onboard distortion based on fetzer valve

[thetragichero]

i have a buudy of mine's bass to rewire, and he expressed interest in some sort of (active) onboard distortion on a push/pull pot
now what he wants is gnarly and frankly obscene sounding distortion, as it 's for a grindcore band
i'd like to use the fetzer valve in it because it's easy and i know it sounds great
to wit: www.runoffgroove.com/fetzervalve.html

i know i'll want to add diodes to ground for more clipping, but i don't want the signal to be too hot in case he runs it in front of a solid state bass amp
my real question is how to adjust the values for maximum clipping while making the output level not too much higher than unity gain
i will probably repost this on the diystomboxes forum at some point as well, but i know fellas such as johnh have done a lot of work with jfets so i look forward to ideas from this forum

[JohnH]

Thats a good circuit, but not enough gain in itself. youll need at least two stages to get there. My best version is this, which could be the basis of an onboard circuit:


BlueJuice overdrive

It has two stages similar to a fetzer, plus a buffer (which can be omitted)

John

[ashcatlt]

Assuming you use two "back to back" clipping diodes (the standard arrangement) to ground, they will limit the peak to peak output voltage to the sum of their forward diode drops. With the usual silicon types this will be somewhere around 1.4V max. That shouldn't distort a typical amp input. Use shotky or germanium for even lower peak output and even more severe clipping from the onboard circuit itself.

Trying to get "unity" out of a distorted like this is kind of weird and somewhat subjective, since the clipping (by definition) changes the dynamic range of the output. It's completely possible to end up with peaks that are lower while the RMS output is greater than that of the clean bypass signal. So which one do you want to match?

I don't think you'll have any trouble with clipping a bass amp, but if the perceived volume boost is too great, just stick a simple voltage divider on the output after the diodes. Maybe a trim pot so you can dial it in to his taste on his rig?

You might also want to think about adding an LPF (cap to ground) after the diodes, just to knock down the harsh high order harmonics generated in the clipping section. This is especially true if it's going into a typical bass amp. They tend to have much greater treble response than guitar amps.

[thetragichero]

finally got the bass together
for the 'distortion' circuit i went with two fetzer valves, j201 and 2n5457, with led and 1n4148 clipping diodes after the first stage and 1n4148 and two 1n4001 clipping diodes after the second
came out as disgusting as i'd hoped!

first i play clean and then pull up the pot for the distortion (of course spacephone added its own distortion to all of it...)
i did not know she had it pointed at me, otherwise i would have put on a shirt...