This pedal is a clone of one on my favorite effects pedals from the 80's... ProCo's RAT Distortion. It's a basic OpAmp distortion pedal using the classic LM308N IC chip that is a fairly simple build for those handy with a soldering gun, have a free weekend, and love rock-n-roll.
The layout we are using was designed by Francisco Pena who has done an excellent job with this and other pedal designs on Tonepad.com. (please refrain from editing the circuit board artwork unless you have permission from the author. (sorry Francisco!!))
The schematic and PCB are available in PDF form at TonePad.com following this link , which also has a great library of other effects pedal and guitar related projects to build from beginner to advanced. You will also need the PDF for Off-Board Wiring for wiring the input/output jacks and power for the pedal.
TonePad.com also carries ready-made Printed Circuit Boards to purchase if you don't have the time or effort to make your own. The PCBs on tonepad.com come with a screen printed component side, tinned solder pads and also a solder mask - all for a pretty decent price considering the cost of supplies you have to buy to make your own! I recommend going this direction if you want as close to error free build as possible.
You can buy the majority( if not all) of the parts from Small Bear Electronics which offers pretty reasonable prices and are supporters of TonePad and several other DIY music project sites. (Also recommended!)
I created the graphics in Adobe Illustrator and have made it available below to all that wish to use it. It is a PDF file saved out of Illustrator so it is easily edited with any vector graphic software that can open pdf's..... have fun with it and go wild.
(I'll post some sound clips and maybe some video soon to give you an idea what it can do.)
Now lets get ready to Rat-n-Roll.....
Step 1: Getting the Goods....
You can cross reference the IC's and transistors online at NTE to find the "generic" NTE version, which are usually easier to find locally. Audiophile types frown upon using NTE parts for high-end projects such as amplifier rebuilds do to inconsistencies in part specs but for projects like this I haven't had any problems so far.
Not stated in the schematic but is assumed knowledge is: All resistors are 1/4 watt , all capacitors need to be rated for 12 volts or higher. (a 12V .01uF capacitor and a 50V .01uF are the same except the 50V can take more voltage and will be bigger in size in case you were curious.)
In addition to the parts listed in the PDF file, you will also need:
- Qty. 3 - 100K-A (Audio or Logarithmic (same thing) Potentiometers - 17mm diameter fit better
(careful not to get Linear taper Pots.... big difference!)
- 3PDT stomp switch (Small Bear Elec, or Ebay)
- 9V battery snap and battery Clip,
- LED and LED holder (if you want a status indicator)
-1/4" Stereo input jack
-1/4" Mono input jack
-22 Gauge stranded hookup wire (preferable 3 different colors)
- 9V "wall wort" power supply (center conductor is " - ")
(this can be a salvaged one rated around 500 milliamps or so)
- power supply connector (female, that matches the connector on the power supply)
-Suitable case (preferable metal for strength and shielding.) Check the web.. Hammond cases are the most popular. I found mine at my local electronics part dealer.
*edit* My last trip back to the store I found out the company and the part # of the case...
The company is: LMB Heeger (they have a website also) and the case is # MDC532
Other Supplies & Tools:
Soldering iron, Solder, Drill/Dremel, Drill Bits, Clothes Iron, Plastic Containers, Rubber Gloves, PCB etchant (Ferric Chloride), Multimeter (for testing/troubleshooting), and cold beer.
Step 2: Etching the Circuit Board...
Once you have your negative (reversed) image then you are ready to iron it on to the copper. I alway clean the board with fine steel wool (00 or 000) and then wipe it down with alcohol right before I begin to get the best adhesion of the tone to the copper. (In these pictures I am etching several boards to use in later projects). Place your image face down and with the iron set to nearly the highest (cotton) setting (no steam) lay the iron in the middle for a few seconds to melt the toner. This will tack it to the copper and stop it from sliding around as you begin to iron and apply more pressure. I generally work with the point of the iron moving from the center outwards, turning the board as I go. The process usually takes only 5 minutes of firm ironing until it is ready. With this paper I can see the toner outline coming through when it is almost ready.
We you are satisfied that you have it all melted then allow it to cool for a minute or two and then throw it in a bowl of hot/warm water to let the paper soak a bit. When the paper starts to buckle I start massaging it to gently loosen it up until it starts to peel. The goal now is to remove all the paper from the exposed copper. It isn't important to remove all the paper stuck to the toner traces just as long as there is no paper blocking any holes or bridging anything. I use an old soft toothbrush and under a running faucet to lightly scrub the stubborn paper particles that still remain.
Next I dry the board and under a decent light source I examine the traces for pinholes or scraped off toner spots and use a Sharpie marker to fill in any gaps or mistakes.
Now it is ready to etch... I like to warm my Ferric Chloride by placing my etching chamber in a bath of hot-ass water, this will speed up the copper removal time immensely. (With Ferric Chloride it is WISE to use gloves since it stains almost everything it touches especially human skin.) Drop the board in a wait a few minutes, then agitate the solution by swirling it around and then let it sit a spell more. Open a cold beer and drink while you are waiting. Repeat until all the exposed copper is de-solved, or run out of beer, then remove the board and rise off with cold water.
To remove the toner you can use Acetone or Car Brake cleaner but I just use a green 3M scotch pad and elbow grease. Voila! You now have a circuit board you made all by yourself!!
All that is left is to drill out the tiny holes and you are ready to start soldering. I strongly recommend using a drill press (Dermel has a nice one) for this process, it will save you lots of curse words that are better spent on crummy football games
Step 3: I Love the Smell of Solder in the Morning.......
Next I move to the Diodes and the Capacitors. Current only flows one way through a Diode so watch the direction so you don't install it backwards. Ceramic and Film Capacitors aren't polarized so they can be soldered in any direction but the Electrolytic Caps (round can looking ones) are polarized so make sure that are placed correctly or they could explode. (There is usually a stripe running down the side of the can with a "-" sign that corresponds to the Negative leg of the capacitor. The circuit board layout diagram always has a "+" sign on it where the Positive leg goes.)
Although not on the parts list, I bought an 8-Pin IC Socket for the LM308 so I could swap it out with other Op-Amp chips later if I wanted to do any mods without having to unsolder anything. You can get a set of 2 at the RatShack for .69 cents... part # 276-1995. After the components are soldered then precede to the wires. I bought a three pack of 22 gauge wire at RadioShack that comes in Black, Red, and Green so I could keep the similar paths color coded for a fool proof assembly.
Solder the Pots to the wires next, paying attention to the order (this is where different color wires helps out. I like to twist similar wire bundles together to keep things organized and keep them out of the way. Remember that in the wiring diagram the pots are shown face down so keep that in mind when soldering.
Alrighty, lets lay out the case and stuff in all together!
Step 4: Case Layout and Stuffing the Beast!
The Quickest and Easiest route is to get a 3 pole double throw switch (3PDT) from Ebay (the cheapest) or other guitar part sites on the web. With a 3PDT switch you can wire a LED without the hassle of adding a separate circuit board with the ByPass circuit. We will be using the Offboard Wiring 5 scheme for this project. This setup will allow for an LED, battery power, and for external 9V DC power.
Determine the best spot on your case for the input and output jacks. I like them in the back to keep the guitar cables out of my way when playing and for this case design it provided me with the most internal space. Use a spring loaded punch to mark the center of the holes, this will give you a center and will keep the drill bit from sliding around as you drill. Drill out these holes and also the holes for the 3PDT stomp switch, the 3 Pots, LED, and External power connector. You can build a cardboard mock-up of your case and play around with the placement of the parts before your drill if you are unsure of the fit. I used some thin gauge sheet metal to fabricate a bracket for the PCB mount that connects to the case via the center pot in order to not have to add any more holes in the case.
Now we solder all the wires to the jacks, switches, and power connectors as laid out in the diagram. Double check all the runs to make sure they are correct since it is easy to mix up wires in all this chaos. I temporarily mounted everything in the case to get an idea of the length of wires I will need to solder all the jacks and switches together.
Once the distances where measured and the wires were cut I took everything apart and soldered it together. I left the wires running to the 3PDT switch a bit long as to give myself some room to run them off to the sides in order to have room for the battery clip in between the board and the switch.
I used a thin piece of plastic to separate the battery from the exposed circuit board traces to protect from any short circuits in case they happen to touch each other.The plastic is bent like the circuit board attachment bracket and connects to the case using the holes drilled for the battery clip which is mounted to the bottom of the case.
Step 5: Rock-n-Roll!
Now plug it up to your guitar and amp and let the world hear the wrath of the DEAD RAT!!!!!
Check back later this year when I build a combo delay and chorus pedal in one case!!!
If you were careful during the PCB soldering and double checked carefully that no excess solder has bridged any traces, and all the solder joints are shinny and clean then you can rule out any board issues.
Some people have had bad LM308 chips that they bought from questionable sources (hmmm..... ebay maybe??) so if you have issues and didn't buy from Small Bear then this might be a good place to start troubleshooting. (This is another good reason to use a IC socket so you can easily swap out chips to test.)
If you have any hum, make sure the grounds are connected correctly. Using a metal case allows you to ground to the chassis which will help out a lot. Also you might want to buy shielded input and output jacks or make a metal shield out of thin sheet metal that covers the jacks to protect against any other electro-magnetic interference.