Mojo Marshall Style 18 Watt Amp Kit Review
- Sep 25, 2011
- in Guitar Kit Builder - Amplifiers
The increased popularity of smaller amps means more interest in small amp kits, such as the 18 watt amp kit we review here from Mojo. Smaller amps, such as the Marshall 18 watt, can be cranked for great saturated tone without entirely drowning out one’s bandmates or for studio use where higher volume levels aren’t ideal. This amplifier was initially produced from 1966-1968 in three versions: the 1958 model with two 10" speakers, the 1973 with two 12" speakers, and what is often considered the standard model upon which imitations are based, the 1974 with one 12" speaker. The amp offered two channels: the normal channel sported controls for volume and tone and the tremolo channel offered volume and tone as well as speed and intensity controls for the tremolo, which could be switched off with a remote footswitch. This amplifier has become very desirable for its great, warm tone, but given the limited production period of the amp, it is relatively rare and pricey when available. To help alleviate this problem, in recent years many boutique builders have offered their own spin on this veritable classic, and Marshall itself reissued the amp, but with a street price of around $2,300.00, the amp is still out of reach for many players. The DIY community has responded by putting together cooperative websites such 18watt.com where builders share schematics and layout plans for variations of the 18 watt amp. Many 18 watt amp kits are also available from such companies as Ceriatone and Weber. In this review I’ll take a look at the 18 watt amp kit offered by Mojo Musical Supply. One of the nice things about Mojo’s approach to amp kits is that the builder doesn’t have to buy all the components at once in one package. For people on a budget like myself, one can first buy the small parts kit ($145.00), which contains the turret board, resistors, capacitors, and minor hardware such as tube sockets; the chassis and faceplates can then be purchased separately ($106.00); and eventually the output and power transformers can be acquired when the project is nearing completion ($62.50 and $84.00 respectively). The components provided in my small parts kit were all of a high quality. I received Sprague atom caps; Tube Amp Doctor and JJ filter caps; Mojo’s own take on the fabled mustard caps used in old Marshalls; and all carbon comp resistors except a couple of metal oxide ones used on the power supply. I should note, however, that a footswitch for operating the tremolo channel was not included in the small parts kit. The turret board was ready to be populated, which helps spare the builder time in putting the amp together. The drawback of a ready-made board, of course, is that it is difficult to deviate from Mojo’s layout. For instance, I decided to depart from Marshall’s older practice of grounding to the back of the potentiometers, so I had to rearrange some of the components to accommodate a ground buss bar. I had read that the ground buss bar would help with quieting the amp and once I had my 18 watt amp kit up and running, and it was indeed a relatively quiet amp. A schematic and a layout sheet were also provided with the small parts kit. No other instructions were available, so this would not be a kit for someone to undertake who had no previous building experience. By contrast, Tube Depot offers an 18 watt amp kit with step by step directions. Given the sensitivity of the 18 watt to lead dress and component layout, it would be best to begin building a simpler amp, such as a PCB-based tweed Fender Tweed Deluxe like Tube Depot offers (see earlier GKB article on this amp here), before graduating on to a more self-directed kit such as offered by Mojo.
Photo 1 - Mojo 18 Watt Amp Kit Front Panel I had experience building a couple of amps previously, so I had little fear in jumping into the build. I should note, however, that the layout diagram doesn’t represent such details as twisting the heater wires to cut down on hum; in fact, the diagram mentions that the heater wires aren’t depicted at all due to diagram clarity. The diagram also doesn’t show such details as where to ground the leads, nor does it represent where the supplied terminal tags go. Here too I deviated from the plan and opted for star grounds. The diagram does indicate an alternative position for a certain capacitor if oscillation is a problem. So that I would be familiar with how various incarnations of 18 watt kits were dressed, I printed up the layout diagrams from Weber and Ceriatone, both of which proved very helpful and both of which are in color, which is a step above the black and white Mojo sheet. I would suggest the same approach for anyone else who may be building the Mojo kit for the first time.
Photo 2 - Top-Down View Aside from some difficulty following the layout drawing supplied by Mojo, I also ran into a problem with the wire supplied. Mojo includes two colors of solid core wire. A whole book could be written about the advantages and disadvantages, both tonally and in terms of ease of workability, of solid versus stranded wire. I appreciated the fact that Mojo included the wire, which helps move the kit closer to 60s authenticity. The problem is that I ran short on wire about two thirds of the way through the build and since I didn’t have any solid wire on hand, I finished the kit with the plastic insulated stranded wire I had on hand.
Photo 3 - Chassis Underside The 1966 Marshall catalogue in which the 18 watt originally appeared claimed that “these models have a distortion-free volume level.” Nothing could be further from the truth; in fact, the amp is popular precisely because it breaks up so nicely when pushed. Unfortunately, the volume needs to be about half way up before that level of saturation occurs, and for most players who hope to use the amp at home, the sound level is too intense. Once the volume is pushed to around two o’clock, the sag effect that so many players love about a tube rectified amp becomes apparent and the 18 watt’s voice becomes huskier and notes bloom wonderfully. Overall it is a very satisfying amp to play despite its spartan controls. Both rich clean sounds and an aggressive crunch tone can be coaxed from the 18 watt.
Photo 4 - Turret Board If pushing the amp to the point of break up is not a possibility, the player can of course rely on an overdrive or distortion pedal, but a better solution might be to use an attenuator. While the speaker will not react the same way as when it is pushed by a loud amp in this situation, at least the amp itself will get much closer to that cranked sound. In the following video I begin with the amp cranked up and using an attenuator to make the recording possible in my residential neighborhood. I then switched to the tremolo channel and then the normal channel. In the video I'm playing my Gibson SG Standard guitar through the amp and a Marshall 1960A cabinet with stock speakers. The recording was made on a Tascam 8-track deck using a Radio Shack microphone. Give it a listen:
Another solution to get natural overdrive at a lower volume is to install a master volume control in place of one of the two inputs per channel, such as one finds on the kit that Ceriatone offers. The last aspect one will want to consider with the Mojo kit is whether to build the 18 watt into a combo or make it a head. Mojo offers all the parts one would need to build a cabinet (except the lumber), and it also offers reasonably priced pre-built cabinets in a variety of styles. I couldn’t be happier with the Mojo approach to the kit, not only in view of the overall price versus the top shelf quality of the components, but also in the step-by-step way one could, say, buy Mojo’s small parts kit to start the amp, but perhaps use transformers of a different supplier. This way, one is not stuck with the particular components a company may choose to assemble into a kit, especially when items such as transformers prove to be such an important link in the tonal chain. I don’t mean to suggest here, though, that the Mojo transformers are in any way inferior, because I think they sound terrific in the amp and see no reason to change them. Overall, I’d highly recommend the Mojo kit to any hobbyist who has some experience in assembling amps and doesn’t need detailed instructions to be successful in building a great amplifier.
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why the mojo 18w sounds not overdrive?