You would think in this world of cell phones, tablet computing and systems on a chip, that the idea of building anything, let alone an amplifier, out of technology the went obsolete in the 70s would be ludicrous, right? Well, not so fast.
As a design engineer with 40+ years of experience, nearly all in solid state design, I myself soul searched over this issue for about 3 months. After all, modern solid state audio amplifiers are near perfect, right? Let me digress and get a little technical here. Solid state devices, mainly transistors, are inherently non linear. However they are DC stable and lend themselves to a feedback based design. Solid state amplifiers tend to be designed around the concept of an Operational Amplifier, an idealized gain block that has infinite bandwidth, infinite gain, infinite input impedance, zero noise, zero output impedance and zero DC offset. The idea is to design a solid state amplifier with the characteristics of an idealized Operational Amplifier, with a gain of 100,000 or so and then use the miracle of feedback to feed back the output to the input with a network to set the final gain to 10 or 20 or so. This cures all ills. It produces an amplifier that has exactly the gain that you designed it for across all frequencies, has zero ohms of output impedance and almost infinite power dependent only on the choice of output transistors and power supply voltage (and current). This approach, if done correctly, has near perfect specifications. It has low noise, wide bandwidth, high output power, and perfect frequency response. It's perfect, right? And cheap. So why would someone want a Vacuum Tube amplifier?
Well, like others before me, I was technically arrogant. First let's start with the definition of perfect. I instantly assumed that solid state amplifiers were better than Vacuum Tube amplifiers because of an assumed definition of a perfect in this context. So, I defined what perfect meant to me as an engineer, it was: An amplifier that has the gain that you desire at all frequencies, has plenty of power and contributes nothing to the signal except to reproduce it and gain it up (make it louder). It turns out that this is not so.. A Guitar Amplifier is an instrument to be played just like any other instrument. Looking at the amplifier from this perspective, opens us up to appreciating a vacuum tube amplifier for its imperfect characteristics.
Vacuum tubes are inherently very linear devices and are amazingly capable of amplifying sound without the benefit of any feedback to control their gain and linearity. A 3 stage Vacuum tube amplifier can reproduce sound from 20Hz to 30kHz at less than 2% distortion with no feedback. So they inherently make good amplifiers by conventional measures. But beyond that, they sound magical. The golden age of Rock, Country, Jazz and Blues, made a tremendous leap when electric Guitars were introduced in the 1940s along with pioneering amplifiers that established the sound that became the gold standards for these genres. By 1959, Leo Fender had refined the Fender Bassman 4x10 amplifier that set the Gold standard and has ever since. Jim Marshall cloned this design and then added his own flavor to it and thus was born Marshall, the sound of rock and roll. By the 80's, all amplifier manufacturers had introduced solid state amplifiers and were obsolescing the Vacuum Tube models. Guess what, the professional musicians would not buy them. The price of used, vintage Vacuum Tube amplifiers began to rise. By the late 90s, all amplifier manufacturers re-introduced Vacuum Tube re-issue models. Why? Because their customers demanded them. Vacuum tube amplifiers are more expensive to produce and have a lower profit margin so the move was not a popular one with the manufacturer's. No self respecting Blues musician today would be caught dead on the stage today with a solid state guitar amplifier. (Bass amplifiers are the exception.) I attend music festivals and the Blues Music awards every year in Memphis Tennessee (the Blues world's version of the Grammys). The stage is packed with vintage Fender and Marshall amplifiers. Also present are Boutique new Vacuum Tube Amplifiers. Joe Bonamassa, the guitar world's most respected rock and blues guitarist and a contributor to Guitar World magazine, plays a Boutique tube amplifier made by my competitor down the Road in Frisco Texas.
Why do Vacuum tube amplifiers sound better? Because they are not perfect by conventional measures. Simply, they add color to the sound. I hear a chiming, a sparkle, when I hear a good vacuum tube amplifier. A magical sound. Because of the lack of overall feedback, they are looser, not as constrained. Not sterile like solid state amplifiers. The frequency response of Vacuum Tube Guitar amplifiers is far from flat, by design, often peaking 10DB or so in the mid ranges with wide swings in the frequency response curves. This is intentional and is accomplished by the tone stack section. Each brand of amplifier has its own tone stack. Marshall tone stacks are particularly drastic by hi-fi standards helping to give them their unique Voicing. Vacuum tube amplifiers have residual distortion even when played at low volumes. This distortion results in lower order harmonics of the input signal. By the nature of the vacuum tube designs, the harmonics are rich in lower order even harmonics. For example the second harmonic is an octave above the fundamental and is almost inaudible. But it adds body to the sound much in the way singers harmonize. These even order harmonics are not present in a solid state amplifier. This is part of the color that I mentioned above.
Another factor is response to overdrive or clipping. Getting a little technical, again, a real challenge in the design of solid state amplifiers is over drive response. When transistors are over driven, and saturate or cut off, the amplifier feedback loop is broken and the amplifier may break into oscillation or simply cease to operate until the overdrive signal goes away. Even if well designed, the amplifier will saturate with sharp edges and a gnarly sound. Engineers try and cover up this behavior by designing the amplifier with a large amount of momentary power capability so that it will ride through the event and the user will not notice. However this overdrive creates a sound that is rich in odd harmonics. Instead of making the tone fuller, it makes the sound harsh and brittle. These sharp edges that are created as a solid state amplifier is driven hard are referred to as edge harmonics. These edge harmonics are not musically related to the input signal and thus sound harsh to the listener. Vacuum tubes behave differently in the presence of overdrive. There is no feedback loop so they never become unstable. They saturate slowly and softly producing that overdrive sound, rich in even harmonic content at first and then as the amplifier is driven harder, adding the odd harmonics, that rock music so loves. However the sound is lacking in the edge harmonics that solid state amplifiers produce. Overdrive does no harm to the vacuum tubes and they can be overdriven all day long. Different stages of the amplifier (preamp or power amp) can be overdriven producing different overdrive sounds. Because of the soft saturation characteristics of vacuum tube amplifiers, they have an effectively larger dynamic range. The output transformer also plays an important part in the overdrive characteristics of the amplifier as it saturates in non linear ways thus adding to the rich sound that occurs during overdrive of the output stage. The master volume amplifiers were designed to allow independent control over these two types of overdrive although musicians generally do not like master volume amplifiers. Manufacturers have been building modeling amplifiers for years with digital signal processors ahead of a solid state amplifier in order to attempt to simulate the rich sound of vacuum tube amplifiers. These amps are feeble attempts to simulate the real thing and have not been able to re-create the rich Vacuum Tube sound of a good guitar amplifier.
Another factor differentiating vacuum Tube amps from Solid state ones is the output stage interaction with the speakers. Solid state amplifiers have a zero ohms output impedance by design. That means that they can drive any arbitrary speaker with any impedance and will happily do so until they burn up due to speaker overload. They are not designed to drive just a 4 ohm load or an 8 ohm load. They simply do not care. In a vacuum tube amplifier, the output impedance is specified and must be adhered to or risk damage to the amplifier. The output tubes, the output transformer and the speakers are linked and are part of the output stage design. If the amp is set to the 4 ohm setting, the output impedance of the amp is 4 ohms and must be connected to (or matched) to a 4 ohm speaker load. This is very important.
In this vein, understand that a speaker is a linear motor. A signal is applied to the voice coil that opposes the magnetic field in the permanent magnet and the coil moves in response to this magnetic force. This movement causes the cone to move air thus making sound. When this signal goes away (think a transient), the voice coil relaxes and the cone moves back to center (the cone is spring loaded so as to force a return to center). Now a fundamental principle of electronics is that any motor is a generator and a generator is a motor. So the linear motor that is the speaker now generates an electric current (known as back EMF). In a solid state amplifier, the zero ohms output impedance shunts or damps that back EMF instantly and the speaker stops in its tracks. In a vacuum tube amplifier, the output impedance is, lets say, 4 ohms. So the collapsing field (back EMF) sees 4 ohms looking into the amplifier. This damps the speaker movement somewhat but contributes to the looseness or unique open non damped characteristic of the Vacuum Tube amp sound.
The sound of a Vacuum Tube amplifier is complex, rich in overtones due to the class-AB drive characteristics of the output stage, loose, and magical all at the same time. The output transformer (not needed in Solid state amps) also richly contributes to the warm sound. In stereo audio amplifier design, the objective is to achieve low distortion and faithful reproduction. In a guitar amplifier, the output transformer is intentionally designed to minimize odd order harmonic distortion. This creates more bell tones, chimes, sonic collisions that result from that harmonic richness. Each output transformer has a unique sound so no two Vacuum Tube amplifiers sound exactly the same. In the early days of Led Zeppelin, it is rumored that Jimmy Page has such a magical sounding amplifier, due to a particular output transformer that he had custom made, that he had the amp insured for 1 million pounds. To hear a benchmark performance of a truly great Vacuum Tube amplifier and a performance that truly raised the bar, listen to the classic 1966 John Mayall, Eric Clapton "Blues Breakers" album. Eric used an original hand wired limited run Marshall JTM-45 amplifier with a Radiospares De-Luxe transformer and KT66 vacuum tubes. Marshall later changed to EL34 vacuum tubes and a cheaper transformer supplier for the JTM-45 and the JTM-45 sound was never the same. George Harrison played a Vox AC30 amplifier with a Haddon output transformer on the early Beatles recordings. This sound still causes goose bumps today. George Harrison drove his Vox amp to overdrive producing that unmistakable sparkle and chime that is heard on the early Beatles performances. The production VOX AC30 amplifiers used a cheaper transformer and did not sound the same. To add insult, the VOX amplifiers changed to a solid state design in that time period. Many US musicians, trying to achieve that Beatles sound, bought VOX amplifiers and wondered why they did not sound the same.
Components in a Crossfire amplifier are individually selected for their sound quality. A Crossfire proprietary bias and control circuit assures that the output stage tubes operate at exactly their optimum operating point for the best sound without damaging the tubes. The output transformers are proprietary and manufactured for Crossfire Electronics by Mercury Magnetics. Vacuum tubes are selected by the manufacturer for specifications provided by Crossfire Electronics. These specifications are proprietary.
I said above that a guitar amplifier is an instrument to be played. I often listen as players play my amplifiers or good ones (vacuum tube ones) that I service. I listen as they adjust the amplifier controls to their taste and then by moderating the pick attack and guitar volume controls, easily go from creamy smooth to edgy and back again effortlessly. The amplifier is truly being played as an instrument. Solid state amplifiers have two regions of operation, clean and gnarly saturation, and it happens abruptly. Whereas Vacuum tube amplifiers have a wide dynamic range of clean to saturation allowing many levels of tuning as the artist plays the amp. All without touching the amplifier. But even at saturation, they never have that edge harmonics effect. Effects pedals are used often to introduce controlled distortion if it is desired.
I am often asked: Can you still buy Vacuum Tubes? Or: Where do you get the vacuum Tubes? Are they still made? There are many manufacturer's of vacuum Tubes in business today. The great European brands like Mullard and Tung Sol are still manufactured by companies that have purchased the rights to both the name and designs. The GE 6L6GT power tube is manufactured by Groove Tubes in Chatsworth California using the original GE designs on the original GE equipment. There are large factories in Eastern European countries as well as in Russia and China. A popular brand, JJ Electronics (formerly Tesla) is located in the Slovak Republic for example. So the vacuum tubes are readily available. I work with a US supplier who individually tests the tubes I order to my specifications. I have done performance testing on these tubes as compared to New Old Stock tubes (from my attic) from American brands such as RCA and GE. The tubes that I purchase are superior to the NOS tubes in several important ways. I also purchase matched, specified gain, output tubes in dual and quads depending on the application. This was not possible 30 years ago unless you purchased 100 tubes and tested them yourself (which no one ever did).
The IEEE (International Electrical and Electronics Engineers professional organization), of which I am a life member, featured an article in the prestigious magazine IEEE Spectrum in 1999 called The cool sound of tubes. You may read the article if you follow the link. There is also a fascinating set of vintage 1942 or so documentary videos about vacuum tubes linked on the links page as well as other interesting links to related topics.