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This is a subjective topic, however, it is the collective opinion of the vast majority of electric players that valves sound better. One result of this opinion is that nearly every solid state guitar amplifier produced is marketed with a claim to reproduce valve sound! A look through any guitar music magazine will show proprietary names such as ValveState and TransTube!
Here are some of the pros and cons of valve amplifiers:
Leo Fender had a background in radio, so its fair to assume he did not design his amplifiers in the 1950s with any thought of controlled and sustained overdrive. Rumour has it that his first design was straight out of the RCA valve handbook!
With the help of field tests with musicians of the day (notably, guitar-pick-melting American surf guitar guru Dick Dale), he clearly intended a physically and electrically reliable design to withstand use and abuse by touring performers. The combination of his electronically robust design and the natural overdrive characteristics of valves produces the trademark guitar sound used to this day. His genius also includes the use of a tonestack tone control design that compensates for pickup tone bias.
Here are some common valves used in guitar amps ...
So what's to stop someone designing a solid state amplifier to work like a valve amplifier? Unfortunately, early attempts are responsible for the appalling reputation solid state amplifiers have in this application. The main objection to these early amplifiers was their harshness and limited dynamic range, caused almost entirely by the super clean sound and wide frequency response up to their maximum output, after which the onset of clipping occurred quite abruptly and harshly.
When the amplifier is played near its maximum output, the attack of notes (immediately after the note is struck) pushes the amplifier briefly into the clipping region, producing a "squashy, spitting" sound at the start of each note. If the amplifier is overdriven well into the clipping region, the type of overdrive was typically very dirty and un-musical.
There was much debate at the time that valve amplifiers produced overdrive with primarily even harmonics (musically related to the fundamental and therefore desirable), while transistor amplifiers produced odd harmonics which do not have a musical relationship to the note(s) played. I don't believe this is entirely valid, because a type of distortion circuit has become popular with heavy metal players in the 80s (and a variation for grunge players in the 1990's) which replicates the harsh clipping of solid state amplifiers. This type of distortion has been made palatable by following it with some very carefully tuned equalisation to minimise the harsh components, without sacrificing too much high frequency response.
Nevertheless, valve designs produce predominantly low order harmonic overdrive (which is musically related), while solid state designs generally produce a full range of harmonic distortion, including the objectionable high order harmonics. This is due to a number of factors, including the transformer output of valve designs, and the large amounts of negative (electronic) feedback required in solid state designs. Valve designs need very little negative feedback, and some good sounding amps don't use any at all!
Another popular theory of the time was that the difference was due to valves being voltage driven, while transistors are current driven. Hopes were therefore high with the introduction of power FETs, because these are also voltage driven. Some companies produced power FET amplifiers with the usual claims of their likeness to valve sound, and again, failed. These early designs still exhibited the transistor cleanliness with abrupt and harsh clipping.
To my ear, the main undesirable elements of a solid state power amplifier are that there is no high frequency roll-off to remove the harshness of the brutal clipping produced, but more importantly, the onset of distortion is just too sudden. Distortion itself is the addition of higher harmonics not present in the original signal, and both the smoother onset of distortion, and their design means they add different harmonics. Valve amplifiers use an output transformer which results in the lower order harmonics being predominant in the early phases of overdrive, giving the characteristic smooth overdrive. More recent solid state power amplifiers have begun to address these design goals with some success by using volume limiting, and flexible preamplifier overdrive circuits. Marshall Valvestate and Peavey Transtube offer some very valve-like dynamics with their solid state power amplifier designs.
One option is to use a very powerful solid state amplifier and speaker system, and produce the required sounds entirely within the preamplifier stages. Indeed, this approach is favoured by some heavy metal players, and for related styles.
Most designs now (valve and solid state) allow the preamplifier to be overdriven, with a master volume level to control the level produced through the power amplifier. This allows the player to use a variety of clean to overdriven sounds at various volume levels. Generally, the player will establish a fixed master volume level to suit the performing situation, and footswitch additional preamplifier overdrive as required. It is common to provide gain in the hundreds, just within the pre-amp stages!
The following diagram shows how the overdrive preamp is switched. It may be built into the amplifier, or contained within a foot-switchable box at the guitarist's feet.
There are many different desirable sounds produced by valve amplifiers, and they vary smoothly from completely clean to "full-on" overdrive. It's not possible to describe every sound between these extremes, but there are three distinctive sounds that occur below, on and above the clipping point:
Clean, where no overdrive is heard. Even when a valve amplifier operates well below its point of audible overdrive, it still exhibits an amount of sound compression, or sustain. Players usually describe this effect as "warmth", although this has nothing to do with warm tone settings (high bass or low treble).
The brown or thick, creamy sound, where the amplifier is pushed into subtle overdrive at the start of notes and chords, but decays into a cleaner sound. This is considered the sweet spot of the amplifier, where sustain and variable clipping occurs over the duration of notes. This sound has so far eluded any solid state preamplifier or power amplifier design I've heard. I would even suggest that anyone who can truly capture this sound in a solid state design can probably make a lot of money from guitarists around the world, although I'm not sure how it might impact the valve manufacturing industry. Without wanting to deter anyone, you should be aware that many have tried and failed. The closest sound I can get uses a limiter followed by an overdrive circuit just on the verge of audible clipping, then some equalisation to reduce a little high frequency response. As mentioned above, Marshall Valvestate and Peavey Transtube offer some realistic sounds and dynamics.
Overdriven sounds, with long sustain and smooth overdrive. Modern preamplifier designs are capable of producing many of the popular sounds tonally, with some new sounds not achievable with any type of power amplifier. Many players agree that the sounds are good, but the "feel" (dynamic response) is not the same as a heavily overdriven valve power amplifier.
I believe the sound commonly sought by guitarists is produced in the power amplifier stages of a valve amplifier. For example, Mesa Boogie offers some of the most flexible valve preamplifier overdrives you can get, and the tones available are truly awesome, however, I don't hear that creamy compression until you start to push the output stage (ie turn up the volume!).
Several recent designs are hybrid, with a valve in the preamplifier section capable of being overdriven, and a solid state power amplifier intended to be run within its power capability. An overdriven preamplifier valve does sound different to the popular solid state overdrive circuits, however, it does not sound the same as an overdriven valve power amplifier to me.
Another field developing at the moment is digital modelling. Preamplifiers such as the Roland GP-100, Line 6 Pods, Axe-FX, and amplifiers such as the Line 6 Spiders convert your guitar signal into numbers which are manipulated by various computer programs then converted back to audio. This already provides some amazing versatility. In addition to simulating many popular amplifier sounds, entirely new ones can be created, and different components can be combined. For example, you could combine a Fender tone circuit with a Vox AC30 output stage played through a Marshall quad box! These amplifiers usually include common guitar effects as well, so all you really need is a guitar (and foot-controller if you play live shows).
Their amplifier simulations range from good to excellent and are very usable in live and recording situations. The versatility alone makes these systems almost a natural choice for musos who need to cover a wide range of amplifier sounds and digital effects with a single setup: for example if you play covers, or need to record quickly and can't afford to waste studio dollars connecting different amps and cabinets. To me, early designs don't quite create the juicy sounds available from a valve output stage, but considering they are first and second generation designs, the future looks (sounds?) bright! The Fractal Audio Axe-FX is probably the first unit to accurately produce valve amplifier tones.