How to Do a Depth Mod on Marshall Amplifiers

Written by dave n.
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How to Do a Depth Mod on Marshall Amplifiers
Remove the RCA cables of an amplifier that has a analogue reverb unit to avoid damage to the cables, reverb unit and chassis when removing the chassis from the enclosure. (phono plugs 1 image by PeteG from

Depth modifications, or "depth mods," often are sought after by amplifier owners who seek a tone that is tighter or more aggressive. Marshall amplifier owners can complete a depth mod on almost every Marshall amplifier on the market. Completing a depth mod produces an amplifier that is unique and can deliver new tones that previously were not achievable.

Skill level:

Things you need

  • Philips screwdriver
  • Wire clippers
  • 24AWG wire
  • New 50k Ohm potentiometer
  • .0047uF capacitor
  • Soldering iron (25W +)
  • Solder (60/40 blend)

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  1. 1

    Unplug the speaker connector cable from the back of the amplifier and any reverb and power connectors, such as RCA or universal connectors, from their respective sockets. Unplug tubes from their respective sockets in the amplifier if the amp is tube-powered. Remove the chassis of the amplifier from the enclosure with the screwdriver.

  2. 2

    Locate the output jack inside of the chassis on the circuit board. Disconnect the live lead from the output jack with wire clippers or a soldering iron. Remove the feedback resistor connected to the output jack lead. The feedback resistor is near the output jack and can be found by following the connecting wires or traces on the circuit board.

  3. 3

    Strip the conduit of the two pieces of 24AWG wire 1/8 inch. The wires must be long enough to reach from the output jack to the feedback resistor.

  4. 4

    Mechanically connect one of the ends of the wire to the live lead of the output jack. Mechanically connect the other available end of the wire to one of the outer legs of the new potentiometer. Mechanically connect the middle leg of the potentiometer to the outer leg that is also mechanically connected to the new wire from the output jack. Mechanically connect the .0047 capacitor to the open outer lead of the potentiometer and the middle lead of the potentiometer as well. Mechanically connect the second wire to the lead where the feedback resistor was connected and the outer lead of the potentiometer connected to the new capacitor.

  5. 5

    Solder all of the mechanical connections with a soldering iron and solder. Heat the connection with the soldering iron tip, and then apply solder. Remove the soldering iron as soon as solder begins to flow.

  6. 6

    Turn the knob of the potentiometer to a desired resistance. Replace and secure the chassis in the enclosure with a screwdriver.

Tips and warnings

  • Switch the leads from the feedback wire and output wire on the outer legs of the potentiometer if the new potentiometer is operating in reverse.
  • Be sure that the amplifier is completely disconnected from any power source prior to working in the chassis. Amplifiers can carrying lethal levels of voltage even when unplugged and turned off, especially tube amplifiers.
  • Properly discharge the filter capacitors of the amplifier prior to working in the amplifier.
  • Consult a professional technician if unsure of capability.

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