This Week on the Breadboard: The LPD Sixty8 (with some mods)

gtt said:
Can I ask another diode question? So you have the 2N7000 clipping elements around U1B and U2B. What's up with D1 and D2 after them and before the treble control? Why the second set of clipping diodes? Does most of the distortion come from these or the 2N7000s? Is there any usefulness in putting a switch to optionally cut them out?
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This is a Marshall emulator. In a real tube amp, distortion happens in more than one place. The preamp tubes make asymmetric distortion (clipping). The 2nd & 3rd stages approximate the preamp distortion. The Ge diodes approximate the phase splitter / output stage more-or-less symmetric clipping. As the signal level varies, the three distortion elements (Q2, Q3, D1+D2) contribute different types and amounts of distortion. This causes the various harmonics to fade in and out producing a complex and hopefully pleasant tone.

One could put in a switch or pot to fade out the Ge hard-clipping diodes. Look at the Emperor of Tone for an example. IMO, this pedal has enough knobs and switches.
 
As Chuck says you can add a switch or a pot to the hard clippers and in the past I have added a switch to the hard clipping diodes in circuits similar to this. But in this case I wouldn't because it's not a high gain circuit at all and I doubt you would gain (excuse the pun) anything.
 
With the booster, this is a pretty high-gain circuit.

Here's the difficulty when adding a switch or pot to the hard clipping diodes: the volume coming out of the hard clipper is no longer limited by the diodes. Disconnecting or fading out the diodes raises the volume substantially. We then have to move the LEVEL pot so it is before the next amplification stage, otherwise we risk saturating that stage. It's all doable, but we have to look at the big picture to make it work. If you're breadboarding, you should try it.
 
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