Bass Harmonic Jerkulator

[Isaiah]

So, I built the Harmonic Jerkulator for my bass and I must say,
I was disappointed by the results. Great for guitar, horrible for bass.

So, I'm on a quest to build a HP/HJ for bass guitar.


After some tweaking, I ended up using a 2N907A Si for the PNP and an AC187 Ge  for the NPN.
All resistor values are the same as the HJ. The input and output caps were changed to 0.1uF and 1uF respectively.

This is fine for fuzzy sounds at full gain settings, but what I really like is setting the gain between 9-12 O'clock
and just achieving overdrive.
Unlike a lot of overdrive circuit I've tried on bass it doesn't add large amounts of harmonics relative to the fundamental - I think it's more the treble frequencies that are overdriven, however the bass is cut a little but I don't mind that too much.

However, it seems to be a little noisy, but I don't know if that's because it's on a breadboard.




Does anyone have any Bass-specific Harmonic Percolator/Jerkulator configurations they'd care to share please?

Thanks,
Alex

[Isaiah]

I think one thing that I like about the gain set quite low (as I described above),
is that it sounds like just the Ge NPN is being overdriven.
With the gain at full, I believe the Si PNP (2N907A) is overdriving itself/distorting as well.

[mac]

What input and output caps are you using?

For example, my Bazz Fuss, which is intended for bass, has lower input caps.

mac

[Isaiah]

So, after some lengthy experimentation -

NTK514F - Ge PNP (I forget the gain, 30-50ish)
AC187 - Ge NPN
Resistors between NPN's collector and +V, and PNP's collector and Ground are both 100k (The resistor for the NPN could be a trim if you like).
Biasing resistor for the PNP is 1M and for the NPN is 100K.
Input cap is 0.022uF but could perhaps be up to 0.1uF.
Decoupling cap between PNP and NPN is 0.068uF.
Output cap is 0.1uF but could perhaps be up to 1uF.
22uF cap from emitters to Ground
No clipping diodes.

To be honest, it sounds best at full gain (input volume control set to 11) on bass.
Probably more usable on guitar I must say.

Oh well, if anyone else builds this and enjoys it, do let me know.

Alex

[mac]

There is a large thread about the Harmonic Percolator,
http://www.diystompboxes.com/smfforum/index.php?topic=55987.60
And a resume of my findings,
http://www.diystompboxes.com/smfforum/index.php?topic=49035.0
I experimented with 2 Ge transistors because it sounds a lot better than a Ge and a Si. The problem is that the leakage can be a problem. It is useful to use trimpots for the 4 bias resistors. If the AC187 is leaky then lowering the collector resistors may help to fine tune the circuit. Also a small resistor between emiters, about 1-2k.
The rule seems to be: moderate leakage for the PNP (100-200uA), very low leakage for the NPN (<50uA).
I did not tried for bass, but maybe increasing the input and output cap to 1uf or more, and the collector1 to base2 cap may help.

mac