Harmonic Percolator Mods - add yours

[edvard]

OK, so I breadboarded a Harmonic Percolator per George Giblet's schematic (props, G!) and although I substituted some parts, I was still happy with the sound.
As I detailed in a post here I got the irresistable urge to experiment and so here is the final fruit of my labors.
That is, this is what I eventually settled on as 'good enough' (which is an understatement).


Parts in Red were added, and parts in Blue were changed.
Ra and Rb were added for 'lift' to the gain and volume control so it doesn't completely shut off at 0.

So, first off, let me 'splain a few things...

I play an '84 Fender Bullet S-3 (one of the last ones made in America) and my choice of a '53 Fender Deluxe or a Peavey Bandit 65.
I cut my own picks out of the sheet metal covers from old busted VCR's and second hand computer cases.
Milk jugs and bits of sheet pvc also get picks cut out of them when I'm in the mood for using plastic.
Most times I just run my gear through a Tascam Porta5 four track (motor's busted, but it's makes a handy mixer) through my computer to my stereo to headphones.
On my computer I'm running Xubuntu 9.04 with the real-time kernel and Jack Rack for effects, mostly eq and cab simulation.
As soon as I can swing it, I'm going to build either a Condor or Daniel's MultiCab.

I've built two HPs from Alfonso Hermida's schematics (one for myself, one for a friend) and was never quite happy with them (now I know why).
I've also tried Mr. Escobedo's Harmonic Jerkulator and liked it better but it still wasn't quite "there".
Coming to this forum, I stumbled on George's schematic and discussion of it and found the missing ingredient - a Germanium transistor.

On my first breadboard of this I used a 2SB173 in the position of Q1 and a BC239 at Q2 with the Germanium diodes in the clipping section.
On first listen, it was very 'fizzy' with some nice gravel in the low end. Not exactly the Shellac tone I was waiting for, but usable.
I could definitely hear the potential.

I figured the fizziness was coming from the Ge diodes, so I swapped those out for Silicons.
Immediately the tone opened up, the fizziness went away and although the distortion had a 'soft' quality to it that I've come to expect from germanium designs, it wasn't 'mushy'.
Very nice.

My itch to experiment further drove me to find somewhere in my little brain a foggy recollection of somebody somewhere saying you could simulate germanium transistor leakage in a silicon design by strapping a diode from collector to base.
I know that's wrong, but something told me to try it and that's where Da in my circuit came from.
I put in a BC309 at Q1 with the diode and was quite pleased with the results.
This configuration had a little more gain, and a little more midrange came through at full up on the gain control.
The pick attack was very quick with lead lines almost playing themselves and chords bloomed with harmonics that just begged to be fed back.
Once again, very nice.

I must also say this is the first distortion pedal I have EVER built that had usable tones at low input settings.
In fact, that's what led me to try the 2N4403/4401 combo.
At full up, there's no 'crackle' like when I tried BC556/546, but some folks might like that.
At a quarter up, it's got a very nice chimey tone with just a touch of breakup.
At 12 o'clock it almost nails the 'Albini sound'.
Very aggressive without a lot of fuzz on the top and a big but tight gravelly low end.
To get closer I'd need an aluminum guitar with out-of-phase pickups.

I'll post soundclips when I get a chance...



Further mods:
Using a 100K ouput pot instead of 50K tames the midrange a bit, especially at high gains.
Take out R1 and leave just the diode (ala Bazz Fuss). Lots of gain with an interesting texture to the tone.
change R1 to 100k for a little less gain.
Switch the directions of the clipping diodes for a different texture.
Try different transistor combinations for more or less gain and different texture.
I found that 2N3906 does NOT work in Q1. It hisses like a leaky bike tire and the sound is weak.

Any more?

[theehman]

That's one tiny schematic.   

[edvard]

Sorry, problems with Photobucket.
Should be fixed now.

[mac]

My mods are:
1. both transistors Ge,
2. a 20K-B pot as a variable resistor between emiters to get nasty octaves.
3. exp with the feedback and output cap, increase output cap if using Ge diodes.

mac