Marshall 1974x (18W)

8 December 2008

The Marshall 1974x (better known as the Marshall 18W) was an amplifier produced by Marshall from 1965 to 1968. At only 18W and being a combo amp with a single 12” speaker the amp was a change of pace from the loud 50W-100W Marshall stacks of the time.

Though the 18W was never super popular during its production years it became very popular years later. With its small size, lower power output and great classic Marshall tone it became a big hit with players. The 18W has also become popular with guitarists and DIYers wanting to build their own tube amp. With its fairly simple circuitry and great Marshall tone the 18W lends itself nicely to building.

For these reasons I set out to build my own Marshall 1874x (18W) clone. Originally the 18W had two channels both having Volume and Tone controls, additionally one channel had a tremolo effect with Speed and Intensity controls. When many people decide to build 18W now they remove the channel with tremolo and replace it with a master volume equipped Treble, Middle, Bass (TMB) channel. This is what I will be building, a slight variation on the classic 18W design.

I decided to buy the chassis, faceplate, transformers and cabinet from Trinity Amps and then source the rest of the parts myself.

Parts:

  • 1/2W carbon composition resistors for the signal path.
  • SoZo capacitors (mustard cap work a likes)
  • Sprague electrolytic capacitors
  • Alpha potentiometers
  • Cliff jacks
  • Ceramic tube sockets
  • Carling switches
  • PVC wire
  • Trinity Amps designed Heyboer made power and output transformers

Build:

 

Design:

As mentioned before this particular 18W is the TMB version. What this means is the original Tremolo Channel with Tone, Volume, Speed and Intensity controls has been replaced by the TMB Channel which has Gain, Volume, Treble, Middle and Bass controls.

The amp uses three 12AX7s, two EL84 output tubes and an EZ81/6CA4 rectifier tube. The amp has two channels as mentioned, a TMB channel and a Normal channel. The TMB channel is the classic Marshall channel with Treble, Middle, Bass, Volume controls as well as an added Gain control. The Normal channel has Tone and Volume controls and comes straight from the 1974x design. The phase inverter and output section are true to the 1974x design.

The magic of the 18W comes from the phase inverter and the output section. The preamp of the 18W (both the Normal Channel and TMB Channel) are fairly low gain. What this means is most distortion is not coming from the preamp but rather from the preamp pushing the output section into clipping (distortion). So removing the tremolo channel and replacing it with the TMB channel is not a big deal as the preamp is really for sculpting the tone and not for generating distortion. The preamp is more manipulating the original signal and boosting it up so it can be passed on to the output section where the “real” 18W tone comes from.

At 18W of output power, with no negative feedback and being a cathode biased amp one can start to see the 18W is going to have a unique output section. All these factors lead to a fairly low volume design that has great compression and feel with a very musically pleasing distortion tone. These design elements along with the EL84 (Marshall normally did not use EL84s) tubes are what make the 18W tone what it is.

Schematic:

18w-siii-schematic

 

 

 

 

Results:

After finishing the wiring of the amp I put the chassis in its lovely 112 combo cabinet. I decided to use a Tone Tubby San Rafael for the speaker. The Tone Tubby San Rafael is a 25W ceramic speaker with the signature Tone Tubby hemp cone. Though the San Rafael is not a Celestion speaker like an actual Marshall 1974x had it is a high quality low wattage ceramic speaker. I am very pleased with the San Rafael. It is not super expensive and seems to have a balanced frequency response (balanced guitar frequency response) with good clarity and note definition. As the amp and speaker got played more the tone has really opened up and become livelier.

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