18 Watt Plexi

NOTICE: This info is provided for DIYers experienced in amplifier building. Please read this first.

NO WARRANTIES: The information contained on this webpage is provided "AS-IS" and with no express or implied warranties of any type, including to the accuracy or completeness of this document. DISCLAIMER OF LIABILITY: Ramble FX specifically DISCLAIMS LIABILITY FOR INCIDENTAL OR CONSEQUENTIAL DAMAGES and assumes no responsibility or liability for any damage suffered by any person as a result of the use of this information. USE AT YOUR OWN RISK: This amplifier operates at LETHAL VOLTAGES that may be present after power is removed.

This is for reference only and nothing here is presented for sale. 

This is an amplifier design that takes the classic Marshall plexi amp (e.g. 1987x) and makes it a more reasonable 18 watts by using the Marshall 1974x (or Watkins Dominator) EL84 based power amp. It's a modified version of a design by Richard Johnson found on the 18watt.com forum, with a number of tweaks and added features. The result is easily recognizable Marshall plexi tone all the way down to bedroom levels, yet also deceptively loud for 18W.


  • 18W Cathode biased EL84 class AB power amplifier

  • Parallel/Series input valves switch. This amp has two input paths with separate triode gain stages and volumes. One is labeled "Treble" and the other "Normal." A switch configures them in parallel or series. In parallel, the brighter "Treble" and darker "Normal" volumes can be blended together. In series (cascaded), the "Treble" triode feeds into the "Normal" for more gain.

  • Post phase inverter master volume. This is magic in this amp. It uses a dual gang potentiometer to place a master volume right before the output tubes. This allows the phase inverter to create distortion.

  • Variable voltage regulator. This is a form of power scaling that lowers the voltage to the power tubes, causing them to distort at lower volumes. Not great on its own, but together with the master volume it can be useful.

  • Dual rectifier switch. Choose solid state diodes or an EZ81 tube. The Marshall plexi has solid state diodes, the 1974x has a tube.

  • Passive serial effects loop.

18W plexi amplifier
18W plexi amp layout
18W plexi amp schematic.png