What Is Plexi Tone? The Sound Behind Classic British Rock

Plexi tone is the sound of the classic British 50- and 100-watt heads of the late 1960s: bright, open and mid-forward, with a crunch that comes from turning a simple, non-master-volume circuit all the way up. It cleans up when you roll back your guitar's volume knob and thickens into sustain when you dig in. Today "Plexi" is shorthand for that circuit and that feel, not for one specific amp.

Where does the name "Plexi" come from?

The name comes from the control panel. Those late-sixties heads had a gold front panel made of acrylic, better known as plexiglass. When the panel later changed to metal, players started calling the earlier amps "Plexis", and the nickname stuck to the sound as much as to the look.

What made those amps special was not the panel, of course, but the circuit behind it. It was a fairly simple design: a couple of preamp gain stages, a tone stack, a phase inverter and a pair (or quartet) of power tubes, usually EL34s. There was no master volume. The only way to get the amp to distort was to turn it up, which meant very loud.

What does Plexi tone sound like?

A good Plexi sound has a few clear traits:

• Strong upper mids. Chords and single notes cut through a band without needing a lot of treble.
• An open, uncompressed feel. Even when it crunches, you can hear the individual strings in a chord.
• Touch sensitivity. Pick softly and it cleans up, pick hard and it growls. Rolling back the guitar volume takes you from rhythm crunch to almost clean.
• Moderate gain. By modern standards a stock Plexi is not a high-gain amp. Its classic sound is crunch and classic rock overdrive, not saturated metal.

Much of that character comes from the power section working hard. When the EL34s are pushed, they add compression, harmonics and a slightly loose, "bloomy" low end. If you want to understand that difference better, read our article on preamp vs power amp distortion.

What is inside a Plexi-style circuit?

Plexi-style amps share a recognisable layout. The guitar signal goes into 12AX7 preamp stages, which add gain. A cathode follower then drives the tone stack (bass, middle, treble), and a phase inverter feeds the power tubes. A presence control adjusts the high end in the power section.

The original amps often had two channels with two inputs each: a bright one and a darker one. Players found that linking the channels with a short cable, so both were used at once, gave a fuller sound with more gain. That "jumpering" trick is part of the Plexi story.

The power tubes matter too. The EL34 is closely tied to the British crunch sound, with its forward mids and early breakup. Our guide on EL34 vs 6L6 vs 6V6 explains how it compares with other power tubes.

How do you get Plexi tone at a sensible volume?

The classic problem with a Plexi is volume. Because there is no master volume, the amp only really sings when it is loud enough for a large stage. Over the years, players and builders found several ways around that:

• A master volume. Adding a master control after the preamp lets you turn up the gain while keeping the overall level down. The trade-off is that more of the distortion comes from the preamp rather than the power tubes. See master volume vs non master volume for the details.
• A boost pedal in front. A clean boost pushes the preamp harder, which gives more gain and sustain from a Plexi-style channel.
• Power reduction. Lowering the output of the power section keeps more of the power-tube feel at lower volumes.
• "Hot-rodded" circuits. Many modern Plexi-style designs add an extra gain stage so you can reach heavier overdrive without relying on volume alone.

Plexi-style tone in Kool amplifiers

Several Kool heads are built around Plexi-style circuits, each taking the idea in a different direction.

The Marvell has two independent channels. Channel 1 is '68 Plexi Clean, a super-clean take on the original '68 Plexi with a big, open sound that works well as a platform for pedals. Channel 2 is Modern Plexi Overdrive: a '68 Plexi-based circuit with a tube-driven preamp booster, running from subtle '70s crunch to contemporary overdrive with a tight low end. Each channel has its own Gain and Master controls, and the footswitchable Loud & Louder attenuators set the overall output between 20 and 50 Watts R.M.S. Combined with the channel masters, that makes lower volumes possible.

The 1984 draws on the late '70s Plexi sound and feel. Its clean channel goes from sparkling clean to an organic edge of breakup, while its Plexi Overdrive channel moves from '60s-style crunch into compressed '80s-style crunch and overdrive. It has independent master volumes on both channels and comes in 18 Watt and 50 Watt versions.

If you want a separate crunch sound and a very high-gain channel as well, the Marvell Deluxe adds a third channel to the Plexi clean and crunch voices.

Your next step

If classic Plexi crunch is the sound you are after, start with the 1984. If you want a super-clean Plexi channel plus modern overdrive in one head, look at the Marvell. Not sure which one fits your playing? Contact Kool and describe your guitar, your pedals and the volume you usually play at.