Preamp and Power Amp Tubes: What They Are and What They Do

Since their invention way back in the early 1900s, vacuum tubes have seen use in a wide variety of industries and applications, from old CRT televisions to computers to microwave ovens. When amplification was introduced to the music industry, these quickly became the cornerstone of countless genres. The sweet, singing sustain of a tube amp is synonymous with rock and roll, country, and blues, to name a few. Let’s take a basic look at these little devices that give our guitars such a big sound!

An all-tube guitar amplifier like Carvin Audio’s V3, Legacy, X100B, or Vintage Series comes equipped with both preamp and power amp tubes. While these two are different in both construction and function, they work together to both shape and amplify your guitar’s tone.

An all-tube Carvin V3, equipped with five 12AX7 preamp and four EL34 power tubes.

While there are many different types of preamp tubes, the most common type is the classic 12AX7. These are generally much smaller than power amp tubes and serve the main function of shaping the overall character and tonal characteristics of your guitar sound. They take the signal from your guitar’s pickups and pre-amplify it (hence their name!) to be sent to the power amp section. Really overdriving a preamp tube and hitting it hard creates richer harmonics, distortion, and sustain. Backing off the gain can give you a shimmery clean sound. You can think of the preamp tubes as the tone shapers of your guitar amp. From a gain staging perspective, these dictate the basis of the tone you want to send to the final output stage.

Preamp tubes today are mostly of the triode type in guitar amplifiers, meaning there are three parts to the tube. These three parts are the grid, cathode and plate. The grid is the input where the lower level signal controls the electron flow from the cathode to the plate. The output signal is usually taken from the plate for increasing the gain of the input signal. In some buffered effects loops or for buffering a high impedance gain stage or EQ section, the output may be taken from the cathode pin, but this pin does not supply any gain to the input. There are other preamp tube types like pentodes, which have five internal parts adding some screens for more efficient control of the electron flow. These other preamp tube types have mostly faded out of use for guitar amplifiers since the early ‘70s. The pentode preamp style tube more emulates a solid state device’s performance, but the triode’s unique tone character has prevailed as the most common type used in a guitar amplifier’s preamp section.

Power amp tubes are much larger than preamp tubes. These take the preamp signal you have dialed in and amplify it to speaker levels, controllable by your amp’s master volume control. While they don’t shape your tone as directly as preamp tubes, different matched sets of power amp tubes can further shape the sound of your amp and provide different levels of touch responsiveness to your playing.

Power amp tubes in most guitar amplifiers today are typically not of the triode type of tube like the preamp tubes. They are usually pentodes like EL84s and EL34s or what is called a beam tetrode or pentode like 6L6 and KT88. The beam type was designed to get around the patented pentode power tube. The new design had some saying it was a five part device and other saying it had four parts, so you will see Beam Tetrode and Beam Pentode names associated with these tubes. There are triode power tubes and by a circuit change you can wire a pentode tube to a triode mode, but this introduces a large loss in efficiency. Carvin Audio’s Vintage 16 has a triode mode switch, and it goes from 16 watts with two EL84s running in normal pentode mode to 5 watts running in triode mode. The tone and distortion character is very different between the modes.

In the next post, we will cover a few of your various options in power amp tubes and their signature tonal characteristics.

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