Introduction: "Screaming Ex" Booster Pedal
I wanted a clean guitar booster pedal to give my VOX AC15 tube amp a bit more "oomph" and make it scream and rage. Hence the "Screaming eX" booster was born.
It's a non-distorting (hence "clean") two-transistor circuit, delivering a gain of (on full throttle) about 24 with the current choice of components. The non-distorting part is important, because that means the character comes from the overdriven amp and not the pedal.
This being my first go at a "custom" guitar pedal, I'm sure I made lots of beginners' errors. Still, it came out pretty OK.
Step 1: Build the Circuit
The booster is a "classic" two-transistor amp in which the first transistor Q1 is a JFET for high input impedance. Q2 is a general purpose small signal PNP. Think 2N3904, etc. Nothing is really too critical about this circuit.
But Kajnjaps, I here you say, can't we just use an opamp for such a menial amplification task? Yes, I would reply, but where would be the fun in that?
The output (collector of Q2) is fed back(R5) to the source of the JFET to stabilize the DC operating point. Note R2 and R4. These make the gate voltage of Q1 dependent on the operating voltage. In this way, your operating point at the collector of Q2 shifts a little with the supply. If you use another type of JFET and find the collector of Q2 not to be approximately midway the supply voltage, then adjust R2 or R4. Always nice to work properly for a wide supply range (9V...18V).
C6, R9 and R10 together with R5 determine the AC amplification. Given the signal voltage (that's AC) at the source of Q1 almost equals the gate voltage, which is the input, the amplification is given approximately by: vo/vi = (1 + R5 / (R9+R10) ) = 2...31. It turns out a bit less in practice. C5 strips the DC from the output and R8 is a bleeder to avoid pops when plugging in. C4 is there to limit the amplification for high frequencies
The circuit consumes a meager 5...10 mA, depending on the supply voltage. The connections for the 3pole-double throw (3PDT) stomp switch and LED are shown in step 5.
The circuit is soldered on a piece of breadboard. I won't give a layout for the components as a) it depends on the form factor of the components you use and b) it's a fun challenge to find a layout which results in the least amount of space taken. Just don't go much larger than what is shown, or it won't fit in the 1590A enclosure.
Hint: be smart and shorten pot R10's axis to size before you start soldering.
Step 2: Design the Front
The front was designed for the Hammond 1590A enclosure in Inkscape and is based on a "rage comic" character I found floating around the Internet. The svg file is attached so you can adapt it as you see fit.
Normally, people use water decal paper to make nice pedal fronts, but I had trouble finding this. So I resorted to these transparent self-gluing ink-jet labels.
Set your printer to a gazillion-pixels-per-inch-super-photo-quality and print a few tests before using the labels.
Step 3: Drill the Enclosure
Print out the decal on regular paper and stick it to the enclosure with tape. Then mark the holes with a pointy object (nail, punch etc..). You need holes for the pot (6mm), LED (3mm), jacks (10mm), 3PDT stomp switch (12 mm) and power supply jack(12 mm, kinda depends on the brand).
The power plug and the pot will be "competing" for space, so drill the power plug hole close to the bottom of the box (near where the bottom plate will be). This creates more room for the pot.
Note I ended up placing the LED on the left instead of on the right as in the picture. No biggie
After drilling, do a "dry fit" of the components. It's a tight fit in these 1590A boxes, but that doesn't matter once it's closed and no one has to see the wiry mess.
Step 4: Spray Paint the Enclosure and Apply the Front Sticker
No pictures here, because I simply forgot to take em, but here's how it goes:
- Sand box lightly (no. 500) to remove oxides etc.
- Degrease and de-dust using rubbing alcohol.
- Place enclosure and bottom seal on large piece of cardboard.
- Spray paint a thin layer from a distance of 30cm...40cm, preferably outside.
- Let dry.
- Really, go find something else to do and let dry.
- Repeat five more times from 4 onwards. Light sanding in between for first three layers, no sanding for last layers to get a nice finish.
- Cut out printed sticker (round the corners) and apply it to the enclosure. Avoid air bubbles underneath.
- Cut the wholes in the front sticker with a sharp knife.
- Spray paint with clear varnish about 5 times. This protects the front and gives the front that nice look.
No sanding in between. Remember: thin layers.
Step 5: Assemble the Pedal
You're almost finished!
Assemble the board, jacks, power supply and switch. Solder the connections as in the diagram. You might want to measure out the 3PDT switch beforehand as I'm not sure all brands have the same internal connections.
The LED and 2k2 resistor are soldered together and the LED is secured to the enclosure with some hot glue or epoxy. to avoid short-circuits, I placed some electrical tape in the top-right corner,
Stick some anti-gliding rubber "feet" on the bottom lid. Before closing the lid, test the pedal.
Also, before tightening the parts, put some electrical tape on the beaks of your pliers, to avoid damaging the finish of the enclosure.