplifier that doesn't belong to you, or you are buying a used one, bear in mind that its transformer may not be the original, which would mean the sound produced isn't the amplifier's original sound either. It is therefore very important to check that the transformer is the original one that came with the amplifier (if it isn't, check the unit's specifications in order to buy one that matches them).
A LITTLE BIT OF CRUNCH
What about distortion? More powerful with transistors, but with a less round and much colder sound; much less "musical."
To distort a sound, the amplifier clips the input signal. 100% tube amplifiers use several stages in order to increase the gain to the point where the tubes saturate, producing so-called odd harmonics, which are much more attractive to the human ear, as opposed to the even harmonics that result from distorting with transistors and with gear combining both elements, which we will discuss next.
So why can't I get that sound with my Marshall? Basically because it will be a model that houses tubes, but with the addition of transistor circuits that contribute to clipping the input signal. For this reason not as many stages are needed, and the evolution of this system has led to the appearance of hybrid gear (usually with a tube preamp, especially in Marshall).
TUBE EFFECTS
Often, in the quest for the perfect sound, many effects modulation devices integrate tubes. To give just one example, Vox's Tonelab series multi-effects (see the ISP test bench from May 2004) feature a 12AX7 tube.
There are loads of effects on the market like Vox's (one of the pioneering brands in tube effects), as well as tubes used in built-in effects (ECC81, in the case of the built-in reverb in Fender amplifiers).
CONCLUSION
In the world of audio, if you care about good sound, it's clear: tubes. But be very clear that you are going to have to pamper your gear, as well as change the tubes (normally every 1,000 hours of use for the power amp and 2,000 hours for the preamp; beyond that there is no guarantee the sound will keep its tonal response, color and gain), clean them and avoid rough handling, and watch out for microphonic tubes, since with use they could cause feedback. Try it, and if you like it, you know what to do: go tube!