Around 1965, Marshall introduced a few amps built around an 18 Watt EL84 powered circuit. While these models were intended for practice, the sound was much better than other small amps. Unfortunately, only a few hundred of these combos were manufactured over the next three years. These rare amps were given the model numbers 1958 for the 2X10" combo, 1973 for the 2X12" combo, and 1974 for the 1X12" version.
The features were two channels with Volume and a single Tone on each, along with Tremolo on one channel and an optional Reverb. The 18W amps consisted of 3 12AX7 type tubes and a pair of EL84 power tubes in push-pull configuration. Legend says Jim Marshall bought the circuit from Watkins Electric Music. Watkins sold a very similar amp they called the "Watkins Dominator."
An internet group for cloning these amps has been established at www.18watt.com. On this site, you will find a verified schematic for the 18 Watt combo. After hearing the sound samples there, we decided to give this circuit a try as a distortion stompbox with FETs in place of the tubes.
We opted to recreate the normal channel. The circuit consists of only one preamp stage, a volume and unique tone control setup, the phase splitter and a single power stage. There is no negative feedback, so distortion will be much easier to achieve. You may notice we used 8k2 as the value of R17 on the 18watt.com schematic. The reason for this is to scale the voltages of the associated JFET to the voltages of the amp. We did find that the sound was closer to the amp when we did this, instead of arbitrarily choosing a value. Please feel free to experiment with this resistor on your own.
Although the components designated do not look to be typical Marshall, they do produce results very similar to the soundclips of the amp. Great rock and blues sounds are readily available with this circuit.
Possible Mods:
Use 1M pots for Volume and Tone. It has been documented that some 18W Marshalls were fitted with these instead of the 500k pots specified on the schem.
Add the treble bleed cap to the Volume control to brighten the amp. 220pF is a good starting point, but other values will also work well.
Experiment with different JFETs (use sockets!) in any or all positions.
Try different values for the 4n7 and 10n caps on the Tone control to change the response. Sockets may also be helpful here. The 100k resistor is another point for tweaking.
I can't embed this video unfortunately, but you can view it here
Got to build this one ....Sold my Watkins Clubman recently & miss it. It was the 6 watt version of the Dominator
ReplyDeleteI have a soft spot for ROG designs. Can anyone testify how this one stands next to the Thor, Umble, Azabache or other amp emulation designs by ROG?
ReplyDeleteWouldn't bet my late grandmother on it, but i historically haven't build any ROG circuit that was bad... :)
Delete+m
+1
Deletegonna build this this weekend...
heiko
Bias Q1 and Q3 drains at 4.5v. Bias Q2 drain to 7v
DeleteBuilt this and it is working fine. Bias as per recommendations. I am interested in this pedal because one of my Amps is a 18Watt TMB from Ceriatone. I am now looking foreward to trying out all possible modifications to see what tones are possible. Thanks very much for putting this up.
ReplyDeleteThanks for verifying!
Delete+m
Got a question about this pedal. The master pot/circuit is really noisy when my pedal is used on Valve Amps. I tested on a Marshall & Fender valve amp. This noise does not occur on solid state amps. Is there a solution of how to remove this noise without changing the tone of the pedal. It sounds good on a Fender Tweed Deluxe 5E3 except for that Master pot /circuit noise/hum????
ReplyDeleteFixed the noise and am Very Happy. Unusual problem. Because I was short of 1meg and 8.2k resistors I joined 2 resistors in series to make up the required value. I noticed at the 1meg and one of the 8.2k resistors the ones I made up were sitting high off the vero board in a upside down V shape with the join high off the vero board. Just speculating but I believe a magnetic/electric field might have been formed here interfering with the other components on the vero Board and the Valve Amps were picking this hiss/hum/noise. Anyway fixed now and I am pleased. PS. I wont be joining Resistors like this in future builds.
DeleteLove this. Putting in a request for the Runoffgroove Condor sim to match it
ReplyDeleteThanks IvMark
Built this one. Also getting weird noise, but I didn't do the resistor thing above. Pedal sounds amazing though, thanks for the layout :)
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ReplyDeleteBuilt this one late in May right after building the Plexi Drive. I like them both a lot but decided to give the Eighteen its own enclosure first. Tuned all drains to 4.5V since the effect is just too clean with Q2 @7V. More recently, I dropped the drain voltage of Q1 and Q2 a bit, for a little more distortion/sag. The first half of the gain pot's sweep is really clean though - I'm seriously thinking about using a 1M pot for it. Great circuit.
ReplyDeleteUpdate: Fine-tuned my built these last couple days and I got a major improvement. My pedal was veeery noisy, so I decided to try some shielded wire, after succesfully using it in another build. Didn't work. After messing with the voltages a bit more I came to the conclusion the trimpots were to blame - they just didn't hold a value - as I turned them, the voltages went everywhere and there was this static noise (guess I got a bad batch from Tayda, this happened before). So I removed them and used regular pots to trim the drain voltages - voila, no background white noise! 100K is a waste. I would have used 20k trimpots here. Biased the drains as per ROG specs.
ReplyDelete100KA for the master vol is also a waste, IMO - 25K B or 50K A works better - with a 100K A you get a drastic amount of volume in the first 10% of rotation. 1M B for gain is just right, also. With the 220pf treble bleed cap it's bright enough for me, so, I'm keeping it (use a socket and try with/without the cap and different values). The tones are very comparable to the Plexi Drive now - The Eighteen being slightly bassy-er and more midrange-y that the Plexi in the "non-bass boost" mode and the later being chimey-er or just ever so slightly scooped.