Which clone is closest to the original, and do we have schem's/layouts lying around anywhere?
I have searched, and watched a couple of albini vids.
I think we are looking at:
A. TimE's Harmonic Jerkulator
B. Doug Hammonds Harmonic Speculator
C. The Brick
Matt
This is possibly the schem for it. I say possibly because there has been speculation that it is not correct.
(http://www.diystompboxes.com/pedals/Percolator.gif)
I have read that those schems have errors in and are not the same as the original. Something to do with the trannies being NPN or PNP. In the above i think they are both NPN, and a lot of people say there is one NPN and one PNP?
Matt
I suspect that the original mad scientist (as albini likes to think of him) who created the Percolator and chuck collins (http://pages.prodigy.net/chuckcollins/percolator.html - my friend has one and it sounds excellent, BTW) fine tune each circuit to taste... of course I could be wrong. I made Tim's jerkulator and modded the crap out of it to try to get it close to the percolator's sound... never really got there, but I liked the end result anyways. I never tried the brick or speculator... although I always meant to.
Oh, and yes, one npn and one pnp... one Si and one Ge...
I posted about Hermida´s schem V2.
http://www.diystompboxes.com/smfforum/index.php?topic=49035.0
I used several trnasistors, including a all Ge version that sound the best.
The trick is to use trimpots for r5 and r7. If I remember well the npn should be a Si or a Ge almost no leakage, <50uA. If the npn is leaky maybe trimpots for r1 and r4.
Once you find the G point it sounds amazing.
mac
Gut shots, revised schematic, layout, etc.
http://www.diystompboxes.com/smfforum/index.php?topic=36110.40
It's Percolator BTW :icon_wink:
Seen it before. It looks very much like a cascode amplifier (common emitter + common base amp), with a complicated feedback.
RLBJR65: I couldnt find one schem or layout that worked in there. According to that thread link you posted MartyB?? put one in his gallery, but theres none there now.
Anyone have anything?
Quote from: mattpocket on April 04, 2007, 09:40:30 AM
I have read that those schems have errors in and are not the same as the original. Something to do with the trannies being NPN or PNP. In the above i think they are both NPN, and a lot of people say there is one NPN and one PNP?
Matt
In those 2 schematics Q1 is PNP Q2 is NPN. If you look at the arrows on the emitters Q1s points towards the base so it's a PNP. Q2's points away from the base so it's NPN.
Those schems are correct then?
If so, has anyone built one from this schem? I want to know that they are good?
Matt
Use the input of version 1, but follow version 2 from the 1st tranny back (including the 220K between the base and collector). I made one a long time ago and it sounds very Big Black-esque. I never got it to work with the 1n34 at the front.
Sorry, I was not aware MartyB took his stuff down :icon_sad:
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
Quote from: RLBJR65 on April 05, 2007, 06:54:07 PM
Sorry, I was not aware MartyB took his stuff down :icon_sad:
I believe DanN used this schematic for his PCB layout http://users.rio.com/senorris/junk/Percolatorplay.gif
That bottom schematic is only correct if you change the Q1 transistor to an NPN. It would appear that some of the harmonic percolartors were made with 2 transistors that were the same (NPN and Si?) and there were others made with Q1 being a PNP Ge transistor which is basically the same schematic as the top one only with the input Diode to ground removed. R6 is something like a 4.7K too.
--john
Quote from: johngreene on April 05, 2007, 07:13:57 PM
That bottom schematic is only correct if you change the Q1 transistor to an NPN. It would appear that some of the harmonic percolartors were made with 2 transistors that were the same (NPN and Si?) and there were others made with Q1 being a PNP Ge transistor which is basically the same schematic as the top one only with the input Diode to ground removed. R6 is something like a 4.7K too.
? Dan posted in that thread that he used a 2SB77 GE for Q1 that's a PNP. http://www.datasheets.org.uk/datasheet.php?article=357683
Quote from: RLBJR65 on April 05, 2007, 08:19:20 PM
Quote from: johngreene on April 05, 2007, 07:13:57 PM
That bottom schematic is only correct if you change the Q1 transistor to an NPN. It would appear that some of the harmonic percolartors were made with 2 transistors that were the same (NPN and Si?) and there were others made with Q1 being a PNP Ge transistor which is basically the same schematic as the top one only with the input Diode to ground removed. R6 is something like a 4.7K too.
? Dan posted in that thread that he used a 2SB77 GE for Q1 that's a PNP. http://www.datasheets.org.uk/datasheet.php?article=357683
I would be surprised if he did and it worked. From the pics on the web that Dan's bottom schematic corresponds to, you can see that the 2 transistors are very much the same in size and shape and are wired in a cascode type arrangement. If you look at the picture of the harmonic percolator that Chuck Collins has on his website, you can see that Q1 is different in size and shape and you can definitely tell that the emitter tab is connected to the second transistor. You can also see that the traces under the board match the ones from the harmony central thread except the holes are drilled differently to support the 220K resistor connecting to the other side of the transistor. Amongst several other things.
I'm pretty sure that the schematic that is shown on Barge Concepts webpage is correct for the Harmony Central thread's pictures.
--john
Surprise surprise! I did and it did!
Ha! It's been so long...
I can't find the one I built before so I just made another. I used a BC549C for Q2.
For Q1 it worked best with a silicon NPN with E to the 1K/20K/(I used 1uf film) junction.
For laughs, I threw a PNP 2SB187 into Q1 with C to that 1K/20K/1uf junction. Not quite as much growl, but it worked fine. The tranny was just a handy pull knocking around a part drawer.
I'm with John on what was in the photos, and the 2 NPN is what I would run with if I was going to put this in a box. I like fuzz and weird stuff, so this thing does not make the cut for me.
Quote from: Dan N on April 05, 2007, 10:00:38 PM
Surprise surprise! I did and it did!
Ha! It's been so long...
I can't find the one I built before so I just made another. I used a BC549C for Q2.
For Q1 it worked best with a silicon NPN with E to the 1K/20K/(I used 1uf film) junction.
For laughs, I threw a PNP 2SB187 into Q1 with C to that 1K/20K/1uf junction. Not quite as much growl, but it worked fine. The tranny was just a handy pull knocking around a part drawer.
I'm with John on what was in the photos, and the 2 NPN is what I would run with if I was going to put this in a box. I like fuzz and weird stuff, so this thing does not make the cut for me.
Heh, I guess I should retract the "I would be surprised if it worked" especially after the discussions of using transistors 'upside down'. It was just a wrong thing to say. But the fact that it didn't have as much 'growl' falls in line with what is expected when you use a transistor with emitter and collector swapped.
--john
I am really confused now. Just to clarify.
Both of those schematics posted above are correct? Which on should I use.
What tranny setup am I going for? NPN/NPN? PNP/NPN? What trannies (part numbers) do you suggest?
Thanks guys,
Matt
Thanks John I had not seen that schematic. Could be like 4 versions if all the schematics are correct ???
Matt, I'm sure I have MartyB's layout around here someware. Pretty busy this weekend (family comming to my place) but If you want it I'll see if I can dig it up for you.
Thanks Dan does yours sound like the clips at HC?
Q1 low gain?? NPN??
Q2 med to high gain NPN 2N3904, BC549, 2N5088, etc.
I planned on building MartyB's or Dan's version but never got around to it. I have built both the Jerkulator (with clipping diodes added) and the Speculator AKA The Brick. Unfortunately I don't have either anymore, one of them I gave away and the other I tore apart. Both sounded okay but I don't recall either of them sounding like the clips at HC.
Richard
Quote from: mattpocket on April 06, 2007, 04:12:31 AM
I am really confused now. Just to clarify.
Both of those schematics posted above are correct? Which on should I use.
What tranny setup am I going for? NPN/NPN? PNP/NPN? What trannies (part numbers) do you suggest?
Thanks guys,
Matt
I think version 2 is the one you want, but without the input diode. The input cap to ground is 100pF and the input coupling cap is .05uF. 90K should be 82K (or 91K). R6 should be something like 4.7K.
I think the schematic on Barge Concepts is how it was originally designed. Only because the original description of the Percolator describes it as being very low currenthttp://pages.prodigy.net/chuckcollins/harmonic.jpg (http://pages.prodigy.net/chuckcollins/harmonic.jpg). So a common method of reducing current consumption but have 2 transistor stages is to stack them so they share the same bias current. Make the junction between them AC ground and you have an input emitter follower and a output buffer with gain. Many VCOs are constructed this way. The bottom transistor is a colpitts VCO and the top transistor is a buffer with gain. The stack uses the same amount of current as just one device. The designer probably thought the original design was lacking in gain so he changed the bottom transistor to a gain stage but did it in such a way that he could use the same PCB artwork. Just had to drill a couple of holes differently.
Compare the traces (visible through the PCB) of this photo:
http://pages.prodigy.net/chuckcollins/oldpercolator.jpg (http://pages.prodigy.net/chuckcollins/oldpercolator.jpg)
And this one:
http://i14.photobucket.com/albums/a333/mbet/CIMG3329.jpg (http://i14.photobucket.com/albums/a333/mbet/CIMG3329.jpg)
You can see the traces under the board are the same but the components around Q1 are different.
--john
It is pretty tricky. It is an NPN and PNP gain stage, both are common emitter, with the common point for both emitters set as an AC ground with that big cap. Given the variances of transistors and all that, they were definitely tweaked to get the biases right. I have seen pictures of the insides of one on HC, and it was all sorts of whack components (glass encased resistors for instance). Cool circuit, could be tweaked to be repeatable, but it should work if you play with the bias resistors to get the voltages ballpark.
Isn't The BRICK the HP in disguise?
John, thanks for the link to that photo. Besides a headache, I get somethng like this (reworked Barge schem):
http://users.rio.com/senorris/junk/notanotherschem.gif
Richard, mine does not seem to break up as much as the clip. The first one I built was much closer.
Chris, maybe he was tweaking values as he went. I suspect he was, eh, "different".
Today I tried different transistors, and pretty much kept getting a rather fat distortion. Q1 being PNP ge or NPN si did not make a whole lot of difference. One note, PNP si did not work at all.
The dc circuit is very simple. As I said before, placing trimpots for the 4 resistors is the way to go. Then use whatever transistor you want, but 2 germaniums are better.
mac
I have a couple dozen 2n531 germanium transistors lying around. They're actually two transistors -an AC127 and an AC125 - paired in the same case, with five legs. I have always intended to use one to build a Harmonic Percolator. Problem is, I can't work out the pinout...the diode testing routine gives me nothing. If anyone can help me out with the pinout, I'd certainly be happy to part with a few.
Chris
2N 531 is a single transistor in a TO-5 case -with only 3 legs.
Maybe there is a little mistake with the numbers ?
analogguru
Quote from: markm on April 06, 2007, 12:06:37 PM
Isn't The BRICK the HP in disguise?
It's Doug's Harmonic Speculator which is his take on the HP.
Could be wrong but I seem to recall he was getting some grief over the name so he changed it to The Brick because of the way the finish on the enclosure turned out.
I found a few notes I made when I built
Doug's version.
To my ears it sounded better with a low gain PNP GE and High(ish) gain NPN SI.
Don't know what PNP I used but the Hfe was only 36 and the NPN was a 2N5089 Hfe 591.
Really vague about the tone but I wrote that it was a nice OD to about 1/2; Distortion 1/2 to 3/4; 3/4 and up fuzz.
2n531 spec sheet:
http://www.datasheetarchive.com/search.php?q=2N531
Five legs. Two transistors. Paul Perry told me they were probably used in portable radios...
Chris
Well, I told the dude I'd build him one, so I guess its time to order a million and one trannies and hit the breadboard!
See how things go.
These things are two knobbers aren't they? Do you think it would be worthwhile adding a tone control of some description?
Matt
intersting:
My transistor-databook from 1981 says:
2N 531: Ge-PNP 15V/25mA/100mW ß: 25-30/1mA fg: 3,5 MHz Case: TO-5 (standard pinout)
Manufacturers: CSR-Industries, Electronic Transistor Corp.
My transistor-databook from 1968 says:
2N 531: Ge-junktion (planar) 15V/..mA/100mW ß:25-30/1mA fg: 1,5 MHz Case: TO-9 (similar to TO-5)
Manufacturer: General Transistor (Division of General Instruments)
hmmm....also the TO-9 case is known for maximum 4 legs...
Do you have a picture of this transistor ?
analogguru
Here's my take:
- Definitely at least two versions.
- For the second version Dan's "version 2" mark-up of Alphonso's circuit looks correct (after tracing against the PCB). The clear things look like resistors close-up
The 101 cap is 100pF and the 503 cap is 50nF. The only thing I would question is the 220k resistor which could just as well be another 737k. The top transistor is an NPN silicon (say Alphonso's 2n3545) and the bottom is a PNP germanium (say Alphonso's 2N404A).
The circuit to me looks fairly straight forward. The large 47uF cap forms a low impedance DC source. The bottom transistor, Q1, forms a CE amplifier with a bypassed emitter. The top transistor, Q2, forms a CE amplifier with a bypassed emitter. The output from the Q1 feeds the input of the Q2. The result is a two stage cascade amplifier without feedback.
The two transistors are biased at the same collector current due to the series connection.
The "manual" quoted supply current is ~50uA, which puts the collector of Q2 at about 9V - 50uA*91k = 4.45V, and the collector of Q1 at about 20k * 50uA = 1V. The biasing of Q2 is fairly stable being a feedback bias. Assuming the gain of Q2 is about 100 at the low currents we are dealing with the base current of Q2 will be 50uA/100 = 500nA and the voltage drop across the 737k will be 737k * 500nA = 0.37V. That means the transistor is bias with a very low CE voltage. The base will sit at 4.45V - 0.37V =4.08V and the emitter will sit at 4.08V -0.6V = 3.48V.
Now the biasing of Q1 implies that the GE transistor is leakage is the primary source of base current for Q1. Remember the collector current is small here 50uA so the base current will be small and the leakage could easily bias the transistor. The connection of the unknown 737k/220k resistor between Q1's B and E implies that some leakage current must be shunted away - this makes the biasing dependent on the transistor, which is bad for reliable circuit builds. The resistor connection in Alphonso's circuit between C and B makes more sense when Q1 isn't leaky, and here Q1's CE voltage will be quite low. There's no point guestimating the leakage but I suspect the circuit may pull a little more than the assumed 50uA and that the collector of Q2 is a little lower than 4.5V in both versions of the circuit.
It appears both Q1 and Q2 are biased with very low CE voltages (which makes the circuit less sensitive to biasing issues).
(Forgot to mention, the 2u2 + 1k network lowers off the gain of the first stage quite a bit above 70Hz or so, pretty much all guitar frequencies. The other circuit with the 2x100nF's would appear to have more gain.)
I got curious too now and gave it a try on breadboard, fed in a sine wave and ckecked the output with a software spectrum analyzer.
I used the Barge schem which should be indentical to Dan's version 2 mark-up of Alphonso's schem (besides Q1).
Si PNP as Q1 didn't work regardless of the orientation. A low leakage Ge PNP (AC128, hfe about 80) as Q1 like in Dan's/Alphonso's schem worked.
The only way of getting approximately the harmonics as indicated in the data sheet of the Percolator was with a NPN as Q1 like in the Barge schem. Also output levels were higher than with the AC128. Also I prefer the sound with the two NPN's.
I tried different NPN's (2N2222, 2N2369 and 2N3904) but there wasn't a real difference.
Don't know how it is supposed to work but with two NPN's the harmonics seem to be OK and the sound with guitar too.....
FWIW......
Markus
> Si PNP as Q1 didn't work regardless of the orientation. A low leakage Ge PNP (AC128, hfe about 80) as Q1 like in Dan's/Alphonso's schem worked.
That makes sense. The mark-up circuit can *only* work if leakage is present so it can never work with Si PNP's; also if a PNP doen't work it is unlikely to work in the reverse direction.
>The only way of getting approximately the harmonics as indicated in the data sheet of the Percolator was with a NPN as Q1 like in the Barge schem.
The Barge version (2xNPN) has got to work, at least to some extent, whether it is the original is another matter! In fact the equal transistors seems to point even more towards the clear C-B resistor being the same value. I had another look at the PCB's, in the image CIMG3335.jpg you can just make out the orientation of the transistors and it does imply the Barge circuit is correct for that PCB. The two transistors look the same but it's hard to be sure.
The thing that is crazy about the barge version is the 1k + 2u2 is loading an emitter follower (ie. a buffer) it doesn't really make sense to do that. I can only guess that someone fiddled around until they got a version that worked!
The pics corresponding to the other circuit show different looking transistors for Q1 and Q2, so I guess that version does use PNP.
As far as the PCB pics go there's two working circuits: barge NPN+NPN and the PNP+NPN with the 2x100nF (circuit not entire clear yet). The Alphonso/Dan NPN+Leaky PNP in ckt works but there's no PCB pic for that version.
QuoteThat makes sense. The mark-up circuit can *only* work if leakage is present so it can never work with Si PNP's; also if a PNP doen't work it is unlikely to work in the reverse direction.
I have to admit it was just trial and error. ;)
So I put in trannies for Q1 and checked if there is any orientation in which they worked.
QuoteIn fact the equal transistors seems to point even more towards the clear C-B resistor being the same value.
Will give it a try with two 1M trim pots. Maybe the harmonics change?
QuoteI had another look at the PCB's, in the image CIMG3335.jpg you can just make out the orientation of the transistors and it does imply the Barge circuit is correct for that PCB.
Thanks for that hint! :) I missed that pic...
QuoteI can only guess that someone fiddled around until they got a version that worked!
Sounds like my approach :icon_biggrin:
Quote
That makes sense. The mark-up circuit can *only* work if leakage is present so it can never work with Si PNP's; also if a PNP doen't work it is unlikely to work in the reverse direction.
When I bradboarded the HP I found that a pnp 2sa49 leaking 20uA did not worked as well as a pnp 2sa101 leaking 100-1500uA, both with hfe around 50.
And a npn 2n388 leaking 30uA worked better than a npn 2sd352 leaking 150uA, both hfe near 50.
Moderate leakage for pnp, and none to very low leakage for the npn if Ge.My all Ge did not have the final gain of the (pnp Ge low gain, npn Si high gain) pair, but sounded more defined, fat and fuzzy.
mac
Interesting commercial development on the Percolater:
http://cgi.ebay.com/Harmonic-Percolator-INTERFAX-exact-duplicate-HP-1_W0QQitemZ110112284343QQihZ001QQcategoryZ41416QQssPageNameZWDVWQQrdZ1QQcmdZViewItem (http://cgi.ebay.com/Harmonic-Percolator-INTERFAX-exact-duplicate-HP-1_W0QQitemZ110112284343QQihZ001QQcategoryZ41416QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)
Quote from: dano12 on April 07, 2007, 12:11:31 PM
Interesting commercial development on the Percolater:
http://cgi.ebay.com/Harmonic-Percolator-INTERFAX-exact-duplicate-HP-1_W0QQitemZ110112284343QQihZ001QQcategoryZ41416QQssPageNameZWDVWQQrdZ1QQcmdZViewItem (http://cgi.ebay.com/Harmonic-Percolator-INTERFAX-exact-duplicate-HP-1_W0QQitemZ110112284343QQihZ001QQcategoryZ41416QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)
That's this guy:
http://pages.prodigy.net/chuckcollins/percolator.html (http://pages.prodigy.net/chuckcollins/percolator.html)
nothing new.
Quote from: George Giblet on April 07, 2007, 11:23:51 AM
> Si PNP as Q1 didn't work regardless of the orientation. A low leakage Ge PNP (AC128, hfe about 80) as Q1 like in Dan's/Alphonso's schem worked.
That makes sense. The mark-up circuit can *only* work if leakage is present so it can never work with Si PNP's; also if a PNP doen't work it is unlikely to work in the reverse direction.
>The only way of getting approximately the harmonics as indicated in the data sheet of the Percolator was with a NPN as Q1 like in the Barge schem.
The Barge version (2xNPN) has got to work, at least to some extent, whether it is the original is another matter! In fact the equal transistors seems to point even more towards the clear C-B resistor being the same value. I had another look at the PCB's, in the image CIMG3335.jpg you can just make out the orientation of the transistors and it does imply the Barge circuit is correct for that PCB. The two transistors look the same but it's hard to be sure.
The thing that is crazy about the barge version is the 1k + 2u2 is loading an emitter follower (ie. a buffer) it doesn't really make sense to do that. I can only guess that someone fiddled around until they got a version that worked!
Not really that 'crazy'. It provides some high frequency filtering. I honestly don't think an emitter follower would find a 1K + 2.2uF as much of a load.
Quote from: George Giblet on April 07, 2007, 11:23:51 AM
The pics corresponding to the other circuit show different looking transistors for Q1 and Q2, so I guess that version does use PNP.
As far as the PCB pics go there's two working circuits: barge NPN+NPN and the PNP+NPN with the 2x100nF (circuit not entire clear yet). The Alphonso/Dan NPN+Leaky PNP in ckt works but there's no PCB pic for that version.
From what I have seen, the barge NPN+NPN and the Chuck Collins PNP+NPN are the only two circuits that have been verified. All other schematics I have seen were drawn in error. IMHO.
--john
> Not really that 'crazy'. It provides some high frequency filtering. I honestly don't think an emitter follower would find a 1K + 2.2uF as much of a load.
There's no doubt it will drive it but from a design point of view it just doesn't look right. At 50uA, re for Q1 will be about 500ohms and that means the output network will form a divider of x 0.66 at high frequencies. With the 2u2 cap the shelf EQ will start boosting frequencies at about 70Hz and shelf off at about 50Hz ie a bass boost. They are wierd frequencies and a buffer stage with a gain loss in a distortion looks a little counter productive too - of course it's only a best guess.
The circuit 'notanotherschem.gif' corresponding to the 'oldpercolator.jpg', with the NPN + PNP is starting to look more like it. The only thing wrong there is the missing parts (and is the 10k resistor in series with the diode correct?). It seems that's the one Chuck Collins is pushing. On that one I'm sure the PNP and NPN emitters go together; you can see the emitter tag on the PNP. Sensible R1 and R4 could be worked out but that's not going to make it 'original' though.
Now what about those traces by Alphonso? where do they fit in?(!!!)
Quote from: George Giblet on April 07, 2007, 01:09:17 PM
> Not really that 'crazy'. It provides some high frequency filtering. I honestly don't think an emitter follower would find a 1K + 2.2uF as much of a load.
There's no doubt it will drive it but from a design point of view it just doesn't look right. At 50uA, re for Q1 will be about 500ohms and that means the output network will form a divider of x 0.66 at high frequencies. With the 2u2 cap the shelf EQ will start boosting frequencies at about 70Hz and shelf off at about 50Hz ie a bass boost. They are wierd frequencies and a buffer stage with a gain loss in a distortion looks a little counter productive too - of course it's only a best guess.
So maybe a bass boost is what he was going after. Impossible to determine what his motives were, but it just doesn't seem -that- unusual to me.
Quote from: George Giblet on April 07, 2007, 01:09:17 PM
The circuit 'notanotherschem.gif' corresponding to the 'oldpercolator.jpg', with the NPN + PNP is starting to look more like it. The only thing wrong there is the missing parts (and is the 10k resistor in series with the diode correct?). It seems that's the one Chuck Collins is pushing. On that one I'm sure the PNP and NPN emitters go together; you can see the emitter tag on the PNP. Sensible R1 and R4 could be worked out but that's not going to make it 'original' though.
Now what about those traces by Alphonso? where do they fit in?(!!!)
Version 2 of Aphonso's schematics is almost identical to the Chuck Collin's picture. So I believe that one. The 10K resistor in series with the diode is in the CIMG3335.jpg picture. So it is probably correct but since that resistor is different in the oldpercolator.jpg pic I have a feeling that it varied quite a bit over the life of the pedal. I doubt that the 220K went to ground as shown in Aphonso's version 1 schematic. If you connect that 220K to the collector of Q1 and make Q1 an NPN, you have the Barge schematic. The 2.7K is probably not correct but it really wouldn't matter much except for current consumption.
I didn't see 'notanotherschem.gif' anywhere. So I can't comment on it.
Two versions? Could it be that they switched over to a two NPN (si) design later to rid the effect of issues with thermal runaway (ge transistor)?
Quote from: vanessa on April 07, 2007, 02:18:30 PM
Two versions? Could it be that they switched over to a two NPN (si) design later to rid the effect of issues with thermal runaway (ge transistor)?
If I had to guess I'd say the two NPN version came first because the board is laid out for it. The board with the Ge PNP has traces for the collector feedback resistor but isn't drilled for it. It also has the pads for the input diode and isn't drilled for it. The 220K resistor uses one of the pads that was for the 1K + 2.2uF. The other pads for these parts are there but not drilled.
--john
> I have a feeling that it varied quite a bit over the life of the pedal.
Yes, it definitely looks that way.
> I didn't see 'notanotherschem.gif' anywhere. So I can't comment on it.
It's basically this one. I've marked up all the corrections. It is now complete and should at least work.
http://www.geocities.com/george_giblet/effects/percolator_later.png
It does look like this version has patched components onto an old PCB so I agree with you that this is probably a later incarnation.
I think the barge version is correct for that (earlier) version.
http://www.bargeconcepts.com/bp/largeschem.bmp
Quote from: George Giblet on April 07, 2007, 09:46:26 PM
http://www.geocities.com/george_giblet/effects/percolator_later.png
In my opinion, your schematic is -the- schematic for what everyone considers the harmonic percolator. You should put your name and a copyright notice on it. ;)
--john
> You should put your name and a copyright notice on it.
I purposely left it off! I can't take credit for it too many people have contribute in some way shape or form - including yourself (thanks for the comments by the way)!
O.K., I pulled and switched parts to match the later schematic. I left the 20K and used film for the 0.1's. To my ears, it kills the earlier version. Sleeping people keep me from further testing.
Way to go, guys! I believe we got us a Percolator!
edit- Again...
Quote from: George Giblet on April 07, 2007, 10:17:17 PM
> You should put your name and a copyright notice on it.
I purposely left it off! I can't take credit for it too many people have contribute in some way shape or form - including yourself (thanks for the comments by the way)!
Seriously, I think you should. I don't have any problem with it. You made the effort to commit it to schematic. I was too lazy to do it. So you should claim credit for what you publish. It is that simple. If I actually -wanted- any credit, I would have published a schematic myself.
However, if you would think it necessary to give me credit in your 'copyrighted' schematic, I would be honored.
I think this is the true 'goal' behind DIY. It is just to receive credit where credit is due. Nothing more.
My opinion.
--john
I wonder what would happen if the input were inverted, incoming signal to q2 base and from its collector the 100nf to q1 base.
mac
Quote from: mac on April 08, 2007, 12:32:50 AM
I wonder what would happen if the input were inverted, incoming signal to q2 base and from its collector the 100nf to q1 base.
mac
uh, wut?
OK, John you have twisted my arm.
Here's an update with credits. I've also cleaned a few things up and put the control names on the schematic.
http://www.geocities.com/george_giblet/effects/percolator_later_rev1_1.png
(I will pull the uncredited schematic).
> O.K., I pulled and switched parts to match the later schematic.
Cool Dan, thanks for testing it out.
One idea which kind of detracts from getting the schematic right: I suspect the unit will sound OK with a silicon PNP, it should work although tweaking the 220k on Q1 might improve the sound.
I did some simulations to check the bias points, gains and general behaviour:
- The circuit behaviour is fairly independent of the 220k resistor at Q1 C-B.
- 10k to 20k on Q1's Collector seems to maximize the gain. So the 20k looks pretty good from that respect.
This resistor does change the swing to some extent so it will change the clipping characteristic and sound.
- The cap between Q1-C and Q2-B tunes the low frequency response. LF Cutoff is currently around 120Hz.
- The circuit seems to hold-up as it stands with both silicon and germanium transistor in the Q1 position.
- Biasing looks quite stable. I wouldn't be too worried about changing the 750k to 680k or 820k.
Haven't done any side by side comparisons of Ge vs Si in the Q1 position yet.
QuoteHere's an update with credits. I've also cleaned a few things up and put the control names on the schematic.
Thanks a lot!! :)
I compared both versions on breadboard. The 2x NPN with two 2N2222's and the PNP/NPN with different trannies for Q1 and again a 2N2222 for Q2.
Again a bit trial and error.....
A AC128 (hfe 70) or AC188 (hfe 150) for Q1 defintely sounds better for me than a 2N2904 (hfe 75).
Interestingly, with both the AC128 and the AC188 for me it sounds better if the trannie is used the other way round (i.e. emitter to 20k, collector to emitter of Q2). Don't know if it makes any sense to use it the other way round ;)
Changing the 20k to higher or lower values didn't really change the sound.
Comparing the two versions it's hard to say which one sounds better.....
Regards,
Markus
Thanks Guys!
I get up much earlier than anyone else in the house so I whipped up a layout and built one this morning. I used perf but followed this PCB. http://aronnelson.com/gallery/albums/album133/Harmonic_Perculator.gif
I have not had much time to experiment with it and probably won't until next weekend. Sounds good so far though.
Here is what I used.
Q1 - 2N404A Hfe - 76
Q2 - 2N5088 Hfe - 486
D1 & D2 - 1N60
R4 - 22K
Had to make the 91K and 750K came up with 90.8K and 750.1K. Can't get much closer than that :icon_biggrin:
No 50K pots so I used the closest I had an oddball 30K . Shouldn't matter much with this circuit right?
QuoteHaven't done any side by side comparisons of Ge vs Si in the Q1 position yet.
I've done it, but it seems that no one read my post ???
mac
Quote from: mac on April 08, 2007, 03:12:16 PMI've done it, but it seems that no one read my post ???
I think we all just have to try it ourselves. DIY fun and games...
>I've done it, but it seems that no one read my post
I read the link to your old post,
http://www.diystompboxes.com/smfforum/index.php?topic=49035.0
but I thought you were changing Q2 with Si and Ge?
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 transistors (only speculating).
I noticed you played with the diodes too - that's another area to try!
One interesting thing is Alphonso's V1 has a low R1 (on his circuit) and V2 has a high R1. The new version I put up is in between these.
I havent made any circuits that needed biasing before...
What does it mean?
When I have seen it on other circuits, ge fuzz's and the like, it has been done by tweaking a trimpot to change the resistance between two points. What does it normaly involve? Something across the tranny or something? Enlighten me please!
I had a quick look at the schem and I cant see a trim pot is it set with the 220k resistor across the tranny?
Matt
Quote from: mattpocket on April 09, 2007, 05:41:20 AM
I havent made any circuits that needed biasing before...
What does it mean?
When I have seen it on other circuits, ge fuzz's and the like, it has been done by tweaking a trimpot to change the resistance between two points. What does it normaly involve? Something across the tranny or something? Enlighten me please!
I had a quick look at the schem and I cant see a trim pot is it set with the 220k resistor across the tranny?
Matt
Biasing is setting up a network of resistors to make the power run the right way thru a transistor so it will work. Different types of transistors have different biasing req's.
You should probly read these articles.
http://www.diystompboxes.com/smfforum/index.php?topic=34611 (http://www.diystompboxes.com/smfforum/index.php?topic=34611)
http://www.diystompboxes.com/smfforum/index.php?topic=48578.msg360118\#msg360118 (http://www.diystompboxes.com/smfforum/index.php?topic=48578.msg360118%5C#msg360118)
Biasing is simply the setting of the DC conditions of a circuit. The DC conditions is the DC voltages and DC currents in the circuit. When people talk about biasing they are usually focussed on one particular aspect. For example in a tube amp, biasing often refers to the DC current in the output tubes. However depending on the discussion biasing can refer to the DC voltages on the plates of the preamp tube. There are of course other DC voltages and currents elsewhere in the circuit, and these are bias currents and voltages, but they aren't of primary interest. In effects biasing often means the adjusting something in the circuit to get a particular (desired/wanted) voltage on the collector of a transistor or drain of a JFET. Sometimes you see people talk about adjusting or changing the biasing of a circuit, all that means is you are changing *something* which will affect the DC conditions in the circuit - eg collector voltage or collector current.
Here's a few examples of transistor circuits to show how playing with the parts can affect the biasing and behaviour of the circuit.
http://www.geocities.com/george_giblet/effects/bias_example.png
In the rightmost circuit the 470k resistor is changed to 220k which causes the transistor collector current to increase and the collector voltage to drop from 4.7V to 3.6V. The gain of the circuit just happens to stay the same as the leftmost circuit.
In the bottom circuit the 4.7k resistor has been increased to 8.2k. The collector voltage is about the same as the rightmost circuit. So the "collector bias voltage" is the same. However changing the bias in this way also changed the gain of the circuit.
What all this means is:
- Biasing is affected by the various parts of the circuit.
- The behaviour of a circuit depends on all the parts even though a particular aspect of biasing is the same. In the bottom circuit the transistor collector current (which can be called a bias current) is lower than the rightmost circuit. The reason the gain changes is *not* because the transistor current is different but the fact that *in that circuit* the gain is determined by the external parts and not the bias conditions.
QuoteThere'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. ;D
mac
>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.
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
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
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
A great thread from Harmony Central:
http://acapella.harmony-central.com/forums/showthread.php?t=1008997&highlight=interfax
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
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.
> 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.
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! :icon_eek:
Matt
Quote from: RLBJR65 on April 05, 2007, 06:54:07 PM
Sorry, I was not aware MartyB took his stuff down :icon_sad:
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 :icon_eek: I neither have the layout creator installed here or will it let me do such a thing...
Matt
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
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
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.
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 (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?
Quote from: alteredsounds on June 27, 2007, 04:19:20 PM
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 (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 (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 (http://www.diystompboxes.com/smfforum/index.php?topic=55987.msg432556#msg432556)
--john
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.
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)?
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
thanks mac!
what does this effect do anyway?
:icon_question: :icon_lol: :icon_lol:
FUZZ!
NONONO!
it "percolates" harmonics. ;D
anyway watch the video clip that was in an earlier post. :)
Distortion/Fuzz.
Lots of highs!
>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)?
I'd say no, this circuit is different to "normal" circuits. Two main reasons: the emitter voltage for the top transistor isn't zero volts so an equivalent to mid supply/4.5V would be (emitter voltage + 9V)/2, and, even that won't work because the transistors are deliberately biased with low emitter to collector voltages.
The sound is unique, it filters harmonics according to the original ad.
Something I'm going to do when I have some free time is to invert the circuit. That is, the input to a Si npn low gain, around 50-70 like a 2n2369, and then a higher gain Ge pnp, gain near 150-200. The output from the pnp collector. Positive ground, or if there is no oscillations negative ground.
Just for fun.
mac
i built this yesterday, it has a really awesome grinding sound. I can't say it's really unique sounding though. I was unable to get the whistling sounds that albini gets in that video from earlier, but i get some oscillation when the harmonics are turns fully clockwise (this unit tends to be fuzzier turning harmonics counter clock wise right?).
i think i'm having problem finding that sweet spot, but i have noticed there are a range of useful sounds when biasing with pots, and the harmonics really do pop out on this thing! i wish i had some kind of recording device so that i could post some of the mayhem i have been creating with this thing!
Anyway, I haven't put any diodes in it yet, i am thinking of making a toggle switch to choose between two sets of diodes, any suggestions?
I think that a rotary switch could do it.
Some ideas:
ge+1k, ge (the standard)
si+ge, si (more volume)
ge+schottky 1n5819 ,ge
none
mac
i built tim's harmonic jerkutator a month or so ago. i didn't add the diodes because of the volume drop. i think it was only the second pedal i had made so i wasn't all that sure of what i was doing not that i am now. anyway i really like it because it is so different than my other fuzzes. i like the turn the fuzz up and the volume to around 10o'clock and use it on top of another fuzz or the amps distortion. it gives my "that guitar sounds like a bass" a nice nasally type of sound for when i want to stand out a bit.
i have only been sneeking peeks at this thread so i guess i should go back and get caught up but, is this going to end with the ultimate hamonic perulator layout/schematic?
I'm sure that there are some layouts and schems in the gallery. Most of what was discussed here is the effect of diodes and transistors bias for the Hermida and Barge versions. My contribution was the all Ge Hermida version, which IMHO sounds a lot better than the Ge-Si one.
Since we talked about breadboarding first using trimpots for the 4 bias resistors to find the sweet spots, all layouts in the gallery can be used. Just replace the trimpots values with the nearest resitors.
Adding a switch to change diodes is easy.
mac
i'm gonna jump into this when i get back from my little "camping" trip. the super hp...
I still havent finished mine! haha
Did anyone find the Si-Ge hermida version to be very harsh and trebley? Mine is. I placed a cap across the diodes to smooth that out, and added a tone control, but it cuts a lot of volume when you use it. Is there a way to boost the volume before the tonestack? Some arrangement with diodes maybe?
Matt
what diodes did you use? I haven't put any in mine yet and all i get is a really thick loud bombastic fuzz! anyway to boost the volume
use an si+ge, ge combination as in mac's earlier post.
You mean add a Si to make assymetric clipping?
I brought a load of different diodes, and just tried them out. The Ge's I used fattened up the sound bass wise, but they farted a littlen and seemed to gate the sound slightly. I tried a fair few different Si's, and I finally settled on 1n914 I think, but dont quote me on that. They were a pretty similar sound to 1n4148, but ever so slighty smoother, without farting out. You can also try putting a cap in parallel with the diodes to smooth it out, it made mine a lot more useable, with a more fuzzy type of response, but still retaining that harmonic grit sound.
Hope it helps!
Matt
yeah an si and ge in series assymetric with a si. if you use more diodes this allows for more output this is explained on the general mods page i think?
Maybe lowering the PNP collector resistor like the barge version may help to boost volume, somewhere between 100k and 22k.
mac
i put the germanium diodes on last night and i think it really does sound like the harmonic percolator in the albini clip. sounds a whole lot more trashy than without the diodes and does seem to accentuate the harmonics further. I think I'm going to leave it just like this!