Harmonic Perculator

[mac]

There's some interesting results about the leakage.  (Using references from Alphonso's circuit) You mentioned that R5 and R7 only have a subtle effect, they appear to me only to have a subtle effect too but I haven't spent a lot of time with it.  If biasing changes due to leakage were the cause of the sound differences I would have expected the R5/R7 adjustments to compensate to some degree.  However the leakages you quoted are quite large so perhaps the leakage can be an issue.  The base currents in the transistors are going to be in the order of 0.5uA to 1uA.  If you take your 2sa101 the base referred leakage will be about 151uA/42 = 3.6uA!  whereas the 2sa53 would be 0.38uA.  Your 2sd352 NPN will have a base refered leakage of 151/50 = 3uA  which indicates the leakage is relatively high compare to the current through R5.  I suppose in summary since you like the high leakage Q1 configuration better it might be worth lowering the 220k resistor for low leakage

First, thanks for the hint.

I'm not sure but lowering the feedback resistors too much decreased the final amount of distortion.  And with some npn Ge at Q2 it did not helped at all. But I need to verify this.  I did not measure voltage drops when I tried it so as to know ic and ib. Que idiota!!!
I guess, and I'm not an E.E., that reducing the collector resistors let some more ic into the transistors, and more ib. So one could make ib to be of the order of the leakage. The 22K in your schematic or a pot can be more helpfull.

I'm going to breadboard it right now and start teawing the bias. I'm going to try different Ge so as to have a picture of what the resistors should be as a function of the leakage and gain. Maybe try to solve the dc bias considering leakage.
If you have any ideas now is the time.

mac

[George Giblet]

>owering the feedback resistors too much decreased the final amount of distortion.

Yes that's a side effect!

For the high leakage case,

>Maybe try to solve the dc bias considering leakage

The high leakage could be diverted using a resistor across B and E perhaps values as low 47k; but I'd start with higher values.

[mattpocket]

Im confused now... yet AGAIN! haha

I thought that we had decided on a suitable circuit, with component values earlier on? What are you guys debating now?

Does the last schem (the credited one) not sound suitably like the original?

Matt

[mac]

Matt,

When I fisrt tried the HP, both Hermida V2 and Escobedo's version, I had to tweak the resistors and find suitable Ge & Si to make it sound good. They sounded out of focus, like a tipical misbiassed circuit. I do not know others experiences but this is what happened to me, and so I posted my results. My bet is that this is somehow like the Fuzz Face. If you have good low leakage Ge you may be lucky and have it working good using the original schematic. But the true is that you most likely need to use a trimpot to adjust the bias.
The schematic is not wrong from the dc point of view. If you have the correct parts, like maybe the guys at Interfax had, it will work correctly. If you see Hermida's V2 you'll notice a 737K resistor which is not a std value, possibly indicating that Hermida used a trimpot. Also I do not remember if 90K was a std value in the past.

If it is the original schematic, I do not know. That's another story. All I can say is that when I fine tuned it, very close to V2, it sounded fantastic and different from the rest, and to be honest, I do not care if it the original.

Now, with the info George gave us, I'm goint to experiment with it once more.

mac

[mattpocket]

So is this the schem you used?

http://www.geocities.com/george_giblet/effects/percolator_later_rev1_1.png

edit: actually, i think you used this one, am I right: http://www.diystompboxes.com/pedals/Percolator.gif

Matt

[Sir H C]

A great thread from Harmony Central:

http://acapella.harmony-central.com/forums/showthread.php?t=1008997&highlight=interfax

[mac]

Matt I used Hermida's v2.

Sir HC, thanks for the link. I've never saw that kind of glass resistors. I dl all the photos to my computer. At first glance it seems that Hermidas schem is very close.

mac

[Sir H C]

Usually those glass resistors are seen for 10 meg and higher (I have some 100 meg resistors like this) because the glass when clean of fingerprints is high enough resistivity to keep the resistor from reading much lower.  Odd to see them used on lower values.

[George Giblet]

> I thought that we had decided on a suitable circuit, with component values earlier on? What are you guys debating now?

> Does the last schem (the credited one) not sound suitably like the original?

I believe the "credited" circuit is correct.   The issue here is whether transistor leakage has an impact on the sound and if it does can the circuit be tweaked to compensate.

Circuits where transistor leakage affects the behaviour are a pain because people can do their builds with the same (correct) circuit yet they get different results.  The fuzz-face is a perfect example.  There's many instances of good ones and bad ones.  With the fuzz face. the circuit can be tweaked by adusting the second collector resistor so the second transistor is biased with a collector voltage of about 4.5V to 5V.  The circuit is not longer the original but the circuit behaves as a good original.

[mattpocket]

Thanks guys, you've been a great help so far...

I am building one of these very soon as a favour for a friend... he knows his stuff about this pedal... a lot more than I do, and that scares me, I might get what I think is a great sound, near to the original, but he might think its rubbish! haha

I am going to have a good look through this thread today and select some nice trannies and decide which schem I am going to use... is there a layout for this yet? I will be doing one, so if anyone would like it afterwards I could post it up... or maybe its time to start a layout gallery of my own!

Matt

[mattpocket]

Sorry, I was not aware MartyB took his stuff down

I believe DanN used this schematic for his PCB layout http://users.rio.com/senorris/junk/Percolatorplay.gif

You did a nice job with your LoFoMoFo layout give the Percolator a try. Have you tried bancika's DIY layout creator yet?
IMO it makes the job much easier. http://www.storm-software.co.yu/diy/index.php?project=software

Sorry, I only just noticed this post dude!

Yeah, I have the layout creator, and it does make things easier, if not being a little slow, and the wires need to be able to be reshaped, but I havent been following the never ending thread at the top of the board...

Unfortunately, I do most of my DIY research and posting here whilst being at work I neither have the layout creator installed here or will it let me do such a thing...

Matt

[mac]

I began experimenting with the HP once more, here some results of the dc bias.

rn=npn collector resistor
rp=pnp collector resistor
r2=npn feedback resistor
r1=pnp feedback resistor
Measured at VCnpn=4.5v

1) q1=2sa101 (42, 151uA) / q2=2sd352 (59, 191uA)

rn=22k ; rp=20k ; r2=680k ; r1=5k yes 5k!

2) q1=2sa101 (42, 151uA) / q2=2n388 (55, 41uA)

rn=87k ;  rp=88k ; r2=680k ;  r1=330k

3) q1=2sa101 (42, 151uA) / q2=bd139 (220, --) Si

rn=83k ;  rp=80k ; r2=680k ;  r1=330k

4) q1=2sa101 (42, 151uA) / q2=2n1114 (163, 300uA)

rn=87k ;  rp=84k ; r2=680k ;  r1=300k

5) q1=2sa49 (56, 15uA) / q2=2n1114 (163, 300uA)

rn=90k ;  rp=88k ; r2=150k ;  r1=39k

6) q1=2sa49 (56, 15uA) / q2=bd139 (220, --) Si

rn=90k ;  rp=88k ; r2=500k ;  r1=22k

7) q1=2sa49 (56, 15uA) / q2=2sd352 (59, 191uA)

rn=18k ;  rp=18k ; r2=120k ;  r1=8.2k

I guess this covers many situations.

In each test I tuned the circuit to sound not oversaturated, which is the case of a pair of med gain Si. First playing with the guitar volume reduced at 3/4 and then at max. Some may like the sound when the transistors are oversaturated, I don't.
When tuned properly the fuzz is incredible, rich in harmonics and the tone is very different from other fuzzes. When rolling back the guitar volume the crunch is amazing, even harmonics I guess.

My bet is that this circuit works as per the schematic if:
a) a Ge pnp hfe near 50 leaking around 100-200uA, and
b) a Si hfe around 200 are used.

I strongly suggest breadboarding using trimpot, and the using the nearest resistors when soldering. Tweaking the feedback resistors let fine tune the circuit.

If using a npn Ge it is more important the ratio leakage/hfe than the leakage itself. See test 2,3 and 4. When this ratio is low, test 1, both feedback resistors have to be reduced to near 10-22k and so the feedback resistors as George noted above. I lowered only one, the pnp, but one can play with both to find a good setting.
The tests using a 10X less leaky Ge pnp, 5, 6 and 7, did not sounded as good as the others. To me it was oversaturated as in the case of both Si. I think that 15uA is almost no leakage, the circuit sees this low leakage Ge as a Si.
Also, the feedback resistors do not alter the voltage drop across the collector resistors, and the circuit is temperature stable.

I wonder if a resistor between emiters or a from emiter to gnd may help when using all Si, and a very low leakage pnp Ge.

mac

[mac]

I added a resistor between emiters, in this order: npn emiter ->resistor->47uf to gnd->pnp emiter.
The effect is that only the pnp emiter is fully bypassed by the 47uf, and the npn emiter sees a resistor and a cap in series to gnd, so the npn gain is lowered. That allowed me more control over the transistors' hfe & leakage, and some interesting tones.
For example, in test 1, making this resistor 2.2k let me increase the pnp feedback resistor to 220k, the original value. Near 10K it was like a Green Ringer. Interesting mod. Going to experiment tomorrow. Maybe now two Si may work better.

mac

[rocket]

I have breadboarded it yesterday, used the summed up schematic with an ASY25 (pnp, Ge, hfe? substancial leakage) and a BC107B (npn,Si, hfe~340).

First I found that there is no difference if I omitt the 220k bias resistor of T1, obviously it is biased by the leakage.
Second, like some other guy here I found that reversing T1 (emitter to the 20k resistor, collector to the virtual ground at the 47uF cap) gives much more output.
It sounded good either way and I liked it more without the diodes. I have to investigate and tweak some more.

[alteredsounds]

Very interesting thread that seems to have died.  I came across it whilst considering building one of these.

One of the posts seems to say that the Barge Concepts schem is very close to being correct to the original build, the only reason I vaguely question this is because of the quite substantial differences that Steve Albini finds when comparing it to his original:

http://www.youtube.com/watch?v=aVUN9j9ihEM

True, many variables come into play but they do not sound 'that' similar to my ears.

Anyone had any joy building one or any more info about this pedal?

[johngreene]

Very interesting thread that seems to have died.  I came across it whilst considering building one of these.

One of the posts seems to say that the Barge Concepts schem is very close to being correct to the original build, the only reason I vaguely question this is because of the quite substantial differences that Steve Albini finds when comparing it to his original:

http://www.youtube.com/watch?v=aVUN9j9ihEM

True, many variables come into play but they do not sound 'that' similar to my ears.

Anyone had any joy building one or any more info about this pedal?

Where it may be true that Barge Concepts schem is very close to the 'original' build. Chucks Collins seems to have the one that is closer to the version that Steve Albini likes.

http://pages.prodigy.net/chuckcollins/percolator.html

I make the comparison between the two in this post:

http://www.diystompboxes.com/smfforum/index.php?topic=55987.msg432556#msg432556

--john

[Zero the hero]

I've built hte George Gilbert's version of the Harmonic Percolator and this pedal has become THE fuzz pedal I use.
I have no sound samples but it sounds very similar to the original pedal that Steve Albini showed in this video serie. I actually mounted 4 pots for looking for the sweet spot then, once trimmed, replaced with 1% resistors.
Build notes / PCB and layout on my site.

[theblackman]

Hi I am about to attempt this build, when biasing the transistors, should i just be aiming for 4.5v at the collector of the NPN? (Also at the emitter of the PNP)?

[mac]

The Barge version uses a 100k above and a 22k below so you may get or may not get near 4.5v at the npn collector.
In the Hermida version  with both 91k near 2/3*vcc at npn collector most likely. Maybe 4.5v at both emiters.
Breadboard first with pots or timmers, dial the best tone and then replace with the nearest resistors.

mac

[theblackman]

thanks mac!