How to modify a jcm 900

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How to modify a jcm 900
The plate resistor is a cylindrical circuit component. (circuit board image by Michael Shake from

The JCM 900 is a series of guitar amplifiers manufactured by the Marshall company. It was introduced in the '90s as a successor to the JCM 800. There are various models in the series, with varying amounts of volume and power. The JCM 900 comes as a combo amp or as a head. The combo incorporates the preamp and speaker in one unit while the head can be connected to any speaker cab. You can modify various parts of the amp. Depending on the type of mod, you can improve the tone, increase the distortion or expand the EQ range.

Skill level:

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Things you need

  • Screwdriver
  • Soldering iron
  • Replacement tubes
  • Replacement output transformer
  • Replacement plate resistors

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  1. 1

    Swap the tubes. The tubes play an integral part in the tone and colour of the amplifier's overall sound. Changing the tubes has a distinct effect on how your amp will sound and perform. The Marshall JCM 900 series amps come with the following tube combinations: The 100-watt JCM 900 4100 has four 5881 power tubes and three ECC83 preamp tubes. The 100-watt JCM 900 5400 has two 5881 power tubes and three ECC83 preamp tubes. The 100-watt JCM 900 2100 SL-X has four EL34 power tubes and four ECC83 preamp tubes. The 50-watt JCM 900 2500 SL-X has two EL34 power tubes and four ECC83 preamp tubes. Swapping for a lower gain tube enables you to drive the amp harder without achieving distortion. Swapping for higher gain tubes enables you to achieve more distortion at lower volumes. 'The Tube Store' website has a conversion chart that illustrates the gain factor of various tubes. This is a handy guide when trying to find a tube upgrade that suits your tone. To swap the tube, unplug the amp and disconnect it from the power supply. Unscrew the back and pull the stock tubes out. Don't twist them, lift them upward. Then slot in the replacements and enclose the back.

    How to modify a jcm 900
    Tubes influence the tone and character of the amp. (old vacuum tubes image by Dmitry Rukhlenko from
  2. 2

    Upgrade the output transformer. This is quite an advanced mod. Remove the amp chassis from the head or combo. The output transformer is the largest metal component in the circuit, located between the tubes and the speaker wires. Unscrew it and melt the solder connection with a soldering iron. Install your replacement by screwing it in and resoldering the wire connections. Output transformer upgrades improve the frequency range and dynamic spectrum of the amp. You can get output transformers from amp specialists or electronics stores. Mercury Magnetics make upgrades for guitar amps.

  3. 3

    Adjust the value of the plate resistor. The gain stage of the JCM 900 has a plate resistor, which dictates how much signal passes through to the output stage. Altering the value of this transistor by replacing it with a higher or lower value transistor will change the gain capability of the amp. Remove the chassis so that you can work on the circuit board. Refer to the wiring schematic for your amp to identify the plate resistor. Remove the transistor and install the replacement.

Tips and warnings

  • The plate resistor mod requires a working knowledge of electronics. Don't attempt it if you are inexperienced.
  • Wear latex gloves when handling tubes. Oils from your skin should not get on to the glass surface.
  • Never perform any mods while the amp is plugged in.

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