AMPLIFICATION: TUBE OR SOLID-STATE?
There is no shortage of opinion about the answer to this question. Most people do not really care; they just want to hear music as they go about their business, and pay as little as possible to do so. However, there are those that care deeply about the sound they create, or the sound system they listen to, and this piece is for them.
Usually this debate centers on the premise that one type of amplification system is clearly superior to the other. The creation of the transistor was the start of the debate, and it has continued for decades. In this article, I would like to take a balanced look at the subject. Unlike many articles you can find, I will not be using mathematics, physics or the science of psychoacoustics. Each of those fields requires its own College degree to talk about intelligently. I simply want to compare and contrast the key features of both Vacuum tube and solid-state amplifiers, and offer my observations.
Specifically I want to look at:
- Amplifier Construction and Operation
At the most basic level tube and solid-state amps are the same. They all have a chassis, electrical components and an enclosure. Instrument amplifiers may have internal speakers while most home audio requires separate speakers. The big difference is in the details.
Tube amplifiers use a high voltage DC pathway to produce amplification and carry a much lower level AC music signal. This blend of AC and DC requires the use of addition components to isolate potentially lethal voltages from the user and the equipment. To deliver their potential, tubes need high voltage. Solid-state devices generally do not need high voltage so their power supplies are far simpler and much cheaper to make. Both solid-State and Tube amps have power transformers to run their power supplies. Tube amps generally raise the incoming voltage, rectify it, clean it and send it to the circuit at various voltages, from 6 volts to 500 volts. Solid-state amps reduce the incoming voltage, rectify it, clean it and send it to fixed power rails that service all the devices attached. These voltages are typically in the range of +/- 5 volts, 12 volts, 15 volts. The highest voltages in a Solid-state amp typically come from the output devices that drive the speakers.
While we are on the subject of output devices, let’s look at the second biggest difference between solid-state and Tube amps. Solid-state amps do not require an output transformer. High voltage vacuum tubes have a high output impedance and require a transformer to decouple the high voltage DC from the much lower power AC music signal sent to the speakers. Speakers will not run on DC, they will melt. The output Transformer also changes the high impedance signal of many thousands of ohms to a low impedance signal in the 2 to 16 ohm range to match with the speakers you connect. The transformers are the most expensive components in any amplifier. By eliminating a transformer, solid-state gets a huge cost advantage.
That leaves us with the Elephant in the room. Tube amps need vacuum tubes and that affects the entire design and construction process. Tubes are electro-mechanical devices that run on high voltage and produce a lot of wasted energy in the form of heat. In order to operate, a vacuum tube moves a controlled flow of electrons from its cathode to its anode or plate. A red-hot filament heats the cathode and a cloud of electrons forms. High voltages of varying levels feeds different elements within the tube to direct the flow of the cloud, while a small AC control voltage regulates the flow volume. Hence, the British term, valve. This process generates heat in vacuum tubes that could be 200 degrees Celsius. Water boils at 100 degrees C. As such, the enclosure for a tube amp has special requirements for safety, to protect the user and innocent passersby.
As scientific research improved, the use of a combination of dissimilar elements combined on silicon wafers to create voltage amplification occurred. Home audio and musical equipment manufacturers jumped on this new technology. So did NASA and the U.S. space program. Most builders of vacuum tube equipment, viewed solid-state devices as a threat to their business or an opportunity to improve their bottom line if they modified their designs.
Since solid-state construction can be smaller, lighter and less expensive the elimination of the vacuum tube seemed to make sense. This opened the door to mass production and printed circuit boards. Most amplifiers today use some type of printed circuit board to mount their required components; both for solid-state and for Tube designs. Early printed circuit boards just eliminated the mounting points for components and the wires that joined them. Components were soldered to the boards by hand. Modern solid-state amp construction is usually done on multi-layered printed circuit boards. The components are installed by robots and then soldered in an automated process known as wave soldering. The primary goal is to make them fast and cheap by eliminating work done by people. You can get much higher quality solid-state amplifiers but the price rises quickly if you demand higher quality. Some guitar amp builders are still using non-conductive component cards with eyelets or turrets to mount their components. This is well suited to vacuum tubes but not so much for solid-state.
When it comes to cost, solid-state amplification really shines. Things like transistors, FET’s, rectifiers and integrated circuits are all machine made in enormous quantities using high-speed assembly lines. This reduces the price of individual components to pennies. Since most designs do not use an output transformer there is a huge saving on that one item. After purchase, solid-state amps have virtually no ongoing maintenance costs. They run cool, unaffected by vibration and will operate for days without a care. Failure in solid-state amps usually occurs very early in their life span, due to manufacturing defects.
Vacuum tubes on the other hand, require a lot of manual labour and are produced in relatively small batches, making their cost to produce hundreds of times higher than equivalent solid-state devices. The average tube amp has ongoing maintenance costs. Tubes are fragile and subjected to intense cycles of heating and cooling. Vibration and heat will degrade tubes and they are susceptible to developing stray noise and microphonics. Over time, the cathode coating material will be depleted and tubes become weak. Tube replacement is inevitable and in the case of power tubes, technical skills are required to ensure the amp is correctly set up for proper, safe, operation. Most people do not have the skills to work safely on high voltage equipment like a tube amp, and must hire somebody to do this work for them.
Both solid-state and vacuum tube amplifiers are available as both cheap consumer products and very expensive professional grade equipment. After purchase, tube equipment will cost more to own.
So far, I have compared the manufacture and relative costs of vacuum tube vs. solid-state equipment. It would seem that solid-state has everything going for it. Cheaper, lighter, cooler and less expensive to operate. If this is true, why is vacuum tube technology still around after all these years?
Here is the rub. Within that group of audio enthusiasts and musicians that really care about their sound, there are those that think vacuum tubes simply sound better and those that think solid-state is just as good or better. Solid-state enthusiasts can provide highly technical data that supports their argument, as can those that favour tube based designs.
There are really two sub groups when it comes to amplifiers. Those designed to create music and those designed to reproduce music. Both use completely different audio sources as inputs and very different speakers to handle the output. Musicians want amps that can provide a wide range sound, from sparkling clean to highly distorted and harmonic laden. Home audio enthusiasts want amps that will reproduce the original source material as closely as possible. They do not want to create distortion they want to reproduce the distortion made by others.
Distortion is one of the key selling points for tube amp lovers. Solid-state amps do not seem to distort as musically as vacuum tube designs. The primary reason is that when you drive tubes hard the distortion comes on gradually and presents itself as gradual compression that blooms into distortion. The high voltage supplied to vacuum tubes ensures that the output of the device seldom exceeds the voltage that runs it. Solid-state amps use lower voltage supply rails to operate and the output can exceed the supply voltage. When that happens, they do not start to compress and gradually clip. They simply cut off the output signal at that level. Sine waves become square waves immediately and the sound is not pleasant. Solid-state distortion in its pure form sounds something like bees in a tin foil bag. Not the best for musical instruments and completely unacceptable for home audio enthusiasts. The nature of clipping and distortion also affects the harmonic content of the output signal.
But the main reason both musicians and audiophiles alike love the sound of tubes is their even-order harmonic distortion. The primary difference is even-order versus odd-order harmonic distortion. Perhaps a lesser known type of distortion, harmonic distortion of tubes is what fills out the sound and adds warmth. Without getting too technical, all amplifiers will have sympathetic distortion related to the original signal. Tubes have mostly even-order harmonics (referred to as second, fourth, and sixth). Solid-state devices have more odd-order harmonics (third, fifth and so on). It is the even-order harmonics that will provide positive embellishments to the original signal, making it sound fuller. A technical article written by Russell O. Hamm published in 1973 in the Journal of the Audio Engineering Society described this as a choral or singing sound. This is largely what provides the “tubey” sound, the full, deep, warm sound tube amplifiers are known for. The odd-order harmonics produced by solidstate amplifiers produce a edgy or cut-off sound. Often this is viewed as more “accurate” sounding, but the reality is it is also largely the cause of listener fatigue. It is not natural distortion or add to the original signal positively, and good ears with tire of it quickly.
Dynamic range is another point of contention between the camps. Solid-state does not seem to handle peaks or transients in the music signal as well as tubes. Essentially, I am talking about sound that goes bang or thump. This is likely due to that fact that tubes naturally have soft clipping and compression to handle transients and smooth those peaks and valleys. The amplification of bass guitar is an exception to this general rule. Low frequencies require more power to amplify than high frequencies. A tube amp designed specifically for low frequencies requires a lot of output tubes and massive transformers to generate the power that most bassists require for live performance. This makes them very hot, very big and very heavy. The Ampeg SVT is widely regarded as king of the bass tube amps but it only delivers 300 watts while modern solid-state sound systems can deliver thousands of watts in a much smaller, cooler package by using advanced operating modes such as class D.
If you are a skeptic, you should look at the direction taken by manufacturers of solid-state guitar amps. The advertisements usually feature genuine tube tone as a major selling point. In order to deliver on this selling point additional solid-state devices were developed that more closely mimic the clipping characteristic of tubes. Helper circuits create things like asymmetrical clipping and distortion on demand, using simple diodes. The technology has really gotten good over time. The advent of amplifier modelling is perhaps the biggest chance for solid-state amps to sound like tube amps and has been a real bonus for recording applications. Just add a computer to your amp and season to taste.
However, there is a cost associated when manufacturers create proprietary devices and circuits to emulate the sound of tubes. When the product ceases production, any proprietary parts also cease production. Most popular tube amps are variations of circuits that have been around for more than 60 years and use a standard range of components. In their glory days, manufacturers of tube gear did use oddball tubes to fit their designs and today you will occasionally find amps that use tubes no longer in production. Most currently produced tube equipment uses standard, popular, vacuum tubes in their construction. There is not enough sales volume to warrant the creation of new tubes due to their high production cost. If you have a tube amp today, you are likely going to be able to get replacement tubes in the future. What happens when your modelling amp loses its CPU? Are they still making it?
Finally, we should compare the ease with which a user can change the sound of their amplifier. When it comes to modification there is nothing better than a tube amp. Since the tubes are socketed, they can easily be removed and replaced by users. This starts you down the road to the land of tube rolling. The quest for a tube that sounds different or better than the one you used before it.
Many folks consider tube rolling to be one of the best reason to own a tube amp. They are on a quest for the ultimate sound and tube rolling feeds the need. Output tubes do require some tech chops to replace if they are using a fixed bias scheme. For example, a 6L6GC would appear the same from tube to tube. Because they are made by hand, a lot of variation is introduced due to differences in the alignment and spacing of the tube elements. If you were to grab a random selection of these tubes and install them, each would operate differently in the circuit. Some may run too cold and some may run too hot. The bias voltage has to be set to compensate for the variations and get the output tubes running at the desired operating current. Some high-end tube amps have test points and bias adjusters available to the user. If you know how to use an electrical multi meter and a screwdriver, you can probably bias the amp yourself.
If you want to tweak a solid-state amp, you have to open it up. Advanced solid-state circuits are more complicated and easier to destroy than tube circuits because they have a very low fault tolerance. Simple static electricity can kill solid-state devices, as can the heat from an improperly used soldering iron. If you want to tweak a solid-state amp, you will need a qualified tech to do so. Tubes fit into a socket only one way, while solid-state devices are easy to install improperly if you do not know what you are doing.
Comparing the two, solid-state amps are cheaper to purchase and cheaper to operate than their vacuum tube rivals. The humble tube amp is more expensive to buy and operate due to the increased cost of parts and the fact that they require ongoing maintenance for optimum performance. People will pay the extra money for tube amplifiers because the sound makes it all worthwhile.
This has been a very simplistic review of the subject. I have tried to highlight the key points so the reader can explore those points in more detail and to the technical depth that they desire.