We have seen the difference between the circuit valves and the transistor. Now we will analyze the valves, because it's important to understand the operation as well as the differences between the various models, in order to combine the amplifier and the valves in the best possible way to obtain the preferred sound. In an amplifier there are four types of valves with different functions, which we will analyze one by one. In the preamplifer, which often has more stages, more than one valve is used. If reverb or vibrating effects are present, the higher level models use valves, while the more economic versions usually use ''solid state'' circuits for these effects.
These preamplify the input signal. The most common are the 12AX7, used in Europe with the name ECC83 (on an industrial level it is cataloged as 7025). Even if the names are different, technically it's the same type of valve, exept for the fact that it is produced in Europe, America or even for military purposes. Today, with no more European and American production, the different types practically indicate the different characteristics. The 7025 generally has a higher gain and more brilliant sound, while the other two types have a richer middle sound and a warmer tone. The old Fender used the 12AY7, characterized by a slightly inferior gain and a softer sound.
These valves pilot the end of the pre but also the internal reverberation for instance. The 12AT7, the ECC81 or the 6201 are usually used. Some also prefer the 12AX7 as dephasing for a more compressed and aggressive sound.
The grinder valve has the job of converting alternating current into direct current inside the equipment. The most common is the GZ34, called the 5AR4 in America, while on many vintage Fenders the 5U4 or the 5Y3 can be found.
The choice of one type of valve to another depends on the characteristics of the circuit and from the voltages being used, therefore its not a choice of the user. The process of rectification through the use of the valve, compared to the process through diodes, causes a slight decrease of power in the amplifier, but gives a softer sound, more harmony (especially at high volume),
a better distorsion and a natural compression of the sound that slowly increases the output level. The last thing that can be said when comparing the two systems, is that it makes the tone warmer.
The job of these valves is to supply the loudspeakers with energy, which determines the overall power of the entire system. There are two very common models: the 6L6, used as ampli such as Fender, Ampeg, Gibson, Mesa Boogie etc., and the EL34, usually used by Marshall. Lately Marshall has been mounting the 6L6 - model 5881 - due to availability problems of the EL34.
The tone characteristics of the two types of valves are rather different. The 6L6 is brilliant, quite linear and has a good margin of clean sound before saturation. The EL34 is more aggressive, with a major emphasis on band extremes (a little more nasal) and a tendency towards major distorsion.
Another model, with a sweeter tone and less power, is the EL84, made famous by a legendary Vox AC30 amplifier, used mainly because of its soft bass, slightly bronzed averages and transparent highs.
In the United States the 6V6 for low power amplifiers is very common. It has a warmer distorsion, clearer tone and more sweet with respect to its much more powerful cousin, the 6L6. Pushed to the limit it has a very typical sound, which is adored by many guitarists.
After having seen the types of valves, we will try to understand how a valve is made. This electronic device is made up of at least four active elements: a heater (the filament), a cathode, a grill and a plate (plate or anode). All these components are in a glass container under vacuum, to avoid burning the various parts.
When it is heated, the cathode begins to emit electrons, passing from the cathode (negatively charged) up to the plate (positively charged). The function of the grill is to control the flow of the electrons, acting itself as a valve.
A low voltage signal, produced by the string of the guitar that vibrates within the magnetic field of the pick-up, causes a considerable flow of current from the cathode to the plate. The bias control inside the amplifier regulates the amount of voltage for the grill. The bias must be correctly adjusted, as this balances the valve with respect to the circuit in which it is found, producing a clean and powerful signal. When the signal nears its maximum power, the valve gradually begins to react less and less to the original input signal, by compressing and saturating it. The distorsion of the valve (clipping) gradually intervenes, producing a low order distorsion, complementary to the original signal, giving a warm sound.
The valves are important but delicate elements: they needs to be protected from sudden impacts. When the amplifier is turned on, they must warm up in order to provide maximum yeild. Contrarily when the amplifier is turned off, they need to cool off before being moved.
Periodically valves need to be replaced. A valve that is ''dying'', can cause the following problems: loss of bass or high sounds, unusual noises, very little sustain and rapid decay of the sound, mixed chords
(especially for bass), and the sound comes and goes. Replacing this as soon as you start having these kinds of problems will increase the life of the amplifier, and ensure that it operates properly.
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