I used NPN transistors with a high beta (>400), I think they will give higher input gain and more output gain in the final stage.
I had a very anoying noise in this circuit from a cheap power supply. Added a 56 Ohms resistor between supply 9V and circuit and the noise disappeared completely. It seems that the 100uF capacitor was not enough and needed a little resistor to absorb the noise.
Ommited the two bottom rows with no function.
Yesterday a made a new, more drastical change: I swapped one of the two rectifier diodes (1N4148, Vf = 0.7V) with a BAT46 Schottky diode, with Vf = 0.3V, just to experiment with this component.
The electrical result is an asymmetrical rectification of the input signal, as can be seen in the oscilloscope:
The audible result is a more natural sounding octave effect. Usually octavers have to be combined with overdrives or fuzzes in order to be bearable. Otherwise they sound too robotic and are unusable, at least for me. With this change, even if visually does not seem to be very different from its symmetrical counterpart, at least for my ears it sounds like a very dynamic distortion, and of course gets improved with some kind of light distortion after it.
Out of curiosity, I bought a second hand SD-1, just to try it as is, and try some of the mods people have been doing to this pedal for the last decades. Some mods try to transform it into a TS9 or even a TS808, so you can turn this relatively cheap pedal into a more expensive one. But this box has its own personality, the design is very similar to TS9 but with some remarkable differences:
Clipping is asymmetrical, two diodes forward and one backwards (in the direction of the operational). Asymmetrical clipping sounds different than symmetrical one, some people describe it as harsh or hairy of fuzzy compared to the latter.
There is no capacitor in the feedback loop of the clipping amplifier. This capacitor smooths the clipping a little, giving the TubeScreamer part of its particular tone.
Tone control is very similar but with different component values and with a capacitor (C6) in the feedback loop of the tone control operational amplifier. This capacitor is the first thing I have seen every mod removes because it sucks a significant amount of bass frequencies to the signal.
After trying the pedal for a while, I quickly noticed what most people complain about, this makes the guitar sound thinner. This has not to be bad in every ocasion, especially in live gigs situations where you want to sound in a different spectrum space than the rest of the instruments. But ok, I want more bass too. In this clip I recorded the original sound of the pedal, playing with a RS420 (humbuckers) and a Fender Blues Deluxe amp (please don’t pay attention to the music, just the sound 😉 ):
To correct this defect or feature, there are multiple mods out there, usually people remove capacitor C6 and change values of R6 and C3, which connect the feedback loop and the negative operational input to a voltage divider, in a similar way to he TS9 connects those to ground. lowering R6 and raising C3 values results in more bass response from the clipping circuit. See below the schematic:
I chose to make this changes in order to get a nice tone from the pedal:
C6: instead of removing, changed it with a 100pF capacitor from its original value of 10nF.
C3: raised from 47nF to 100nF.
R6: lowered to 3K3
The other section where people make changes is the clipping circuit. Many mods aim in the direction of getting symmetrical clipping, but I didn’t want another TubeScreamer. Instead of that, I wanted to enhance the personality of the effect by exaggerating the asymmetry of the clipping section. Asymmetry comes from using a different quantity of diodes in each direction, or from using diodes with different specifications, especially with different forward voltages (Vf). Vf is different for the following types of diodes:
Ge diodes: low Vf, around 0,3V
Si diodes: usually around 0,6V
LED diodes: depends on the color, 2V for green ones, 1,6 for red ones.
So using a Ge diode in one sense and a LED in the other, we get asymmetrical clipping, right? Right, but Ge diodes are expensive, hard to find and unstable. I read about simulating Ge diodes with schottky diodes in this article:
Ge diodes are used in some mods also because of the smooth I-V curve the exhibit. Schottky diodes have a low Vf too, but the I-V curve is more abrupt than the Ge one. By adding a resistor in series, you can get a similar response, at a lower cost and with more stability.
In my mod, I have replaced D5 and D6 (in series in the original circuit) with a resistor (10 Ohms) and a schottky diode (BAT46) with a Vf of 365mV. And D4 has been replaced with a green LED with VF=2V. You can see the measures below:
The components used:
And the changes to make:
For the clipping section, that I suppose will be subject of further modifications, I have replaced the original components with sockets. In the picture below you can see the other components replaced too:
Here you can see a detail of the changes in the clipping section:
And the result in the following record. I was not trying to play a song, just to get all the possible tones from the pedal. Please forgive me if I get sloppy:
I can perceive these differences after playing for a while:
More volume at the same “level”.
Much more bass frequencies, without being too much for a band situation, I think (has to be tested in a band situation).
More of the asymmetrical character: “harsher” when playing harder, keeping dynamics (I think more than the original, maybe I am somewhat subjective).
I think the sound changes more than the original when moving the drive control: from a smooth, almost TS9-esque overdrive at low gain levels to almost fuzzy and even octave-like at maximum gain.
If you are interested in this mod, please try it, test it and give me your opinion in the comments section. If you improve it, tell me how. If you like it, give me a “like”, and if it’s possible for you, mention my blog to yout friends 🙂 Mods have no copyright, but we like some recognition, don’t we?
Edited on March 16 2019:
Some days ago I put a red led instead of the green one. Now the overdrive is more usable and versatile, although the octave effect is less pronunciated.