Put two audio engineers in a room and you’ll probably get three different opinions on the relative merits of tubes vs. solid state. Pop knowledge and marketing slogans extol the virtues of the tube’s warm sound and forgiving nature, while others point to the greater neutrality and accuracy possible with solid state. As witnessed with many subjects, prevailing wisdom usually tells just part of the story.
A tube is a device that amplifies a signal by controlling the movement of electrons between a filament and a plate in an enclosure with the air removed. The evacuated air and cylindrical enclosure give the device its name, the vacuum tube. This enclosure is usually made of glass, though metal and ceramic are used in some types of tubes. In the UK, the devices are generally called valves, or thermionic valves, referring to the release of electrons by the heated filament in a process known as thermionic emission. The negatively charged electrons are then attracted by a positively charged plate, and controlling their flow controls the gain of the device.
 At the heart of solid-state electronics is the transistor. It can be a discrete device, or it can be one of many miniaturized transistors in an IC, or Integrated Circuit. Like a tube, it also can control gain between two of its terminals based on a voltage applied to a third. Solid State refers to the current traveling through solid semi-conducting material like germanium, or more commonly, silicon, rather than the electrons traveling in the space of a vacuum tube. The advantages of solid state from a manufacturing point of view include a reduction in size, weight, price, and power consumption, and an increase in flexibility, control, lifespan, and manufacturing automation.
There are many practical differences between tube and solid-state designs for audio, yet often they aren’t as dissimilar as one might expect. A key difference is in the way that each handles overload. Tubes go into saturation slowly so that distortion gradually increases, and you get that familiar “warming” of the sound before hearing obvious and objectionable distortion. A transistor goes from clean to ugly very quickly once you pass the threshold of its ability to cope with a high level. Another important distinction is the type of distortion that predominates with each. The dominant distortion product of a tube is second harmonic, which is more complimentary to the musical signal and in small amounts adds a “fattening” effect to the sound. Transistor distortion is dominated by the third harmonic, which is far less flattering to musical signals.
The warmth and color of tubes is highly exaggerated by advertisers. Adding a small 12AX7 tube to a sterile sounding audio processor will amount to little more than a sterile sounding audio processor with a little light bulb in it. A large, and often more significant contributor to the warmth and color of vintage audio gear and modern neo-classics, from mics to amps to EQs to compressors, is the transformer. Transformers have their own transfer characteristics and distortion products. This is one reason that highly prized mic-pres and EQs from Neve sound as warm as they do without the inclusion of tubes.
Many modern transformerless designs that contain tubes can sound quite transparent, and you would be hard pressed to tell the difference between a tube and solid-state design simply by listening. Many talented designers understand that each approach has advantages and disadvantages, and they often use tubes for reasons other than generating warmth. Some modern manufacturers take the opposite route and design solid-state gear that tries to emulate tube characteristics, sometimes with the added assistance of digital processing. Some trademarked names they apply to this include Valvetronix, Valvestate, and Transtube. Don’t assume that there are valves or tubes in a product simply based on the name. Some of these emulations achieve certain degrees of success, while others just add unhelpful distortion on top of a solid-state tone. Let your ears be the judge.
-Jay Frigoletto Question for Jay? Visit him online at www.promastering.com or www.myspace.com/sslmixer.
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