Author Topic: Pre-build: Harmonic Percolator and mods  (Read 8457 times)


Pre-build: Harmonic Percolator and mods
« on: September 03, 2006, 07:31:48 PM »
I decided to give the HP a try.
I used A. Hermida schem at

At first I could not make the HP (version 2) to sound good. I used a low-gain low-leakage Toshiba 2sa53 (hfe: 45, 17uA) with a BC550C (hfe:500), and 1n60 Ge diodes. The sound was like an over saturated distortion or misbiased transistor. I was going to give up when I replaced the 2sa53 with a Matsushita 2sa101 with a lot more leakage (hfe:42, 151uA). Suddenly it sounded good, as the clip at
I do not know if it is normal but it seems that my circuit is happy with a moderate leakage Ge.
The next step was to replace the 220k and 680K resistors with a couple of 1M-A pots.  That allowed me to get subtle distortion differences. 680k for the Ge transistor and 720k for the Si made the best dist for me. Anyway the std 220k and 680K that appear on other schems are fine.
Then I replaced the diode clipping section and added a second Ge 1n60 diode at the input to form a back-to-back pair. In the clipping section I used 1n4148s and a 1n60 instead of the 1k resistor (1n4148 |< , 1n4148 + 1n60 >|). The addition of the Ge diode at the input resulted on a brighter tone. The changes in the clipping section sounded better to me, both at low and high gain settings. See for yourself.
Next I used a Texas Inst 2n388 Ge (hfe:40, 31uA) and a Matsushita 2sd352 (hfe:50,  151uA), both NPN, at Q2. With the 2ds352 I could not make any good sound, same as above. It appears that Q1 needs some leakage while Q2 almost no leakage. The 2n388 made a quite different dist than the BC500C, more fuzz maybe. I liked both.
I also found that adding a 470k - 2.2M resistor in parallel to the 47uF cap, ie, from emitter to gnd, produced nice tonal variations.

About version 1, it seems that this circuit can work better with low-leakage Ge better at Q1. I'll have to investigate a little more. I'd really appreciate if someone can check this or if it is something that only happened to me.
Surprinsingly both version sounded very close, but I am going to build version to with the mods I detailed above.


mac@mac-pc:~$ sudo apt-get install ECC83 EL84

alex frias

Re: Pre-build: Harmonic Percolator and mods
« Reply #1 on: May 13, 2008, 11:44:54 AM »
I confess to be impressed with Mr Albini's Harmonic Percolator video. And my first atempts to build this mystery but simple beast resulted in very excentric fuzzes. Some better and closer to the shown at the clip. Others, unusable, in my opiniom. The best I've got was obtained using a similar version 2 Hermida's circuit, above referred. 

Q1= 2SB54 (hfe=80) and Q2= BC108 (hfe=480). The cap at the input, going to ground was 100pF, the input to Q1 base was 0.1uF. I didnt't use the diode to the input, but I hope try other experimentations with this idea later. R1= 33K, R4= 100K, R5= 680K. I didn't use the first pot for the Harmonics, first for lazyness, then I've tried it and preferred to use the guitar volume control instead. When I've tested the clipping section using the same as the Hermida'sV.2, already referred, the result was on full splatiness. Better on low settings and worst at the higher gains. After try/error tests I've found the excenctric configuration:
1N4148 - 1N60 - 1N60
1N4001 - 1N60 - 1N60 - R6=4K7

I put a SPST to use or not the clipping section, because both modes sounded good. I will try to do some audio samples.
I've liked a lot the results. It's very unique sounding fuzz. Using it with a very clean amp and my Lace Sensor's Parker P-30, it sounds like a blend of clean and fuzzy guitar. It's very dynamic and using the guitar volume pot you can have a lot of timbres and nuances. But the best use was to put it in front of a "starting-to-saturate" Marshall/Orange/Mesa/Etc. amp.  This configuration was the most interesting, in my humild opinion.
Pagan and happy!


Re: Pre-build: Harmonic Percolator and mods
« Reply #2 on: May 13, 2008, 12:59:16 PM »
if you have the time try the schem that was made by various members here working from real percolators. if you search for it you'll find a really long thread about it.

use gains of around 100 for both transistors. it sounds GREAT
visit for info on (allmost) every effect in the world!

alex frias

Re: Pre-build: Harmonic Percolator and mods
« Reply #4 on: May 13, 2008, 02:27:37 PM »
I'm really happy with the results I've got. What does not mean a finished or acomplished project.
Well, I allways fine tune my pedals as I think it could be better or more flexible.
I don't even know about any other good HP samples, other than the Albini's video or Tonefrenzy. But one thing I really apreciate from S.Albini sound is the bass texture, a quasi octave down subharmonic. Well, that's just an impression. Mine has really good bass texture, but it does not achieve that cool dragon snore. Aside from that, I'm very satisfied with it. I'm buying some 2N404A and some 2N3565, jkust for fun.

In my experiments, maybe because the output level is set high, the cliping device doesn't sound ok, for me. My alternative circuit, with 6 diodes and 1 resistor, sounded rich and full, with no splat sound.

I think the only focused comment about the use of the diode Ge at the input (to ground) was Mac's on this topic. All the others I remember was anything like "it didn't work for me". So if anybody would like to coment about it. I think I will try some Ge's, Si's and LED's this afternoon.
Pagan and happy!


Re: Pre-build: Harmonic Percolator and mods
« Reply #5 on: May 13, 2008, 04:31:11 PM »
Excellent.  There is a lot of myth surrounding this pedal, and I love Shellacs crazy tone.  That video made me really want a Harmonic Percolator.  That "work-alike" he tested, however, sounded TERRIBLE so I was concerned about building one.
Built: Fuzz Face, Big Muff Pi (Stock), Distortion + (Germanium and Silicon versions)


Re: Pre-build: Harmonic Percolator and mods
« Reply #6 on: May 13, 2008, 04:38:45 PM »
Just to throw in two more cents, this is my adaption of this little bugger from last year: