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I've been working on a distortion circuit for a guitar pedal and so far I like the tone but I was wondering if I could get some insight on how I might drive the transistor (T1) harder, increase gain to get a stronger distortion sound. Right now it's a bit too mellow. Is it about getting higher voltage to the base? Where would be a good starting point for swapping out components to experiment? I'm still learning the basics and starting to experiment with transistor distortion. Thanks!

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    \$\begingroup\$ Probably just increase the value of R10. \$\endgroup\$ – The Photon Jul 23 at 2:09
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    \$\begingroup\$ To keep the output impedance of the T1 stage constant, I suggest reducing the value of R13, rather than increasing the value of R10. But the effect will be subjective, so try both. Start with a factor of two. \$\endgroup\$ – elchambro Jul 23 at 3:45
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    \$\begingroup\$ FWIW, as I read the circuit, the distortion is meant to come from the diode network D2, D4, D7 or D3, D5, D9, not from the transistor T1. Trying to get audible distortion from T1 is changing the operation of the circuit entirely. Increasing R10 increases gain and increases output impedance of the T! stage, making the clipping diode network work more strongly. Adjusting R13 will change the bias point of T1, but should not affect AC gain strongly due to C12. But I am not an audio guy, so could be reading this wrong. \$\endgroup\$ – The Photon Jul 23 at 5:06
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    \$\begingroup\$ The OC139 is a rather old part and it looks like your distortion from the diodes might not be as hard as you want. Are you actually using an OC139 and where did the circuit idea come from? On a side note - your name greybrow reminds me of someone from a music site called mp3unsigned. \$\endgroup\$ – Andy aka Jul 23 at 7:10
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    \$\begingroup\$ Side note - Schottky and germanium diodes have a mellower distortion than standard silicon types - maybe try 1N4148 or 1N4001 diodes if you can. \$\endgroup\$ – Tyler Stone Jul 24 at 0:53
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R8 is the primary limit on the action of the diodes; you should try reducing it, probably to zero. But R10 is a bit too large to give a full fuzz tone effect. In fact, everything in this circuit is badly chosen, making it difficult ( and far too much work ) to suggest individual changes. C12 makes T1 run at maximum gain, so getting more amplitude for the diodes would mean using Q1 as a common source amp rather than the source follower it currently is. You should look for a more competently executed design. That said, I will add that this circuit achieves it's effect in two ways: the obvious one of the diode clipper, and the more subtle way that T1 is connected. Grounding the emitter with C12 makes an output that has compressed positive peaks and expanded negative peaks, creating more even order harmonics.

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