Tubes - The Mystery Unraveled

This month we will begin the first of a multi-part discussion on perhaps the most mystical subject among guitar players- Tubes. The amount of information/ misinformation available on the Internet about Tubes is mind-boggling.From rock stars to self proclaimed Internet Gurus you can read how to find the Holy Grail of Tone by following their methods. Unfortunately your results often may not be as claimed leaving you frustrated and dollars poorer.

I speak with many guitar players through my repair business some amateur, some pro but what I find they share in common is most don't understand anything about tubes other than they can provide the best guitar tone.

Let's begin by asking the question what does a tube do?

For the purposes of guitar amps, tubes are devices that take the tiny signal coming from your guitar pickup and amplify it into something that can drive a speaker to make sound. As you are probably aware tubes are not the only method of doing this. Solid state devices such as transistors can do this also, but it is a matter of much debate whether they can do it as well. So what is it about tubes that makes this outdated technology something still desired by guitarists? The first thing we can comment about is that during the early days of the electric guitar, the only way to amplify the guitar was through the use of tubes. This means that the legendary guitar icons created their sound with tube amps and those seeking to emulate them should seek the same equipment. If Jimmy Page used a Supro amp to record Led Zeppellin I and his tone is what you are after, then it makes sense to seek out an amplifier similar to that. Additionally in those days as the first solid state amps appeared there is no question they were inferior sounding to tube amps when it came to guitar tone. So we see the establishment of a mindset among musicians that tubes are the way to go based upon past history. Nowadays solid state technology has progressed and there are many solid state amps that do a great job. Modeling is another new technology that is improving all the time. But it seems to be the general consensus that a well designed tube amp can contain nuances of tone that don't seem to be present in solid state amps.But why is this so?

The answer to this is complex but there are several basic reasons we can discuss here. The first difference is harmonic distortion. All sound is made up of fundamentals and overtones(harmonics). It is the combination of fundamentals and harmonics that give all sounds we hear their unique sonic signature. Harmonics are merely multiples of the fundamental and occur at the octave, fifth, seventh, ninth, etc.Each overtone may also vary in its volume in relation to the fundamental. When a guitar signal is distorted one the distortions created is harmonic distortion. The fundamentals and harmonics of the original signal are distorted and now create further harmonics. Which harmonics are generated is the key to what sounds good to your ear. When there is an abundance of even order harmonics (octaves, thirds,fifths) it sounds pleasing to the ear. Odd order harmonics are brittle and jagged sounding. Here is where there is a difference between tubes and transistors. When distorted, tubes have an abundance of even order harmonics while transistors have an abundance of odd order harmonics. You would think that this would only have an effect on distortion sounds and you wouldn't be able to tell the difference on clean sounds, but even on a clean tube sound there is enough unperceived distortion present to make the sound warmer than what is perceived on a solid state clean sound.
Another main difference between tubes and transistors is that as signal levels are increased tubes start to distort in a more gradual fashion than transistors. this is why you can vary your picking dynamics to change the distortion intensity of the sound or lower your volume control on the guitar and have the sound clean up. This is not done as well with transistors.

But let's move on and talk about the tubes themselves.Here is a simplified explanation of how a tube works. The glow you can see inside the tube is called the heater(also called the filament). The filament sits inside a sleeve called the cathode which is coated with a chemical that emits a cloud of electrons when heated. The cathode is surrounded by several concentrically arranged wire spirals called grids. At the outside of the last grid there is a metal structure called the plate.The plate is charged with a high positive voltage somewhere between 200 and 500 volts DC. The electron cloud emitted from the cathode is negatively charged and I'm sure you remember from high school science class that positive and negative charges attract so the electrons are pulled from the cathode towards the positively charged plate. As the electrons pass through the concentrically arranged grids on their way to the plate, they are influenced by control voltages placed on the grids, one of which is the guitar signal itself. As the control signal varies in polarity(changes between being positively or negatively charged)it interacts with the flow of electrons on their way to the plate. As the control grid becomes more negative it blocks some of the electrons from getting to the plate by repelling them back towards the cathode(like charges repel). The larger stream of electrons flowing towards the plate from the cathode now can vary in step with the smaller signal at the tubes input and the result is that as the signal goes from tube to tube it is made larger.

This is a simplified explanation of what is going on. We will revisit this when we talk about bias. I'd like to also give an analogy to help you understand why the smaller signal at the input to a tube can create a larger one at its output. Think of an aerosol spray paint can. There is an aerosol propellant in the can. The pressure from the propellant will push the paint out of the can when the valve is opened. It only takes a small amount of pressure on the valve from your finger to unleash the much larger pressure of the propellant that pushes the paint from the can. If you think of voltage as a pressurized propellant, electrons as something to be moved like the paint in the can, and the small input signal into a tube as your finger, then you can see how a small signal can control a larger one that moves in step with it.

Next, let's discuss, what the tubes in the amplifier Are doing as an amplifier system.

We start with the tiny signal from a guitar pickup. It gets inputted into a series of preamp tube stages (usually several 12ax7). Each preamp tube stage increases the signal from the previous tube to the point where it is large enough in voltage to the drive power tubes which require a much larger input signal than what comes out of a guitar. The power tubes (EL34/EL84/6L6 etc.) are what create the power that can drive the speaker. Unlike the preamp tube circuit,which only amplifies voltage, the power tubes can increase current also.To create power (watts) you need to have an increase in both voltage and current (amperage). Suspiciously, Ohms law tells us that Volts time Amps = Watts. Since there is no appreciable increase in current from the preamp tubes they are called voltage amplifier stages leaving the power tubes to do just that- create power.

Tubes can not do the job by themselves. There are a host of support components that are in the circuit also. Resistors capacitors, transformers, etc. are all connected to the tubes and shape the way the tube operates. Therefore It is important to understand that it is the circuit in its entirety that creates the tone of an amplifier not the tubes themselves. Each tube model does have some general tonal characteristics unique to itself, but this can be varied over a broad range by the circuit. This is why you get different sounds from different amps using the same model tubes. And although different model tubes plugged into similar circuits may sound different than each other, you can make the same model tube sound vastly different by varying the circuit it is in.

We have only begun to scratch the surface of tubes. Next month we will move on to talk about bias, class A and rectifier tubes, all the buzz words that everyone uses everyday and for a million dollars could not correctly explain to you what they mean. I invite you to e-mail your questions, comments and suggestions.