Preamp vs Power Amp Distortion: What's the Difference?

Preamp distortion comes from overdriving the small 12AX7 stages at the front of the amp; it sounds tighter, more compressed and more saturated, and you can get it at any volume. Power amp distortion comes from pushing the output tubes hard; it sounds bigger, looser and more dynamic, but only happens when the amp is loud. Most great amp tones use some of both.

Where does each type of distortion happen?

A tube amp has two main sections. The preamp takes the weak signal from your guitar, adds gain and shapes the tone. The power amp, with its power tubes and output transformer, turns that signal into enough power to move a speaker.

Both sections can distort. In the preamp, a gain stage clips when it receives more signal than it can handle cleanly. Stack several of these stages and you get more and more gain. In the power section, distortion starts when the power tubes are driven to the limit of what they can deliver. The phase inverter, which drives the power tubes, often starts to break up around the same point and adds to the sound.

If you want to know more about the tubes that do the work in the preamp, read what does a 12AX7 do.

What does preamp distortion sound and feel like?

• Tight and focused. The low end stays controlled, which helps with fast riffs and palm muting.
• Compressed and saturated. Notes sustain easily, and high gain levels are possible.
• Independent of volume. With a master volume, you can get the same amount of preamp gain at a whisper or on stage.
• Shaped by the tone controls. In many amps the tone stack sits after some of the gain stages, so EQ changes the character of the distortion as well as the level of each frequency.

The downside is that a lot of preamp gain can feel less responsive to your picking. Too much of it can also sound fizzy or flat, especially at low volume.

What does power amp distortion sound and feel like?

• Big and open. Chords stay full, and the sound seems to grow as you hit the strings harder.
• Dynamic. Pick lightly and it cleans up; dig in and it pushes into crunch. Rolling back your guitar volume works very well.
• Some sag and compression. When the power section is working hard, the amp can respond with a slight softness or "bloom" on each note.
• Coloured by the power tubes. EL34s, 6L6s and 6V6s each break up differently. Our guide on EL34 vs 6L6 vs 6V6 covers the differences.

The catch is volume. A 50-watt amp needs to be very loud before the power section really distorts. That is the reason many players look for lower-wattage amps or ways to reduce the output of the power section.

How do you mix preamp and power amp distortion?

On an amp with a master volume, the gain control mostly sets the preamp distortion and the master decides how hard the power section works. A few practical approaches:

• Mostly preamp: gain high, master low. Good for high-gain sounds at a controlled level.
• Mostly power amp: gain moderate, master high. This gives a classic, open crunch, but it gets loud.
• A blend: set the gain for the core of your sound and bring the master up as far as the room allows. Even a little power-section drive adds depth.

A boost or overdrive pedal in front of the amp pushes the preamp harder, adding preamp distortion. Time-based effects such as delay and reverb usually sound clearer after the distortion, which is what an effects loop is for. See series effects loop explained for how that works.

How Kool amps give you both

Kool amps let you set the preamp gain and the power-section level separately, so you can choose the balance yourself.

On the Marvell, channel 2 is a '68 Plexi-based circuit with a tube-driven preamp booster. That extra preamp gain takes it from subtle '70s crunch to contemporary overdrive with a tight low end, without needing an overdrive pedal. Both channels have their own Gain and Master controls, and the footswitchable Loud & Louder attenuators set the amp's overall power between 20 and 50 Watts R.M.S. In practice, that means you can let the power section work harder at a lower level than a fixed 50-watt amp would allow.

The Marvell Deluxe spreads the range over three channels: a Plexi clean and mild crunch channel, a crunch and overdrive channel, and a high-gain channel. It also has the Loud & Louder attenuators for 20 to 50 Watts R.M.S.

The Ace takes a simpler approach with one channel. Gain and Master work together to move from sparkling cleans to a sweet, organic edge of breakup and then subtle crunch. Its SVC volume control in the power section goes from 0.1 Watt bedroom levels to the full 18 or 50 Watts, so you can keep the tone and sustain at lower volumes without a power soak.

Your next step

If you want high gain from the amp itself plus control over the power section, start with the Marvell or, for three channels, the Marvell Deluxe. If you prefer a dynamic, touch-sensitive amp that you drive with your hands and a boost pedal, look at the Ace. Not sure? Contact Kool and describe the sound and volume you need.