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Guitar Amps / Effects

DSL 100 Clean/Crunch Footswitch update below!

Whilst playing around with tube amps like the Marshall JCM2000 DSL 100 shown here, I came across a number of issues/gripes and wishes people have with amplifiers. No matter how good an amp is, commercial and design decisions often lead to compromises in the final product. I believe the Marshall DSL 50/100 series, whilst generally being great amps, suffer from some limitations.

Modern high volume production amplifiers make heavy use of printed circuit boards. These dramatically speed up the build process and produce consistant results (assuming the components are kept the same between batches). Older amplifiers made use of hard wired circuits. Whilst they often look cumbersome, wire links were only generally as long as they needed to be and wires could cross each other as many times as was necessary. PCBs are often only printed single sided, this means that components are placed to ease circuit layout rather than in their optimum position. This can also lead to signal paths that are again less than optimum to provide a design that can be built easily. Such compromises cause the amplifiers to sound "different" and deviate from the vintage sounds people strive for. One problem introduced by the modern circuit layout is the FX loop problem descibed here.

Low Volume Marshall DSL FX Loop Problem and Modfication

Many people complain about the effects loop (fx loop) on these amplifiers. Whilst some of the problems I believe are caused by losses in the pedals/fx units themselves I did find the amp sounded horrible with the loop on at low volumes - the sort of levels you might use at home to practice. Try this:

Use a short piece of cable to loop the FX send and return jacks - this enables the FX loop but returns exactly what was sent from the loop. Select the red channel and turn the volume all the way to zero. Play your guitar - notice the trebly sound coming through even at zero volume? As you turn up the volume you bring in your normal amp sound but the tinny sound remains. I found that I didnt want to use the FX loop because of this. If you turn the amp up, you can hardly notice the effect and it sounds much better - but I just couldnt get on with the low volume sound at all.

Eventually I found the cause and the fix to this problem, click here for an in depth explanation and cure.

Marshall DSL 50 / 100 FX Loop Investigation

Whilst the modification above improves the transpareny at lower volumes, people say that the FX loop also affects the tone as the volume is turned up. To understand the problem some tests have been performed to compare the frequency response of the entire amplifier under different loop conditions. This is a work in progress investigation however results will be updated as they are available.

Footswitchable clean/crunch - Update (30/8/09)- Prototype Successful

The DSL (Dual Super Lead) amplifier boasts 2 main channels classic gain (green) and ultra gain (red). These channels can be selected by either the push button switch or a footswitch. Each channel then has two modes of operation, the green channel has Clean/Crunch modes whilst the ultra gain has Lead 1/2 modes.

The clean/crunch channel gives some great sounds similar to more vintage Marshalls, however there is no easy way to switch between them other than using the button on the amp. More problematic however is the massive difference in volume between the two channels, this means that the clean/crunch selection is not easily made in a live situation.

This mod aims to give allow the user to change between clean/crunch modes using a footswitch, either mounted as an additional jack input on the rear (requiring drilling the back of the amp) or replacing the seldom used reverb switch input. More importantly, it allows the user to adjust the levels between the clean/crunch channels to better match them. This can be done by pre-selecting the attenuation of the crunch channel whilst installing the mod (this works best if you dont adjust the gain setting dramatically) or for the more adventurous, re-assigning one of the reverb level pots as the crunch volume giving complete control of both channels. No tracks need to be cut on the PCB making this mod 100% reversable should it need to be returned to stock. The amount of additional wires and solder joints has been kept to a minimum (4 wires need to be soldered to pick up relevent signals for the standard preset option), so that it is almost a drop in replacement for the clean/crunch switch. The majority of required signals are pickup up using supplied patch cables.

The prototype is currently installed into my DSL100 and working well. The PCBs are under development to make this mod as easy to fit as possible whilst mainting a level of flexability. I would anticipate the final selling price to be £40 and installable by anyone competant in valve amplifier repair/modification. Pictures will be uploaded once the PCBs are complete. If you have any queries in the mean time please email me contact me via email.