How to Put an Effects Loop on a Marshall JCM 800

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How to Put an Effects Loop on a Marshall JCM 800
The Darkness perform in front of a wall of JCM 800 amps (Jeff Fusco/Getty Images Entertainment/Getty Images)

The Marshall JCM 800 is 100-watt tube-powered guitar amplifier. It has a distinctively heavy, "rocky" tone. Dan Hawkins, lead guitarist with British classic rockers The Darkness, is one notable JCM 800 user. An effects loop is a circuit that enables you to route your effects so they bypass the preamp. Rather than plugging into the amp via your pedals, you plug in directly and connect the pedals to loop circuit on the back of the amp. This way, your pedals affect the amplified sound, rather than affected sound being amplified after. This approach typically preserves more of your guitar's natural tone and reduces the hum associated with using multiple pedals.

Skill level:
Moderately Easy

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Things you need

  • 3 1/4-inch jack instrument cables
  • 1 patch cable per effects pedal

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Instructions

  1. 1

    Arrange your pedals in the preferred order. The order is a matter of preference as it influences the tone. For example, the sound of a phased distortion is very different from a distorted phase. Connect each pedal in sequence with a patch cable, right to left.Connect a standard instrument cable to the output jack on your guitar. Connect the other end of the cable to either the "Hi" or "Low" input socket on the amplifier. The "Hi" socket has a higher output level and is suitable if you're seeking a thicker, heavier tone.

  2. 2

    Press the red power button down to put the amplifier into standby mode. Standby mode delivers an attenuated current to the tubes, of which there are four in the power section and three in the preamp. By delivering an attenuated current, the tubes are semi-warm when you turn the amp on. This approach is preferable than delivering the full current to a set of cold tubes.

    How to Put an Effects Loop on a Marshall JCM 800
    The JCM 800 has three ECC83 preamp tubes and four EL34 power tubes. (Jupiterimages/liquidlibrary/Getty Images)
  3. 3

    Connect a standard instrument cable to the "Send" jack on the rear of the amplifier. Connect the other end of this cable to the "Input" jack on the first effect pedal in your chain.

  4. 4

    Connect a third instrument cable to the "Return" jack on the rear of the amplifier. Connect the other end of this cable to the "Output" jack on the final pedal in the chain. By plugging cables into both effects loop jacks, you engage the effects loop circuit. This sends the signal post preamp, through the pedals and back to the power amp before being routed to the speaker. The preamp amplifies and colours the guitar signal before the effects pedals do anything.

  5. 5

    Turn the amplifier on by flicking the standby switch down.

Tips and warnings

  • Tweak the "Level" controls on each pedal so there is no jump in volume when you turn them on. Typically this requires each pedal to have its "Level" dial in the centre, but some pedals are more powerful than others and my need to be turned down slightly.

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