How Tube Amps Work | Chapter 1: The Tubes

How Tube Amps Work | Chapter 1: The Tubes

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Welcome to chapter 1 of the tube amp series. In this multi-post series we will be breaking down and rebuilding everything about tube amps. If you finish this whole series (and I hope you do) you will be able to build your own tube amp from soup to nuts. Before we dig into everything, if you’re a fan of everything music please drop your email in the newsletter form on the left for updates and new articles. 

Without further delay, let’s jump onto Chapter 1 – The Tubes.

What Is A Tube Amp

There is good reason tube amps are so highly revered. The tones of countless rock bands and guitar gods have nearly all been generated by a tube amp. There are many different types of tube amps but let’s crack open the basics of what a tube amp really is. 

A tube amplifier (tube amp) is an amplifier that uses vacuum tubes (also known as valves) to increase the amplitude or power of a signal. 

Here’s how it works:

Signal Amplification: The basic function of a tube amp is to take a small electrical signal and amplify it, making it strong enough to drive a loudspeaker or other output device.

Vacuum Tubes: The core components are the vacuum tubes, which control electron flow. When a musical signal is applied to the input of the tube, it modulates the flow of electrons within the tube according to the signal’s voltage. This modulation effectively amplifies the signal.

Warm Sound: Tube amps are often praised for their “warm” or “rich” sound quality. This is due to the harmonic distortion and dynamic characteristics they add to the audio signal, which many musicians and audiophiles find pleasing.

Components: Besides vacuum tubes, a tube amp includes transformers to step up the voltage, capacitors, and resistors to manage the flow of electricity, and the circuits to connect these components.

How Is A Tube Amp Different From Solid State Amp

We will keep this brief to stay focused on tube amps. The technology difference between tube amps and solid-state amps lies in the way they amplify signals and the components they use to achieve this:

Tube Amp Technology:

Vacuum Tubes: Tube amps use vacuum tubes to amplify the signal. These tubes have electrodes inside a vacuum-sealed glass bulb. When a voltage is applied, electrons flow from one electrode to another, amplifying the incoming audio signal.
Analog Warmth: The tubes naturally add a kind of compression and harmonically rich distortion to the sound, which is often described as warmth. This distortion occurs because tubes handle signal peaks in a smoother fashion compared to solid-state components.
Output Transformers: These are needed to match the high output impedance of the tubes to the low impedance of the speaker, which also affects the sound.
High Voltage: Tubes require higher operating voltages, which means tube amps need power transformers to step up the voltage.

Solid-State Amp Technology:

Transistors: Instead of tubes, solid-state amps use transistors to amplify the signal. These can be individual transistors, integrated circuits, or a combination of both. They control the flow of current in response to an input signal, amplifying it.
Consistency and Clarity: Transistors provide a very clean and clear amplification, with less distortion at normal operating levels. They can offer a very accurate reproduction of the input signal.
No Need for Transformers: Most solid-state amps do not require output transformers, as transistors can be designed to work with the low impedance of speakers directly.
Low Voltage: Transistors operate at much lower voltages than tubes, which means the power supply design is simpler and does not require large transformers.
Efficiency and Heat: Transistors are generally more efficient and produce less heat compared to tubes, which can run quite hot and require time to warm up before they perform optimally.

The Tubes

We’ve finally made it to the meat of chapter 1. As you might imagine, the tubes are the foundation of these amplifiers and understanding how they work and the difference between setups and types will have a profound impact on the sound of your amplifier. 

What Is A Tube

As we touched on earlier, a tube is a vacuum-sealed glass bulb containing electrodes that control electron flow to amplify audio signals, imparting a warm, rich sound characteristic to tube amps. But it’s important to understand the components of a tube. 

Inside a vacuum tube, there are several key components, also known as elements:

Cathode: This is the electron-emitting element, which is heated either directly or indirectly to cause it to release electrons.

Anode (Plate): This is the positively charged element that collects the electrons emitted by the cathode.

Grid(s): Positioned between the cathode and anode, the grid controls the flow of electrons. In more complex tubes, there can be multiple grids (such as control grid, screen grid, and suppressor grid) to refine the amplification process and control different aspects of the tube’s performance.

Heater or Filament: This is used to heat the cathode so that it emits electrons. In directly heated tubes, the filament itself is the cathode; in indirectly heated tubes, the filament heats a separate cathode element.

Glass Envelope: This is the vacuum-sealed container that holds all the elements. The vacuum inside allows electrons to flow freely without colliding with air molecules.

Pins: At the base of the tube, there are metal pins that connect the internal elements to the external circuit.

These components work together to control the flow of electrons, which is modulated by the audio signal, leading to the amplification of the signal.

When an audio signal enters a vacuum tube, it’s applied to the tube’s grid, which sits between the cathode and the anode (or plate). The cathode is heated and releases electrons, a process called thermionic emission. The grid controls the flow of these electrons by becoming more or less negative in response to the audio signal. When the grid is less negative, more electrons are allowed to pass through to the anode; when it’s more negative, fewer electrons pass. This varying flow of electrons between the cathode and anode amplifies the original audio signal because the tube can control a large flow of electrons with a very small change in the grid’s voltage, which is manipulated by the incoming audio signal.

Preamp Tubes and Power Tubes

If you look at the back of a tube amp, you will notice 2 different types of tubes. These are the preamp tubes and the power tubes – each of them have very specific functions. 

Preamp Tubes: These are the first tubes in the series and generally are the smaller of the two types. These are used to amplify the initial audio signal from the instrument or source. They’re responsible for the first stage of amplification and can shape the tone and character of the sound. They often add harmonic content and can influence the overall “voicing” of the amplifier. Common types of preamp tubes include the 12AX7, 12AT7, and 12AU7. 
Power Tubes: Larger than preamp tubes, power tubes amplify the preamp’s signal to a level that can drive a speaker. They’re responsible for the final output that you hear, and the type of power tube can significantly affect the amp’s sound and behavior. Common types of power tubes include the 6L6, EL34, 6V6, and EL84.
Here’s a great photo of preamp tubes and power amp tubes in the back of an amp. 
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How Preamp and Power Tubes Impact Tone

The impact on tone in a tube amplifier comes from both the preamp and power tubes, but they contribute in different ways:
Preamp Tubes have a significant impact on the initial shaping of the tone. They are responsible for the primary amplification of the instrument’s signal and heavily influence the gain, clarity, and warmth of the sound. The preamp stage is also where most of the amp’s distortion is generated, especially in amps designed for overdriven sounds.
Power Tubes influence the overall character and feel of the amp’s sound at higher volumes. They contribute to the sonic signature, particularly when the amp is driven hard and the power tubes begin to saturate, adding their own form of distortion which is generally smoother and more dynamic.
In general, preamp tubes tend to have a more noticeable impact on the tone when playing at lower volumes, while power tubes become more influential as the volume increases and they approach their limit of clean power output. For guitarists seeking to shape their core tone, especially for overdriven or distorted sounds, the preamp tubes will be a key factor. For those interested in the feel and response of the amp at louder volumes, or the “headroom” available before distortion occurs, the power tubes will be more critical.

Recommended Tubes

There is no “best” tube for an amplifier. Every guitarist and band will seek a different tone and need a different tube for the job. But chapter 1 would be incomplete without at least a few recommendations. Before you go out and buy a set of tubes (and they can be quite expensive) I also recommend finishing the rest of the tube amp series here on
Some tips when choosing tubes:
Match the Tubes to the Amp: Some amps are designed with specific tubes in mind, so check the manufacturer’s recommendations.
Consider the Music Style: Different tubes can favor certain music styles, so consider what suits your playstyle.
Quality and Reliability: Look for tubes from reputable manufacturers to ensure reliability and consistent performance.
Listen to Comparisons: If possible, listen to sound comparisons or ask for opinions from experienced users. 
Preamp Tubes Recommendations
12AX7: Known for its high gain, the 12AX7 is a favorite among guitarists for creating a warm, rich tone with good sustain. Brands like JJ Electronic, Telefunken, and Sovtek are often recommended.
12AT7: Typically used in reverb or driver stages, the 12AT7 is valued for its clarity and headroom. It’s lower gain than the 12AX7 but can be used to clean up an overly aggressive preamp section.
12AU7: With even lower gain than the 12AT7, the 12AU7 is known for its smooth and mellow sound. It’s less common in guitar amps but can be used for specific tonal adjustments.
Power Tubes Recommendations
EL34: Favored for British-style amps, the EL34 tube is celebrated for its smooth highs and rich midrange, making it popular for rock music. Mullard and JJ Electronic make highly regarded EL34 tubes.
6L6: This tube is often associated with American-made amplifiers and is known for its robust lows and crisp highs. It provides a clean, articulate tone with a solid bottom end. Groove Tubes and Tung-Sol are among the top choices.
EL84: These tubes are used in many classic British amplifiers and are known for their clear, chiming highs and tight distortion. JJ Electronic and Mullard produce some of the most sought-after EL84 tubes.
We’ve covered quite a lot in Chapter 1. In the next chapter of the series we will discuss amp speakers and how they work with the tubes to produce different types of tone and overdrive. Stick around and if you liked this article please subscribe to the mailing list on the left!

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