It’s safe to say that 99% of the time distortion is an unwanted part of our audio signals and could be classed as noise that degrades our product. Distortion can originate from broken or faulty equipment (mics, cables, mixing desks, monitors etc.) and can be really frustrating if you can’t find its origin so learning to identify and isolate distortion is vital. Sometimes however, distortion can be harnessed and used as an effect that can enhance our sounds, and in genres like Metal, distortion is king!


When describing the sonic characteristics of distortion, certain terms will inevitably keep showing up and when speaking to people who utilise distortion as part of their tone, namely guitarists, it sometimes feels as if you need a dictionary to understand what the heck they’re saying. In this extended post I will explain the different forms of useable distortion for electric guitar and take a look at how Vacuum Tube-driven amps can alter the distorted signal, giving the trademark U.K (triode) or U.S (pentode) tones.


Vacuum Tube Amplifiers

If you have had the pleasure of playing through an amp that uses vacuum tubes then you would have no doubt experienced the rich, creamy sound that they are renowned for. A few of you may have noticed in the amps user manual that it may be set up for operation using a triode or pentode configuration and thought what the ?!*#. Triodes and pentodes simply mean the number of parts operating within a vacuum tube, so triode means three parts and pentode means five parts. How does this affect distortion? I will answer that question soon (promise!) but for now we need to understand whats happening inside our vacuum tubes.


Elements of Vacuum Tubes

1. Cathode

As a cathode is heated (the heater is not part of the tube-element count) negative electrons are released that have almost no mass. The negative electrons travel through the vacuum tube at a speed that is close to the speed of light towards a positively charged plate.

2. Plate

The plate is the visible piece of metal within a vacuum tube and is positively charged. With nothing to interfere with the flow of electrons they move uninhibited towards the plate. Sometimes the electrons need to be regulated so as to not arrive at the plate at the same time, for this we use a control grid.

3. Control Grid

The control grid is a loosely wound coil of wire that sits between the cathode and the plate. It supplies a voltage that is slightly more negative than the passing electrons. This helps to neutralise the attraction of the plate and repels the electrons back toward the cathode (known as Bias). Subtle changes in the control grids voltage will have an enormous effect on how much current flows and Randall Smith from Mesa-Boogie sums up the process clearly;

That’s what amplification is, a small change in voltage at the grid causing a large change in current flowing to the plate (Smith, 2011)

The three elements we have seen so far are all elements we would find in a triode. Now we need two more elements to make up our pentode so let’s have a look at what they are.

4. Screen Grid

When using larger power tubes the distance to the plate can be too far to attract enough electrons past the negative influence of the control grid. To attract these electrons there is a fine coil of positively charged wire inserted in-between the control grid and the plate. This lures the electrons past the control grid.

5. Pair of beam-confining shields

These are negatively charged and direct the flow of electrons right towards the plate.

So with this information we now know what makes up the triode and pentode but how does this relate to distortion? How has the triode been labeled as a vintage, U.K-sound and how did the pentode earn its title of having a modern, U.S-sound?


Sonic characteristics of Triodes and Pentodes


The make up of a vacuum tube affects the tone that is produced and this can be exploited to achieve incredible results. The sound from a triode is commonly referred to as ‘British’ or ‘Vintage’ and have some common sonic characteristics that can be described as;

  • Smooth
  • Round
  • Creamy
  • Voice-like

The triode also excels at thick, high gain chord progressions, single note soloing and has a loose, organic sound with rich harmonic content, making it perfect for blues.


Pentodes offer more head-room than their triode counterparts and the sound a pentode produces is known to guitarists as ‘U.S’ or ‘Modern’. Some of the commonly used terms to describe the pentodes sound are;

  • Aggressive
  • Bouncy
  • Dynamic
  • Tight
  • Percussive

The U.S sound (pentode) is also known for its stunning attack and will have amazing definition, even at ridiculous gain settings. It has a tight low-end response has become popular with the heavy metal community.


Gain Structure 

Another factor that ties in with pentodes, triodes and distortion is the level of gain applied. If we divide the gain into sections of low, medium and high then different sounds can be produced. Here is the typical behaviour of what happens when we adjust a gain pot on our amplifiers;

Low gain

  • Clean and least saturated
  • Brighter
  • More upper harmonics
  • Has a three-dimensional sound

Medium Gain

  • Enhanced saturation
  • Reduced upper harmonics
  • Richer, warmer quality
  • Fuller bottom-end response
  • Expressive attack with ample sustain
  • Excellent for solos!

High Gain

  • Heavily saturated
  • Enhanced low and low-mid frequencies
  • Compressed and softened attack
  • Maximum sustain


Putting it all together

We have seen how triodes and pentodes work and have also looked at different gain structures and how they affect our sound, but how do we put it all together? This is the fun bit…play with your amp! If your able to get hold of a tube amplifier then test out different gain settings with the different tube settings (pentode, triode) and be amazed at how almost any distorted guitar sound is possible!

Here are some listening examples of different gain settings using a pentode and triode configuration. The amp used was a Mesa-Boogie 25w Mini Rectifier which is multi-watt. Multi-watt amps are growing in popularity and this one in particular changes from a triode setting when operating at 10-watts to a pentode when operating at 25-watts. There are two channels and they can be set to whatever wattage and gain level you choose…I LOVE MESA-BOOGIE! XO

Triode with low to medium gain.

Pentode with high gain.



  1. I need to get a pentode tube man, that sound is so clear even though its heavily distorted, I can’t get my guitar to be that clear when distorted that hard, when I go too far with the level on my boss distortion pedal the chords just blur into an orange stain.


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