Marshall 1974x (18W) “Plexi”

29 December 2012

I am beginning to put together a Marshall 1974x, better known as the Marshall 18W amp. This all tube powered amp will be built in a head cabinet that can be hooked up to an external speaker cabinet of choice. This 1974x circuit won’t be stock though, I am using the 1974x circuit as a platform to try a few modifications and circuit tweaks/ideas. The end goal of the build is to have a low power Marshall JTM/JMP/Plexi type amp. Lots of big fat clear clean tones and sweet harmonic distortion.

The chassis, eyelet board, power transformer, output transformer, faceplate and head cabinet are coming from the good folks at Trinity Amps. The rest of the parts I am sourcing myself.


  • 1W carbon film resistors for the signal path
  • SoZo capacitors (mustard cap work a likes)
  • NOS BC/Vishay electrolytic capacitors
  • PEC 2W potentiometers
  • Cliff jacks
  • NOS American made noval sockets and Belton octal sockets
  • 22 AWG PTFE Teflon wire, 20 AWG PVC wire and teflon co-axial cable
  • Mountain switches
  • Trinity Amps designed Heyboer made power and output transformer
  • Hammond choke
  • NOS vacuum tubes

Most of the passive components can be seen on the turret board:




The following gallery contains pictures of the build in various stages.


Sound Clips:

Normal Channel: Clean

TMB Channel: Clean

TMB Channel: Dirty

Normal Channel: Dirty




{ 6 comments… read them below or add one }

Rafael January 24, 2013 at 5 h 50 min


i find your site across the trinity forum!
i must say i’m really impressed by your work! puhhh very nice.
i’m build amps aswell, more restauration then builds but i’m looking now for a TC15 or a 18 Plexi.
i’m interessted in your mods, the i would like to have a Marshall with all combination like your build.

can you send my some particulars about the tonggle switches? the boost thing is ok but the jtm45/plexi switch sound marvelous!

thanks in advance and cheers from switzerland!



cmoir February 4, 2013 at 19 h 29 min

Schematic now up!


Chris January 24, 2013 at 9 h 05 min

Hey Raffa,

I am actually in the process of updating this post with the working schematic of the amp as well as soundclips. If you check back soon I should have the schematic up.

By the way the boost switch is kind of part of the JTM45/Plexi switch. The JTM45’s didn’t have the extra boost cap while the Plexi’s did. So when in Plexi mode you should switch the boost switch on to get a truer Plexi sound (you can get Plexi lead or Plexi bass with the boost switch). In summary you can have JTM45, Plexi Lead, Plexi Bass, quasi Plexi (no boost) and quasi JTM45 (with boost).


raffa April 9, 2014 at 4 h 23 min

thanks chris,

for your effort and answers!

i really apreciate that!

cheers raffa


daviedog February 27, 2013 at 0 h 13 min

i gather you started with a trinity ‘Marshall’ and did the aforementiomed mods?

Which one? The 6V6? Could the mods/switches be done afterwards? Prettiest clean s i’ve ever heard..


cmoir February 27, 2013 at 9 h 07 min

I actually started with something similar to what the schematic shows then tweaked from there. I never built the amp quite like a Trinity “Plexi”.

All the mods can be done after building the amp stock (though some are easier to do from the beginging like the 6V6 mod). It’s best to build amps somewhat stock then tweak from there anyways I find, gives you a good baseline to judge things by.


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