Valve Approximator Distortion Pedal

Valve Approximator Distortion Pedal

  • 30 May 2016 | 

  • 15 September 2016 | 

  • guitar, electronics.

My P27 amplifier has a nice sound but its distortion tone is pretty useless for various flavours of heavy rock/metal. I’ve made a few overdrive distortion circuits on my bread board and this is the first one I liked enough to decide to commit to a pedal. The circuit is modelled on the (in?)famous Ibanez Tube Screamer, but with modifications to suit my taste. I also ditched their JFET effects bypass scheme in favour of a triple pole double throw foot switch which also illuminates an LED to tell me when the circuit is engaged (it’s common for pedal manufacturers to use these BOM-heavy schemes because parts are cheap, but labour to wire up 3PDT switches is not). It runs from a 9V battery, but can also be run from a plug-pack. When the plug-pack is inserted, the battery is bypassed. To stop the quiescent current of the circuit draining the battery when not in use, a switching jack socket is used on the input. When the plug is absent, power is disconnected.

Step 1. Throw out the bread board version and re-build the circuit on prototyping board and verify that it actually works:

It's useful to tape the jacks down so the weight of the cables doesn't drag the board off the bench.

It's useful to tape the jacks down so the weight of the cables doesn't drag the board off the bench.

Step2. Buy a cheap die-cast aluminium case, drill some holes in it and paint it:

This was the most expensive part of the whole build at around ten dollars.

This was the most expensive part of the whole build at around ten dollars.

Step 3. Assemble the components in the box:

Wiring the pots to the board was a bit fiddly. The rear of the board was then insulated from the case by hot-gluing a piece of cardboard to it.

Wiring the pots to the board was a bit fiddly. The rear of the board was then insulated from the case by hot-gluing a piece of cardboard to it.

Step4: Profit!

I won't use stick-on labels for a pedal again as they don't stick down well to the orange-peel texture of several coats of spray paint.

I won't use stick-on labels for a pedal again as they don't stick down well to the orange-peel texture of several coats of spray paint.

The top needed a bit of jazzing up. I quite like the lo-fi effect of simply drawing on it with a paint marker.

The top needed a bit of jazzing up. I quite like the lo-fi effect of simply drawing on it with a paint marker.

And there it is, a proper effects pedal for around $30. The most expensive components were the case and the foot switch, which made up the bulk of the cost. The rest of it was jelly-bean parts I already had lying around. It also uses an NE5532 opamp instead of the noisy 4558.

As I said above, it’s basically a knock-off of the Tube Screamer, but with a modified tone circuit and a slightly different feed-back path. I use it to play things like Judas Priest, Black Sabbath and The Offspring. If you turn the distortion right down, you can get a pretty cool ‘dirty blues’ kind of sound out of it as well. Well worth the effort :)