Small improvement to Octave-Up Sick Box

[Mark Hammer]

Prompted by a recent thread, I threw one of these together, and it certainly lives up to the name that the designer - Gus Smalley - gave it.  It does an octave up, and it gets sick....real sick.

It provides a max gain of 371x in the first stage.  That first stage gets split with half going to a unity-gain invertor stage.  As with all split-then-rectify-then-recombine octave units, matching of the two paths yields best octaving.

For a lark, I replaced the 10k feedback resistor in the invertor stage with a 6k2 fixed resistor and 5k pot, wired as variable resistor.  This would go from a bit below to a bit above unity-gain.   I was hoping to be able to have a 3rd control that would provide a clear tonal change.  I didn't get that with the values used, but found I was able to improve the octaving by adjusting the pot.  I imagine that this is really a change aimed at a trimpot, rather than a chassis-mount pot.  But it does get you better octaving when the Gain control is set to something a little more modest.

[Gus]

When the OUSB was first posted one could get most of the parts from Radio Shack that is why some of the part values were selected.  As noted it has a lot of gain.  I would reduce the 100K gain control pot(50K or lower) and/or increase the 270 ohm(1k)for finer control of the circuit.  This will have less gain but finer gain control.  It is very sensitive to gain setting for the octave to work well.

Have anyone tried Si diodes like 1N4148s?

The four diodes at the output are more for limiting the output

Mark have you tested how the octave works over the whole fingerboard?

[DougH]

Another fun trick with this one is to add an "octave" switch to disconnect one of the rectifying diodes. Makes a nice fuzz sound.

[slacker]

Have anyone tried Si diodes like 1N4148s?

It was a while ago but I tried it with Si diodes, it worked fine but from what I remember it lost some of the wildness and it had less of the big compression effect you get at high gains.

[Mark Hammer]

When the OUSB was first posted one could get most of the parts from Radio Shack that is why some of the part values were selected.  As noted it has a lot of gain.  I would reduce the 100K gain control pot(50K or lower) and/or increase the 270 ohm(1k)for finer control of the circuit.  This will have less gain but finer gain control.  It is very sensitive to gain setting for the octave to work well.

Understood.  Since the "implosion" part is fun, I'd suggest using the stock 100k pot, but replacing the 270R resistor with a 1k or 1k5 unit, and a 470R toggled in parallel with it for over-the-top stuff.  I still don't understand how that imploding thing occurs.  I know it also happens with the Scrambler when you push it, and I know that the recovery time seemed to be shorter when I didn't have optimal octaving last night.  I just don't know what is happening to produce it.

Have anyone tried Si diodes like 1N4148s?
The four diodes at the output are more for limiting the output

I didn't, but I could try it out.  I gather the Ge diodes in series provide some crossover distortion, as in the Foxx Tone Machine.

Mark have you tested how the octave works over the whole fingerboard?

Not meticulously, but my recollection from last night is that it wasn't any different than most octave-up units; i.e., octaves below the 7th fret are rare.  The obstacle is not the circuit per se, but the stiffness of the string at different points along the fingerboard, and the likelihood of having an octave audible amongst all the frizz.  Note that I stuck a feedback cap in parallel with the gain pot, just to keep the fizzies out, and also dropped the ground leg cap from 100uf to 47uf..