Mod 2: Alter TS9 and SD-1 Distortion by Swapping Diodes

You can get different shades of distortion by swapping clipping diodes in your Tube Screamer or Super Overdrive. For example, replacing the existing diodes with germanium diodes will yield a compressed, smooth fuzz sound. In contrast, silicon diodes (1n4148, 1n4001, 1n914, etc.) tend to provide a crisper, tighter, more focused sound. LEDs sound warmer, offer a great crunch, and usually make the pedal sound louder.

You can also experiment with different diode configurations. Two types of clipping can be achieved through different configurations: symmetrical and asymmetrical. Asymmetrical clipping—the type of clipping achieved in a stock Boss SD-1 circuit (see Fig. 2)—tends to yield a more dynamic and responsive overdrive resembling the feel and response of an amp overdrive. You can get asymmetrical clipping by putting two series-wired diodes in parallel with one diode oriented in the opposite direction (as shown in the mod instructions).

Fig. 2: Asymmetrical Clipping. A stock Boss SD-1 schematic (left), and an SD-1 schematic with an LED swapped out in place of the original clipping diode to yield a louder, warmer, more responsive feel (right).

To get more headroom out of a symmetrical clipping circuit—the type of clipping achieved in an Ibanez TS9 circuit (see Fig. 3)—you can add an extra set of diodes in series with the original diodes, or you can change both diodes out for LEDs (as shown in the diagrams). However, keep in mind that this will change how much clipping you hear.

Fig. 3: Symmetrical Clipping. A stock Ibanez TS9 schematic with two silicon diodes (left), a TS9 schematic modded with two sets of series-wired diodes running parallel to each other to achieve more volume and headroom with slightly less clipping (middle), and a TS9 schematic with two LEDs running in parallel instead of the original silicon diodes, which yields more headroom and volume, with a warmer response.

When replacing diodes, make sure you orient them correctly. The stripe on the diode always goes on the same side as the bar at the tip of the triangle on the diode symbol that’s stenciled on the circuit board. For LEDs, the short leg goes towards the bar.

Now that you know more than you probably ever wanted to know about diode configurations, we’ll show you how to do some diode mods on a TS9 and an SD-1.

TS9 Asymmetrical Clipping Mod
Let’s start by changing a Tube Screamer’s clipping from stock symmetrical to asymmetrical by adding a diode pair in series.

Photo 6 (left): Diodes 1 and 2 on a TS9 circuit board. Photo 7 (top middle): Wire two diodes in series by making sure their black stripes are oriented in the same direction and then wrapping the middle leads together. Photo 8 (bottom middle): Solder the diode legs together and bend the outer legs for easy installation. Photo 9. (Right) Solder the series-wired diodes’ legs back into the holes vacated in step 2. Note the black heat-shrink wrap protecting the new solder connection.

1. Locate the diodes on your TS9’s circuit board. See Photo 6.

2. Desolder diode 1 (D1) or diode 2 (D2)—it doesn’t matter which comes first. Note: I recommend using a felt-tip marker to mark which components you need to desolder on the underside of the circuit board.

3. Wire two diodes in series—either pair one stock diode with a new one or pair two brand-new diodes—by twisting their legs together as shown in Photo 7. Note: See how the black stripes are both on the left hand side of each diode? This is very important to get right—your pedal won’t work unless they are oriented correctly.

4. Solder the twisted-together legs as shown in Photo 8, and then place heat-shrink wrap or electrical tape on the exposed solder joint (not shown), and bend the legs as shown.

5. Place the series-wired diodes’ legs back through the D1 or D2 holes (depending on which you removed in step 2) and solder them in place. See Photo 9. Note: Make sure the diodes’ black stripes are on the same side as the bar on the tip of the triangle marked on the board.

Now that you know how to place diodes in series, you can read the schematics in Figures 2 and 3 and execute any of them that use series wiring.

Photo 10: On the Boss SD-1’s circuit board, diodes D4, D5, and D6 can be altered in various asymmetrical and symmetrical arrangements for different feels and gain types.

SD-1 Symmetrical Clipping Mod
The SD-1 circuit is different from the TS9 in that it comes standard with an asymmetrical clipping arrangement. Take a look at the circuit layout in Photo 10. D4, D5, and D6 are the clipping diodes. D5 and D6 are already in series with each other and in parallel with D4. If you want to make this a symmetrical arrangement, you can remove D5 or D6—it doesn’t matter which—and place a jumper wire where it used to be.

If you want a symmetrical arrangement with more headroom, I suggest leaving D5 and D6 alone and adding a diode in series with D4, just as we did in steps 3 and 4 in the previous “TS9 Asymmetrical Clipping Mod.” If you want more clipping with an asymmetrical setup, you could also place a diode in series with D4 and D6. You can try many variations of series and parallel pairings of different types of diodes, and it’s a bit easier with the SD-1 as compared to the TS9 because of the SD-1’s setup and its roomier circuit board. So don’t be afraid to experiment—just make sure you don’t put your diodes in backward. If you do, it won’t hurt anything, but your pedal won’t work right. All you have to do is turn them around and you should be good to go.