Tube Screamer Clone Mod

[duhvoodooman***]

For you hardware geeks like myself, I thought I'd provide some additional details on the clipping diode setups. To me, this is the most important single feature within an overdrive pedal, because its primary function is to "clip" the input signal, thus distorting its sound. The debates about using the "true TS-808 chip" in a Screamer clone seem pretty funny to me, since the difference between a NOS JRC4558D, current production JRC4558D, or an RC4558P opamp are very subtle, compared to what you get by changing diode types.

As I understand it, diodes are essentially a one-way electrical gate, allowing current to pass through them in only one direction. Diodes apparently require a threshold voltage to start working, and different types "turn on" faster than others. So when the input signal exceeds the threshold voltage, the diode turns on and restricts the current flow, i.e. "clips" the signal. The turn-on time determines how "hard" or "soft" the clipping is, with slower turn-on giving a softer clip.

For these clipping applications, diodes are typically used in pairs, wired together in parallel but opposite current flow orientations. The opposite polarities allow one diode to clip the positive signal (i.e. top half of the sine wave) while the other clips the negative signal (bottom half). Use of such a matched pair of diodes is referred to as "symmetrical" clipping, since both signal polarities are clipped the same way. However, you can use two different types of diodes or place additional diodes in series on one side vs. the other to give "asymmetrical" clipping, where the two signal polarities are clipped differently. This is reputed to give a more natural or tube-like tone. Such arrangements are very popular in modified TS-type pedals offered by many of the "boutique" pedal makers.

The original TS-808 used two 1N914 silicon diodes for a symmetrical clip. Silicon diodes have a fairly high threshold voltage, and turn on quickly, i.e. "hard". However, I opted to use an asymmetrical arrangement of one 1N914 and two 1N4001 diodes for a somewhat smoother character. This was included as an option in the BYOC Overdrive pedal kit I started with on this project. So, my "stock" TS-808 tone isn't 100% absolutely stock, but it's still very close. The difference is pretty subtle.

For the second clipping mode, I chose 1N34A germanium diodes in an asymmetrical 2-and-1 arrangement. Germanium diodes have a lower turn-on voltage than silicon, but also turn on more slowly, giving a softer clip. The resulting tone is significantly smoother than the stock TS-808, and it reminds me a lot of the overdrive tone I get from my Delta Blues tube amp when I push the clean channel volume way up to saturate the power tubes and run the output through an attenuator to knock the volume back down. I assume it relates to the lower voltage threshold, but running through the germanium clipping diodes really reduces the overall pedal output volume significantly, so you have to turn up the Level knob to maintain overall volume. But it's a really nice, restrained, bluesy overdrive!

The last diode combo I used was two 3mm yellow LED's in a straight symmetrical arrangement. Use of LED's for signal clipping is at the heart of the very expensive ($379!!) Landgraff Dynamic Overdrive pedal, which is little more than a TS-808 with switchable silicon diodes and LED's for the clipping section. People rave about this pedal and some other similar LED-based overdrives because of their aggressive "Marshall Plexi" sound. LED's have a higher voltage threshold than silicon, and they turn on "hard". They cut the signal least overall, so produce the highest (loudest) output of these three combos. I have to admit, I like the sound of the LED clipping mode, especially with humbuckers.