Thursday, March 15, 2012

Tube Screamer Tone Mod

If you have a Ibanez Tube Screamer or clone you know that the pedal has a very pronounced mid range boost which personally I don't like too much. I know this gives the pedal its characteristic tone, but it is pretty easy to change the pedal's tone either by swapping out the capacitor that precedes the clipping stage of the pedal or by adding a switch and additional caps to enable changing the value of the capacitor on the fly. Note that this mod has nothing to do with the tone pot control, which functions as an active tone control. The capacitor in question is the 0.047uF cap at the center left of the schematic below which functions as a high-pass filter and thus rolls off low frequencies.

There are a couple of approaches to changing the value of the this cap. You can solder in a cap of a different value or, as with the B.Y.O.C. "overdrive 2" clone of the Tube Screamer available here the approach is to use a SPDT switch (on-off-on) with three different capacitors which allows you to change the effective cap values on the fly to suit your needs while playing. The schematic below shows the B.Y.O.C. approach where the single 0.047uF cap is replaced with a switch and three caps, the original 0.047uF cap, a second 0.047uF cap, and a third 0.150uF cap. The center (off) position selects the right 0.047uF cap alone. The first on position selects the right 0.047uF cap and the left 0.150uF cap together in parallel. The second on position selects the right 0.047uF cap and the center 0.047uF cap together in parallel. The total value of capacitors wired in parallel is the sum of the individual capacitor's values. So the total values here would be 0.047uF, 0.197uF, and 0.094uF, respectively.

According to this page on modding the Tube Screamer, the following cap values correspond to the following corner frequencies of the high-pass filter:

Capacitor Value--Frequency
0.047uF--720 Hz
0.1uF--339 Hz
0.22uF--154 Hz
0.47uF--72 Hz
1.0uF--34 Hz

The fundamental (open) frequency of the guitar's six strings are as follows:

E--329.6 Hz
B--246.92 Hz
G--196 Hz
D--146.8 Hz
A--110 Hz
E--82.4 Hz

So the stock capacitor value of a Tube Screamer attenuates notes below the 12th fret on the high E string. Higher capacitor values will thus lower the corner frequency of the high-pass filter and increase bass response. Thus the the B.Y.O.C. overdrive 2 approach allows you to chose higher cap values that will give you corner frequencies corresponding approximately to 339 Hz (open high E string) and 154 Hz (open D string). These switching options increase the bass response significantly.

I still wanted more bass however, and as the Fat Bass for Tube Screamers page suggests, an even higher cap value will lower the corner frequency of the high-pass filter even further. My goal was to set this value below the lowest frequency of the guitar, 82 Hz for the low E string. I desoldered the middle 0.047uF cap in the B.Y.O.C. overdrive 2 schematic above and replaced it with a 1.05uF cap which, when combined in parallel with the original 0.047uF cap, gives a corner frequency below that of the low E string on a bass and no attenuation of bass frequencies. When I switch in this cap in the pedal there is certainly a lot of bass, and as the original designers of the Tube Screamer were trying to avoid, probably too much bass at high drive levels. At lower drive levels it sounds good, however, and gives you additional tonal options with this classic pedal.

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