How to Add a Preamp Tube to a Marshall JCM800

Written by simon foden Google
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How to Add a Preamp Tube to a Marshall JCM800
Adding a tube to any Marshall preamp will increase the gain. (l'ampli image by Richard villalon from

Known for its simple control panel, the Marshall JCM 800 is a single-channel, 100-watt all-tube amplifier. There are no on-board effects like reverb or chorus. The JCM 800 has a total of seven tubes: Four EL34 power tubes and three ECC83 preamp tubes. Adding a preamp tube to your existing three modifies the tone and output level of your amplifier. It requires a partial modification of the preamp layout. It's important to note that such modifications, while reversible, will void the manufacturer warranty.

Skill level:

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Things you need

  • Four ECC83 preamp valves
  • Four ninepin tube bases with shields
  • Screwdriver
  • Wiring schematic
  • Drill
  • 10mm drill bit
  • Soldering iron
  • Resistor

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  1. 1

    Turn off your amplifier. If you have been using your amplifier, leave it for about 10 minutes so the tubes can cool down. Moving your amplifier when the tubes are still hot can cause them to shatter.

  2. 2

    Unscrew the back panel. This exposes the tubes and the chassis. Remove the tubes. To connect an extra preamp tube, you need access to the circuit board.

  3. 3

    Remove the chassis. Slide the chassis toward you. It comes out of the cabinet partially. Melt the wire connections with a soldering iron to fully remove the chassis. Melt the solder connections fixing the printed circuit board to the base of the chassis and place the circuit board on your work bench. Consult your Marshall JCM 800 preamp schematic (See Resources) when disconnecting the circuit board wiring.

  4. 4

    Remove the preamp tube base farthest from the power supply. The tube bases are located on the chassis. Use the tube base to draw an outline of the base on the chassis. Using a 10mm drill bit, carefully cut a new hole for the new tube base. The new hole should be as close to the existing three as possible.

  5. 5

    Replace the tube base that you removed and fit in the new tube base. It slots neatly into the new hole. Solder the tube base to the chassis and install the tube shield.

  6. 6

    Consult the schematic to find the value for the circuit board resistor that governs the flow of current to the tube bases. Solder in an identical resistor and wire it to the brass track on the bottom of the circuit board base. Connect the new tube base to the new resistor. Consult the schematic for guidance.

  7. 7

    Reinstall the circuit board and wire it back together. Fit a new set of matched tubes. Don't use the old tubes because the bias will be out. Matched sets are pre-biased.

Tips and warnings

  • This is a complicated modification. Consider purchasing a tube powered effects pedal such as Ibanez Tube Screamer, this adds a tube to your signal chain while mitigating the need for modifying your amp.
  • Consult the schematic carefully when replacing and refitting circuits.
  • If you are unsure at any point, consult a qualified guitar technician.

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