Series Effects Loop Explained: How It Works and What Goes In It

A series effects loop sends the whole signal out of the amp after the preamp, through your effects, and back into the power amp. All of your tone passes through the pedals in the loop, with no dry signal mixed in. That lets delay, reverb and modulation sit after the amp's own distortion instead of in front of it, which usually sounds much clearer.

What is an effects loop?

An effects loop is a break in the amp's signal path, usually between the preamp and the power amp. On the back of the amp you will find a send jack, which carries the signal out to your effects, and a return jack, which brings it back in.

Why put effects there? Because order matters. If you plug a delay into the front of an amp and then use the amp's preamp distortion, the repeats are distorted too and quickly turn into mush. In the loop, the delay comes after the distortion, so each repeat stays clean and defined. The same goes for reverb, chorus and other time-based or modulation effects. If you are not sure where your amp's distortion comes from, read preamp vs power amp distortion.

Series vs parallel effects loops

There are two main types of loop:

• Series loop: 100% of the signal goes through the loop. Whatever is in the loop, and whatever it does to the signal, is what reaches the power amp.
• Parallel loop: the amp splits the signal. Part of it goes through the effects, the rest stays dry, and a mix control blends the two.

A series loop is simpler and behaves the way most pedals expect. You set the effect level on the pedal itself, and modulation effects such as phasers work properly because there is no dry signal to cancel them out. The trade-off is that every pedal in a series loop is fully in your signal path, so the quality and levels of your pedals matter more. A parallel loop keeps your dry tone untouched, but some pedals do not work well in it, and you often need to set them to wet signal only.

What goes in the loop and what goes in front?

A simple rule of thumb:

• In front of the amp: tuner, wah, compressor, fuzz, overdrive, distortion and boost pedals. These are designed to work with a guitar signal and to push the preamp.
• In the loop: delay, reverb, chorus, flanger, and often a phaser or tremolo. These sound best after the distortion.

Rules are made to be broken. Some players like a phaser in front for a more vintage sound. If your amp is set clean and all the drive comes from pedals, you may not need a loop at all. For a clean, pedal-based rig, our article on a tube amp head as a pedal platform is a good place to read on.

Loop level, true bypass and transparency

The signal at the send jack is often stronger than the signal from your guitar. Some pedals handle that well, others may clip or sound noisy. A loop level control lets you match the signal to your effects, and to balance the volume with the loop on and off.

Two other features are worth looking for. True bypass switching takes the loop completely out of the signal path when it is off, so your effects cannot affect the tone. A footswitchable loop lets you turn all the loop effects on or off at once from the floor. Our amp footswitch guide explains how that fits into a live rig.

Some amps use a tube to buffer the loop. That can change the tone slightly. A transparent loop aims to pass the signal without adding its own colour.

Effects loops on Kool amps

The Marvell and the Marvell Deluxe have a series effects loop with relay true bypass. There is a level control on the back, so you can boost, attenuate or balance the loop signal, and you switch the loop on and off with the footswitch. Without the footswitch connected, the loop is on.

No Kool amplifier uses tubes in the effects loop. The loop is ultra transparent, so your pedals and rack effects do not colour the original tone. The 1984 and the Ace also have a footswitchable effects loop.

The Blue Sky is the exception: it has no effects loop. It is a single, very clean channel with lots of headroom, built as a pedal platform. Because the amp itself stays clean, delay, reverb and modulation pedals go straight into the front input. It also has full tube-driven spring reverb.

Your next step

If you use the amp's own overdrive and want your delays and reverbs after it, look at the Marvell or the Marvell Deluxe with their switchable series loop. If all your drive comes from pedals and you want a clean base, the Blue Sky keeps things simple. Questions about your pedalboard and which amp suits it? Contact Kool.