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This is about getting running 2 transistors in parallel to get low gain.  It turns out I got VERY low gain.

I've been building an all silicon version of Aron's Hornet, which is a fuzzface variant.  Along the lines of RG Keen's suggestions in this forum, I have been looking at "piggybacking" transistors to get different hFEs out of them.  e.g. A while back Javacody wrote "I also read something where RG suggested running two trannies in parallel, but leave the collector of the second tranny unconnected, thereby cutting gain in half. "

Simple.  Or so I thought....  Well, my Hornet had Q1 and Q2 hFEs of 130 and 340, so I piggybacked then with 2 extra tyrannies of hFE = 200 and 340, respectively.  The Hornet was now more of a Wasp, and sounded great.  I bundled it up and sent it off to the guy I was building it for.  Then I started to wonder what I had done to the hFE of those transistors.  A few checks revealed some things that STUNNED the *^%@ out of me.

I found that piggybacking two similar transistors (PN100s, hFE=350 and 430) to those I'd used for Q2 in the Hornet gave a hFE of either 6 or 12, depending on which transistor piggybacked on which.   So piggybacking DIDN'T HALVE THE GAIN.  Also, the Hornet worked great with a hFE at Q2 of about 10. (What!!??)

I thought I must be measuring something wrong, so I used RG's transistor testing circuit to measure the hFEs.  Same result.  I also tried different transistors and got variable, though similar results.  2 BC549Bs with hFEs of 280 gave hFE = 5 when piggybacked, and 2 BC547Bs with hFEs of 450 and 425 gave hFE = 25 when piggybacked.  

RG, if you see this, do you have an explanation?  It seems to me that the current must be preferentially flowing through the base-emitter juction that has the disconnected collector.  Also, if people are always recommending hFEs of between 70 and 130 for fuzzfaces, why did mine sound so good (it sounded REALLY good) when the hFEs were presumably as low as 5 to 25 ????  I'll definately try to get that Hornet back and measure the hFEs.

Anybody got any suggestions about this?  I welcome any/all comments, and encourage other folk to have a look at this and see if they get similar results.

Maybe the HFe test on a multimeter doesn't work right when somethng so unorthodox is put to it? Note that the HFe given by a multimeter isn't really the "small signal AC gain", it is something else, which is USUALLY an indicator. maybe one of the EEs here can comment.

As well as the multimeter, I used RG Keen's transistor testor, which simply puts 4uA through the base-emitter and measures the resultant collector current.  The results were essentially the same.

(... oops... don't know ma own strenth...)

I thought the hfe *would* be about halved. The idea is that an identical junction ought to eat about the same current, stealing base bias and signal current in equal amounts to the active transistor.

I'll do some speculating, then go do some research to find the real answer.

======= Speculation======
A bipolar base-emitter junction behaves as though there is a phantom resistor in series with the emitter. This so-called Shockley resistance shows up as added base-emitter voltage and as a gain determining component in transistor circuits. This much is fact.

Since the dummy transistor is getting only base current, not the base-current-multiplied-by-hfe current in the active transistor, the dummy transistor base-emitter may have a slightly lower junction voltage because the current through its Shockley resistance is much lower and it can therefore steal most of the base current, causing a much lower apparent hfe.
====== End Speculation =====

I'll go do some reading, maybe I can figure out what is really happening.

I'm glad it sounded good. There may be a real pony in there.

I tried this in the Yaff I built this morning
 With 2n2222 in Q1 and NTE103 in Q2, Q2 read 4.76v at collector
 With 2n2222 and 5088 in Q1 and in Q2 an NTE 103, Q2's collector read 5.11v
  The sound of the Fuzz became more refined....lol....not exactly a 'labratory' test, as above noted the two configurations were only ear tested [since this morning] and we're not rebiased between takes.
  I refrain further comment, yet to say, to me
  it seems like a Neat Trick, Very Nice yaFF.
  Sure was easy to get bias on this thing, I put a trimpot on Q2 but haven't even had to move it  the whole time, even with 3904 in Q2 and the dual transistor in Q1 !!!
  It's stuff like this that makes it hard for me to stop building for a minute.

Brett,

Thanks for doing so much experimenting on the Hornet.

I need to try what you did. Sounds interesting.

yeah man, that sounds REALLY interesting.

I did some more experimenting last night, following up RG's idea that there's extra base-emitter resistance in the transistor with all leads connected.

What I did was add an external resistor to the "piggybacking" base-emitter, to "lighten" the piggyback.  I found that if I added about 4k ohms, the hFE of two similar PN100s would be halved when doing the piggyback thing.  I was quite surprised that it was such a high resistance.  I don't know why, but I was expecting maybe 100ohms (perhaps because I was thinking of the base-emitter as a freely conducting diode, but it's obviously not).

Next, I thought I'd see if the hFE-reduction effect depends on the amount of current flowing through the junction.  Sure enough, it does.  I modified RG's transistor tester to do this for a piggybacked pair with a 2k resistor.  With 4uA base current I got hFE=50, with 40uA I got 150 and with 80uA I got 220.  So the hFE is dependent on the base (and/or collector) current.  Next thing I think I'll try is the base-emitter plus resistor (1k?) piggyback method in a fuzzface and see how it sounds.  The variable hFE might have some interesting effects.

That's it for now.  Cheers.

I love this forum!

Neat stuff! It sounds like we have the beginnings of a DIY low-gain transistor which includes all the dirty stuff that modern high gain, high linearity devices don't.

I can't get my simulator to work right, so I'm falling back on reading solid state semiconductor physics texts to try to get a more formal explanation of the phenomena.

QuoteWhat I did was add an external resistor to the "piggybacking" base-emitter, to "lighten" the piggyback. I found that if I added about 4k ohms, the hFE of two similar PN100s would be halved when doing the piggyback thing.

Did you add it to the base side or the emitter side? I would expect same results whether base or emitter as long as the collector remains open, but different results (much more effect when it's in the emitter) if the collector of the dummy transistor gets current to contribute.

QuoteWith 4uA base current I got hFE=50, with 40uA I got 150 and with 80uA I got 220. So the hFE is dependent on the base (and/or collector) current.

That's true of all transistors - hfe is an exponential function of the collector current (neglecting losses and parasitics). That's how a differential multiplier cell works - the two transistors have a lower gain at low collector currents than they do at high ones. I'm surprised at how fast your hfe changes though. Maybe you happened to test right on the knee of the exponential curve for that transistor.

Changing gain with collector current is *good*. That's a fundamental means of generating even-harmonic distortion. Bad old transistors (like early germanium ones) did this a lot more than today's devices, and it is worse at low collector currents - that's why the Rangemaster is sitting near cutoff, it compresses the low-current side of things and expands the high-current side.

QuoteNext thing I think I'll try is the base-emitter plus resistor (1k?) piggyback method in a fuzzface and see how it sounds.

I'll be very interested. If this can be tweaked in and we can deduce the principles behind it to make it predictable, we may have made searching for dinosaur-egg germanium devices a thing of the past.

...OK, OK, I'm getting carried away...

QuoteDid you add it to the base side or the emitter side? I would expect same results whether base or emitter as long as the collector remains open...

Yes, I left the collector open on the piggybacker (Q2) and added the resistor on the emitter side as follows:
Base of Q1----baseQ2|emitterQ2----2k resistor----Emitter of Q1

Wow, we are experiencing history in the making.   :D

I tried two bc109's, one with a gain of 510 and the other with a gain of 486. Combined they gave me gain of 8 and they sound awesome!

i tried 2 bc547c's left the collector and everything in there and noticed a little more "amplike" attack and resonance with no loss of distortion ...nice..did the same to q2, a 2n7000 and it just got a little better. not a real huge improvement but noticable. mismatched a bc547 with a bc338 (low gain) and the sound got kinda honky..had to tweak the trim pot almost every addition or substitution..

R.G. how is this different from using say a 1n4148?

 In a Si rangemaster type circuit I built I picked a tip29 from looking at its very curved Hfe vs Ic.

Good question about the 1N4148 Gus.  I'll experiment with one tonight.  Concerning the slope of the hFE va Ic relationship, using the setup mentioned above (base-emitter and 2k resistor piggybacking on Q1) I measured the following: hFe = 50 @ 0.2mA, 150 @ 6mA and 220 @ 18mA.  The hFE of a TIP29 varies from about 50 @ 1mA to 90 @ 100mA, so it's hard to tell which is more "curvey".  I obviously "missed" an interesting region between 0.2 and 6mA where the hFE is "jumping" significantly.

When I get to the shop and buy some more transistors, I'll measure some hFE and Ic values with smaller piggyback resistors (ie at lower "synthetic" hFEs more useful for Q1 in fuzzfaces).  I am starting to realise that the piggybacked transistors have lots more curvature in their hFE-Ic relationship, and that the very low multimeter hFE values (mostly under 20) may be simply a reflection of low Ic.

Lots of questions and few answers (so far)

I did something like that some time ago (adding a Ge diode in parallel with b-e junction of a Ge transitor to make it more suitable for a FF).

http://www.geocities.com/gfr.geo/britface.html

This is good stuff, guys. :D

After building the Hot Silicon and breadboarding the FT70, (and comparing them to my fuzz face) I decided no more Ge sorting for me thank you when it comes to fuzz boxes. It looks like you may be on the verge of a technique that would make a really nice sounding Si Rangemaster. Maybe I will be able to dump Ge's forever now. Good riddance!! :D

Thanks for all the hard work.

Doug

QuoteR.G. how is this different from using say a 1n4148?

I'm not sure it's different at all, except that with more similar junctions and doping profiles (that is, more similar than a 7V junction that's deliberately as thin as possible instead of the 100V, gold doped 1N4148 junction) the curve of junction voltage versus junction current is more likely to interact a lot, which is what is causing the effect... I think.

If one junction is a lot lower voltage (say, 25 mv) than the other at some current, it will steal most of the current, you might not get much effect. I'm speculating again.

Quotedid the same to q2, a 2n7000 and it just got a little better. not a real huge improvement but noticable.

The 2N7000 is a MOSFET. I would not expect nearly the same results from a MOSFET because of the fundamentally different structure.

QuoteYes, I left the collector open on the piggybacker (Q2) and added the resistor on the emitter side as follows:
Base of Q1----baseQ2|emitterQ2----2k resistor----Emitter of Q1

OK. That's how you want it for experimentation. Try connecting the collector to 9V through a 10K-47K or so and see what changes in the sound.

Hey! Fun stuff!  I think I may have just obsoleted my private reserve of germanium devices  :shock:

======= Speculation======
A bipolar base-emitter junction behaves as though there is a phantom resistor in series with the emitter. This so-called Shockley resistance shows up as added base-emitter voltage and as a gain determining component in transistor circuits. This much is fact.

Since the dummy transistor is getting only base current, not the base-current-multiplied-by-hfe current in the active transistor, the dummy transistor base-emitter may have a slightly lower junction voltage because the current through its Shockley resistance is much lower and it can therefore steal most of the base current, causing a much lower apparent hfe.
====== End Speculation =====


I'll concur on that one, RG. Paralleled transistors are common in audio power amps, and disconnecting the collector of one will cause the Vbe to drop relative to the parallel units and "suck" the drive current (from the previous stage). This often damages the Base-emitter junction of the transistor. Of course, the available currents are very high compared to the stompbox application...

It also follows that if you used some small unbypassed emitter resistors (seperate one on each transistor), the beta would approach the average beta of the transistors (as it does in a power amp).

Quote from: Brett ClarkIt also follows that if you used some small unbypassed emitter resistors (seperate one on each transistor), the beta would approach the average beta of the transistors (as it does in a power amp).

I definitely think that you are on to something here, inserting equal value resistors on both emitters would would overwhelm the difference in the Shockley resistance and equalize the b-e junction voltage. Isn't this similar to the equal value resistors normally used on the emitters in differential amplifiers?

This is a great discussion! Though at some point, someone is going to have to draw a picture  :) .

Jay

Just to show how stupid I am, how about mixing a 2N3904 with a 2N7000?  

Glad I'm finishing up one, it looks like I'll be going back to the Tone Bender.   8)

QuoteJust to show how stupid I am, how about mixing a 2N3904 with a 2N7000?

You could probably figure out a way to do something that works with these two but it won't work like we are talking about here. A MOSFET doesn't draw any current through its gate, so if you attached it like we are talking about here it wouldn't have any effect at all, it would be like adding a teraohm resistor from the base to the emitter of the 2N3904, nothing would change.

If I understand the discussion here correctly, we are talking about using a second transistor whose base is connected to the first transistors base, and whose emitter is connected to the first transistors emitter, but whose collector is left floating. Then this two transistor combo is used as a single transistor in a circuit.

Aron - what can you tell us about this "Incredible Stompbox Index"? What goes here vs. elsewhere?

BTW, thanks as always for this wonderful forum...

Guys,

This thread is VERY COOL!

I've made a new forum - the iSi (Incredible Stompbox Index) that will house interesting discussions.

This one is the first since it's pretty interesting. It doesn't really affect the thread although it says "moved".

If there are other very interesting discussions you guys want in there, let me know. These would be live discussion, not FAQ type ones.

Thanks!!!!!!!

there must be some magig in emitter resitors, especially in differential amps. Transient intermodulation distortion was big debate some years ago, when one Finnish bloke discovered it (M. Otala, he was in the team that designed Citation CXX power amp (costed more than a Mercedes Benz car)). After that some finnish electronic mags suggested tips how to make your transistor amps sound better.

By adding those emitter resistors

But that was slightly off topic.

Thanks Jay, I knew that.   :D   I just plug the 3904's and 7000's into the socket the same direction and it works, so I thought...  

Guess I will try a 3904 and 5088.   8)

RG, have you had a chance to test this out yet?

I cannot believe how much better your original suggestion sounds. Now I have to try the resistor thing. I'm not able to picture in my mind though how I connect the resistor?

Does the resistor go between the two emitters?

Quote from: javacodyDoes the resistor go between the two emitters?

In my mind the bases would be connected together, each emitter would have it's own resistor and the other ends of the resistors would be connected together forming the "artificial emitter", and one of the collectors would be left floating.

Remember in the end you have a circuit that will have three terminals just like a BJT and be used as such.

:mrgreen: I just discovered few cool names for pedals that can made if this works :mrgreen:

I think you can name a fuzzface "Miss Piggy" and paint it pink

If I make a Rangemaster I think it can be R.G. Master

:oops: I need a holiday

Quote from: Nasse
I think you can name a fuzzface "Miss Piggy" and paint it pink

You could even buy one of those rubber pignose things from a store that sells masks and halloween costumes etc. and glue it on your pedal. :)

Hi.  Concerning javacody's question about connecting the resistor: Yes, I simply connected it between the two emitters.  Values of between 1k and 10k gave interesting results (1k=lower gain, more piggyback, 10k = higher gain, less piggyback).

Here's a layout for a "Miss Piggy" (thanks Nasse for the cute name), a  Fuzzface with the usual trimmers plus piggybacked trannies for Q1 and Q2:
(http://members.optusnet.com.au/~jethro.dog/mypic105.jpg)

Also, Javacody wrote:

QuoteI cannot believe how much better your original suggestion sounds. Now I have to try the resistor thing.

Thanks for the compliment.  It's good to know that someone else has come to the same conclusion (ie that it sounds very good).

Last night I built a Fuzzface (as per the Miss Piggy, above), and piggybacked (no resistor) 2 x BC547s for Q1,Q2 (synthetic hFE=10) and 2 x BC549Bs for Q3,4 (with no resistor, hFE = 4, with 1k hFE = 7 and with 3.3k hFE = 17).  With zero resistance between Q3 and Q4 the gain was too low, and the fuzz was very soft, but with either 1k or 3.3k between Q3 and Q4 it sounded excellent.  I definately think that a low-gain Si fuzzface is going to find a place on guitarists' pedalboards.

Lastly, I also tried Gus' suggestion of using a 1N4148 diode in place of the piggybacked b-e junction.  In the cases where I tried it, the diode sucked all of the current and the hFE was 0.  Like RG, I figure that having a similar junction in parallel probably increases the chances of the two junctions sharing current in a useful manner.

as soon as i saw the dave barber op amp stacking the same day i was was doing it w/ trannies. i used them in the double bazz fuss and got very pleasing results. my bass plaer loves it. i tried so many combo's that the possibilites were endless. i don't if all this stuff is sweat and voodoo. but i had fun.
            - tom

Very cool Brett!!!

Thanks for trying the diode.  

How is the noise with this type setup?

Gus

What noise?   :D

By the way, the Miss Piggy is exactly the same setup I used, minus the emitter resistors.  I'm very happy with my "compound" trannies gain of 8 and 10 respectively. Sounds amazingly organic.

Concerning noise; I haven't really turned the amp up yet, but there's a couple of things noticeable:
I suspect that noise will be as low or lower than a standard fuzz face, maybe not because the circuit is less noisey, but because with lower gain, there's less amplification of pickup noise, etc.
Second, and probably most importantly, if you count harsh harmonics as noise, then I can definately say that there's a difference (I think better) with the piggybacked transistors.  For example, my test signal at 1kHz gives some fairly unpleasant high-pitched harmonics if using 2 regular high-gain Si trannies in a fuzzface.  This disappears with the piggybacked arrangement.  Instead of almost vertical slopes on the trace, I get a recognisable slope.  With hFEs of 10 and 17 I didn't notice any harshness in the sound until the fuzz control was nearly at maximum.  I'm starting to think that much of the magic associated with Germanium might actually be due to low gain.  There's definately a bit of a "clean Germanium (?)" sound to Miss Piggy.

I will second what Brett said. Brett, what is the gain on the trannies you used? The BC109's I used have an average gain of ~490 to 500. There must be a forumula for this? A way to predict what the compound hfe will be?

I wish there was a formula.  But these things seem to vary quite a bit with the type of transistor. I've had 2 BC547s (hFE=392 piggied on a 491) give hFE=9, while 2 BC549Cs of 430 gave 4.  I did get a hFE of 20 out of 2 trannies with hFE=410 and 480, but I forgot to write down what type they were.  It's interesting that you were very happy with such low hFEs.  It has reinforced my view that quite low hFEs are not such a bad thing. e.g. I liked 9 and 7 in my Miss Piggy more or less as much as 9 and 17.  

I'm off to the shop this afternoon to buy about 5 transistors of 5 different types.  If there's a pattern to piggied hFEs, I'd love to work it out. (I'm a scientist by occupation AND habit).

QuoteThere must be a forumula for this? A way to predict what the compound hfe will be

That's what I'm digging through my thirty-years-old semiconductor physics texts for - a way to predict results.  I'm sure there is a way to do this, but what we need is a way to do it that doesn't involve knowing the n-type and p-type doping diffusion constants ..... aaargh!!

Fortunately, we seem to have a good empirical result - stick a 10K trimmer pot set up as a variable resistor between the two emitters; now tune till you hear germanium. 8-)

QuoteI suspect that noise will be as low or lower than a standard fuzz face, maybe not because the circuit is less noisey, but because with lower gain, there's less amplification of pickup noise, etc.

I think there is an even better advantage on noise. The noise generated in a bipolar base-emitter depends on the temperature (same for each), the current density (we're mucking with that - probably dropping it by half) and the net resistance. We're deliberately cutting the resistance quite a bit. There are moving-coil hifi pickups that use paralleled NPN transistors for getting very low effective Rbe. I think something like that may be working here. In any case, we're using glass passivated modern transistors which are much cleaner, and sound it.

Talk about the best of both worlds! Germanium sound and modern transistor availability and low noise.

what about a way to effect the amount of "piggyback" or remove the piggyback completely from the circuit. maybe a trimpot? FWIW i also noticed 2 2n4401's with all leads piggybacked in q1 sounds pretty good at least in the multiface.

Trust me, try it the way that Brett has it and I doubt you will be interested in "removing the piggyback".   :lol:

If you replaced the emitter-bridging resistor between Q3 and Q4 in the Miss Piggy with a 25k pot, you'd have a neat "pig" or "piggy" control that'd take your gain from low to high (high now meaning maybe 50!)  At low gain I suspect you'd get softer, more transparent, organic fuzz, at mid gain you'd get a kind of "clean Germanium" sound, and at high gain you'd get a tighter, more "constricted" fuzz.  The high gain Si fuzzfaces have a reputation for sounding far too hard/tight - resulting in "congested" or "sticky" feel.

Oops.  I forgot that the bias on Q3/Q4 will change if you change its gain with a pot.  I've noticed this effect already when using 0, 1k and 3.3k resistors in the Miss Piggy.  Each resistor needed adjustment of the collector trimmer to get back to 4.5V.  But I wonder if the bias goes "off" by an acceptable or unacceptable amount with a 10 or 25k pot?

i can tell i'm going to have to build one tonight.  did you have to do this when it's friday in new zealand?  :P  i was going to get drunk!!!  *grin*

now for the serious stuff.  say we increase the voltage across the base-emitter of the compound device, what's happening?

i'm going to call the tx with no emitter resistance Q1 and the tx with the emitter resistor Q2.  both Q1 and Q2 have their depletion regions shrinking at the same rate since no current is flowing and it's a function of the voltage.   as soon as some current starts flowing, we have this problem of the emitter resistance.  the BE junction of Q2 and the resistor form a voltage divider - and the current develops voltage across the resistor, which in turn lowers the voltage across the BE of Q2, which thickens the depletion layer of Q2.

meanwhile, we have this voltage at both bases.  as it tries to go up, Q2's 'resistance' drops at a lower rate to Q1's, but without actually affecting the output signal...  so as we turn the compound device on more, we'd expect less and less current to go through Q2 and more and more of it to go through Q1.  you could check it by measuring the voltage across the emitter resistor on Q2, it should go up at a lower rate than the bases do.

so we in fact have a device that has a softer 'turn on' knee but looks the same once you get it turned on.

i might be way out of line but that's my thinking on it.  :)

so if you want to add a "oink" pot (piggyback), you better have a bias pot huh? i am gonna experiment later with some combos..

RG - I've been thinking about the analysis/modeling issue on this. I'm expecting that, once an analysis is complete, that there will be lots of dependence on parameters that are loosely controlled (like hfe itself) or are completely unspecified (like the variation of hfe around cutoff).

If you come up with something good though, I'll be happy to see it!

Also, it may be that SPICE will give nonsense on this - many of the transistor models have a lot of approximations that are generally valid, but not for "fringe cases" like this.

Take Q1 emitter to outside luf of pot [25k? to start with?]
 Take Q2 emitter to other outside lug
 take the wiper to a socketted resistor to wherever it would go [ground?]
 Then you could turn one up and the other down as it were at the same time.
 Go from the 'front' one to the 'back' one...siamese twinzistors

When this thread popped throught the needle...that this concept is panning out in reality is beyond Kool IMO.
 Siamese Twansistors share the pot !!! ]?]

How about putting a capacitor in parallel with that resistor that is placed in between the emitters. Would that make the hfe lower on the higher frequencies? Some other weird FX?

Has anyone actually compared the sound of this arrangement in a FF to a Ge FF?? This is really exciting stuff! I can't wait to try it out!

Brett and R.G., thanks for all the effort!

Doug

:P  :P  :P
I tried that with the easy face I built the other day 2N3906s for Q1, Q2, Q3 and Q4.

For the first gain stages i used a 2k2 emiter and for the second gain stage a 4k7 resistors.
:D
I had the worst time trying to stop playing!!!  :P

Wonderful tones! No noise, easy to bias, great bass response, bell like tones!
I've just started to use Fuzzface (the easy face is my first ever) and I'm addicted to it...

Thanks for making people all around the world more happy about playing a guitar.

Kleber AG

Brett, R.G.

   I would think that using two of the same devices would be more consistant.  Maybe we should look at devices that have a big change if Hfe vs Ic at the current levels we are working at.

  There is an pro microphone pre that uses 8 matched transistors in the input section.  That was done for noise I believe.

  I stopped  building effects for while still draw out designs in my notebooks.  I have been busy building,modding, learning about etc. tube and transistor circuits for condenser microphones.  Look what happens one of the coolest things in years this is as much fun as the ampage time learning about the rangemaster!

Gus

Very cool thread guys!
you've probably designed the next generation FF!
Thanks, Ed R.

How long till Dunlop/whoever cottons on to it?  :D

I have a Fuzz Face (dunlop reissue) that I modded with better Germaniums. The germaniums I used are OC88's (? Can't remember for sure) and are a little higher gain that I would like (100 to 120) and to me, this si Fuzz Face I just made (which is identical in every other way) sounds far better.

I would say you get Germanium sound without Germanium's headaches.

Yeah, I was thinking the same thing gez. I wish RG would patent it so that the big shots wouldn't get a free ride offa this.

Quote from: javacodyI have a Fuzz Face (dunlop reissue) that I modded with better Germaniums. The germaniums I used are OC88's (? Can't remember for sure) and are a little higher gain that I would like (100 to 120) and to me, this si Fuzz Face I just made (which is identical in every other way) sounds far better.

I would say you get Germanium sound without Germanium's headaches.

That sounds really encouraging. Just what I and a bunch of other like-minded people want to hear. :lol:  :lol:

After messing with the FT70 on the breadboard, I realized I do like the si FF sound. But then again, the lower gain transistors sounded the best. This is all very fascinating- stompbox technology history in the making! :D

Doug

Quote from: Kleber AG:P  :P  :P
I tried that with the easy face I built the other day 2N3906s for Q1, Q2, Q3 and Q4.

For the first gain stages i used a 2k2 emiter and for the second gain stage a 4k7 resistors.
:D

I will give that a try, Kleber, thanks!

Quote from: Kleber AG
I had the worst time trying to stop playing!!!  :P

I *hate* it when that happens... :lol:  :lol:  :lol:  :lol:


Doug[/b]

http://www.freewebs.com/nassetronics/emitters.jpg


I had a dull moment this day and drawed something like this on my calendar corner at job. Sorry if someone can not see the image (my lousy "pages"), maybe pasting the address to new window helps. If not, dont worry , it is just maybe useless thingie. It is just something I thought might be something that I can test those emitter resistors (just a trimpot and a dual-ganged one, halves connected across the single pot. There may be somewhat heavy "interaction"

QuoteThis is all very fascinating- stompbox technology history in the making!

VERY VERY EXciting and I am going to try this. I am excited!!!!!!!!

OK, now U guys, please give credit if you start using these ideas.

I have always had a stock of Ge transistors, but man, not needing them will be GREAT!  :D

QuoteWonderful tones! No noise, easy to bias, great bass response, bell like tones!
I've just started to use Fuzzface (the easy face is my first ever) and I'm addicted to it...

Thanks for making people all around the world more happy about playing a guitar.

OH MY!!! Stop it!!!! It's only 11AM here!!!! I can't wait to try it!!!!

The Hornet will get a face lift!!!! :twisted:

I seem to recall in R.G.'s FF article reference being made to controling the asymetrical vs. symetrical distortion on the Fulltone 69 by adjusting the hFE on Q1 and Q2 which resulted in the "ideal" hFE's of 90 and 110.  Although it appears the hFE's are a lot lower, the "balace" between Q1 and Q2 may have an impact on that ratio.   :shock:

Maybe a dual gang pot, one for each Q is in order.

Hi guys.  Thanks for the thanks.  It's really a matter of being lucky, not smart.  I just made a lucky find when following up a suggestion by the real brains behind this (RG).  It does seem to be a really worthwhile find, too, judging by the response of people who have tried it.

First, concerning versions with emitter-bridging resistors. In the Miss Piggy, above, I nominated 1k resistors for Q1/Q2 and Q3/Q4.  That seems too small now,  I think that Kleber AG's idea is more like what will work well.  Perhaps 2.2k on the first gain stage and 3.3k or 4.7k on the second.
I tried lots of combinations of transistors last night, and a 1 k resistor only added, on average, about 3 to the hFE.  Previously, I have found that 3.3k gave an increase in hFE of about 10, and 4.7k up to 100 (on PN100s with natural hFEs of about 400).

I know that a few of you are probably already ahead of me on this, but I've done some work on piggybacking different types of transistors and across hFEs:  Basically, some types appear to have characterictically higher hFEs when piggied than others.  e.g. 5 PN100s stayed in the range hFE = 13 to 15, no matter what was on what.  With 5 BD139s (a mid power device) the hFEs were 2 and 3 (!).  When a BD139 is piggied on a PN100, the hFE is only 2 or 3, but the PN100 on the BD139 was 14, which leads me to think that some types of transistors are "lightweight piggybackers", and don't drag the hFE as low as others. Of the four types I tested, the BD139 was "heavy" (hFE~ 2 to 3), BC548s and BC549s were "medium" (hFE~5 to 7), and PN100s were "light" (hFE~10 to 15).

Anyway, keep the piggyback testing going lads and lasses!  
Anybody tried this yet in a Tonebender (if it works in a FF, it'll probably work in a Tonebender, right?) or a Rangemaster (I seem to recall that Germanium is prefered in the Rangemaster. Yes??  If so, maybe the piggyback is a goer in there, too) or what else????  Of course, it's gunna have its limitations.  I guess even if it applies mainly to fuzzfaces it's still opening up new possibilities for lots of future distortion pedals.

A bit OT: There's also another story to the Hornet (with piggybacked transistors) that started this thread.  It was my first attempt at a commercial sale.  So it's sitting in the local guitar shop waiting to be sold (on comission).  It's funny, but I feel like getting it back out of the shop because I am getting a sense that it's become part of stompbox history :D  (OK, in a MINOR way, but history all the same).  It's weird...  :wink:

Quote from: brettA bit OT: There's also another story to the Hornet (with piggybacked transistors) that started this thread.  It was my first attempt at a commercial sale.  So it's sitting in the local guitar shop waiting to be sold (on comission).  It's funny, but I feel like getting it back out of the shop because I am getting a sense that it's become part of stompbox history :D  (OK, in a MINOR way, but history all the same).  It's weird...  :wink:

Get it back and auction it on EBay - it's got enough publicity! (future collectors item and all that). :D

Nice work by the way!

What's funny is that I built my Sili Face with two sockets connected together for each transistor after reading RG's comments in the original thread, but was too lazy to experiment with piggybacking right away. It wasn't until I read about Brett's success that I popped the two extra transistors in there. What a discovery. We all really owe thanks to RG for suggesting the idea and to Brett for having the gumption to be the first one to try it. Good job guys! I've been playing my piggy Fuzz Face non-stop for the past 3 days. My main od/distortion sound used to be a tubescreamer. Not anymore!

Here is the layout I used on perf. I used the RunoffGroove layout and modified it a little bit. I'm not claiming this as my own, and I'm just placing it here as a help to anyone who may want to perf it. If B has any objections, I will redraw it.

(http://www.mycgiserver.com/~javacody/images/fuzzface.gif)

It may make sense to not solder together the two collector pins of the sockets, simply leave the second socket's (for the pair) collector unsoldered. I left them soldered and bent up the collector lead on one of the trannies.

EDIT: I also need to thank B Tremblay for the nice layout and great article on RunoffGroove.com. Read the Sili Face article at http://www.runoffgroove.com/sili-face.html

Quote from: javacodyWhat's funny is that I built my Sili Face with two sockets connected together for each transistor...

Can you describe the difference between the stock Sili-Face sound and the piggy-backed version?

Also, you connected the collectors, whereas Brett's Miss Piggy left them unconnected.  Have you tried it both ways and this is better?

There's no need to completely re-draw the layout you posted, but it may prevent some confusion if you add a note that this is your layout for a modified piggy-backed Sili-Face.

The sound differences are very hard to put into words. With your original input resistor, I don't think there would be enough fuzz, so I removed that from the piggybacked Sili. The stock Sili sounds great. I would call it classic si transistor Fuzz. The piggybacked version is ge all the way.  The fuzz is warmer, rounder, and seems to "breathe" more. I have a very hard time translating what I hear into words, but it sounds a lot like Hendrix on the Are you experienced album (of course, I'm playing a strat with Fender Custom Shop 69's, which also really affects that), but I can also hear some Clapton in Cream type sounds with it.

Also, concerning the collectors, I only connected them on the sockets, I bend one collector's lead up out of the socket and no I haven't tried just parallelling the transistors, but I've heard (and had experience with this with op-amps) that two transistors in parallel rounds out the sound a bit, giving the fuzz a more tube amp like sound, whatever that means.   :D

I just tried the schematic that Brett posted using NOS Philipps BC109's. I put 10k trimpots on the emitters of each pair of transistors so I could play with the gain. I tried it with just the two transistors biased correctly and then put in the other two and biased them again.

The results were a marked improvement with the trannys in parallel. There was no loss of distortion and it just sounded better, more germanium like. The brittleness of the SI's were gone and it sounded warmer and fuller.

Guys I think you've definitely found something here. Thank you for sharing it with us apprentices.

Bill

I'll never forget the way she looked the first time I saw her.
 Brett, this schematic couldn't be easier to follow and use to create a board.

I breadboarded Miss Piggy last nite,
but was hoping someone else would post first-

I mean no dis-respect,  to R.G. or Brett,
but there is No danger of this taking the place of Ge transistors.
Yes, it distorts!
but has none of the character of a well-tuned Ge FF.
I really don't think the "piggy-back" transistors are functioning as anything else but diodes to AC ground.
I removed the piggy-back transistors and installed a 1N4148+1N34A.
(in series) in each spot. Same sound.

history is hard to make, I think.
Ed R.

mine does.  i used 12k and 15k emitter resistors to get gains of 90 and 110 and i like it more than my germ ff

personal taste... maybe your 'tuned' Ge ff sounds much better than mine!  :P

Hi again. Regarding

QuoteI mean no dis-respect, to R.G. or Brett, but there is No danger of this taking the place of Ge transistors.

I think that constructive criticism is the way to go, so don't apologise for not liking something.  Like you, I feel that there is a significant difference between piggybacked Si and Ge.  My personal feeling is that *some* combinations of piggybacked transistors sound good and have *some* of the good characteristics of Ge (low gain being part of it).   I was lucky that I used 2 x PN100s in my first circuit.  They offer higher gain (about 20) and seem to work better than many other combinations.  So perhaps people should nominate what trannies and synthetic hFEs they're working with.  If Ed's ff had hFEs of 3 and 3 I wouldn't expect it to sound very good, either.  

Concerning the opposite (high praise):

Quotemine does. i used 12k and 15k emitter resistors to get gains of 90 and 110 and i like it more than my germ ff

This is interesting!  That's what I originally hoped to achieve with piggybacking.  Of course, some Si transistors already offer those gains without piggybacking. e.g. the BD139 (mid-power) has gains from 80 to 200.  I've already used them and liked them (especially for Q1), and refered to their sound in previous threads as "hybrid Si-Ge".  

It also appears that there's some fertile ground between hFE 20 and 50 that's waiting to be explored.

QuoteI really don't think the "piggy-back" transistors are functioning as anything else but diodes to AC ground.

I've only just read this thread and I agree, it's not a new idea.   GFR's had his idea on his page for an while but the idea has been around a lot longer than that.

There's no mystery why the gain drops either.  The voltage drop across a  diode junction increases as the current through it increases.  Without the series resistor, the two parallel BE junctions are forced to have the same BE voltage drop.  The base emitter voltage of the active transistor Q2a, the one with the collector connected is largely determined by it's collector current.  The base emitter voltage of the transistor with the unconnected collector, Q2b, is determined by the base current.  So to rough approximation, it follows that the base of Q2b is about the same as the collector current of Q2a.  In other words it's just a current mirror(!!!!) a very common circuit topology, which has a gain of 1.  If you go through the fine grain detail of transistors the current mirror isn't quite a perfect mirror and you end-up with a gain of 2 or 3, which are the figures which were push around previously.

QuoteI mean no dis-respect, to R.G. or Brett,
but there is No danger of this taking the place of Ge transistors.

Respect isn't an issue - it's a techie idea. It works for some things, not for others. No need to pre-apologize.

QuoteYes, it distorts!
but has none of the character of a well-tuned Ge FF.
I really don't think the "piggy-back" transistors are functioning as anything else but diodes to AC ground.
I removed the piggy-back transistors and installed a 1N4148+1N34A.
(in series) in each spot. Same sound.

history is hard to make, I think.

Every ear and taste is different. I guess more importantly, every guitarist looks for a slightly different set of things for their perfect sound.  As all of us know by now, there is no one thing to satisfy everyone's perfect sound, and in fact different people will perceive the *same* sound different ways.

Is the piggybacked transistor a perfect emulation of germanium? It can't possibly be. But it does seem to make a distortion sound that a number of people like. It's another technique in the tool bag. It side steps some of the things that people have not liked about silicon at some times in some circuits.

And yes, I agree, history is hard to make. In fact my personal opinion is that really interesting history can be best appreciated from a safe distance.  :D

Philosophy discussion over, technical discussion follows:

QuoteI really don't think the "piggy-back" transistors are functioning as anything else but diodes to AC ground.

The original idea was that a junction paralleled with the base-emitter of a silicon transistor would steal an amount of current from the functioning base, and by sheer larceny lower the current gain of the silicon to some lower level that's unobtainable with modern "semi-perfect" devices. I reasoned that equal voltages on the BE junction would drop the current gain by half.

That turned out to be wrong - the effective current gain dropped more than that, by actual test in a base-current/collector-current gain tester. There's something going on in there that lets the paralleled base-emitter steal more than its fair share of base current. I think that is because the paralleled BE junction has no transistor action going on, so there is no Shockley resistance in the emitter, and for equal voltages, the current density in the paralleled BE is higher; so it steals more than the naive half.

In the fuzz face circuit, yes, I'd agree that the action is as it might be with a diode to ground - that is what a paralleled BE junction on Q1 does. However, I think that in circuits with emitter resistors, you would still get a difference in effective transistor gain from the paralleled BE junction. It's still stealing base current, so the composite transistor still can't amplify as much as it used to.

QuoteI removed the piggy-back transistors and installed a 1N4148+1N34A. (in series) in each spot. Same sound.

Hmmm... what led to putting in two diodes, the Si and a Ge?

There is obviously no magic about the paralleled junction being a piggybacked transistor; anything that steals current in the same voltage range as the BE naturally functions will lower gain. I think that the most interesting circuits would have a nonlinear element like a semiconductor junction that was stealing current at the static voltage of the biased original transistor. If the paralleled element needs too high a voltage to conduct ( like, say, an LED) it won't ever get enough voltage from the silicon BE voltage to conduct, so it won't steal base current, and will have little or no effect. If the paralleled element has too low a voltage (like a Ge junction), it will steal almost all the base current and the transistor won't have much normal function at all.

I think it's like a see-saw - the more interesting things happen when the original and paralleled junctions are nearly matched. The simplest way to do that is to stick in another one just like the one that's in there. The current density argument and the various discussions on adding a resistor to the paralleled junction reflect the difference between the active transistor junction and the dummy/paralleled junction.

Anyway, that's what led to my question about why two diodes. Just offhand I would have expected that to boost the conduction voltage high enough to not have much audible effect.

QuoteI've only just read this thread and I agree, it's not a new idea. GFR's had his idea on his page for an while but the idea has been around a lot longer than that.

I'm certain it has. There are not many new ways to connect up NPN junctions after the field has been plowed for fifty years. Maybe it never got the attention it deserved. It took some active experimentation to turn an incomplete speculation into something interesting.

QuoteThe base emitter voltage of the transistor with the unconnected collector, Q2b, is determined by the base current. So to rough approximation, it follows that the base of Q2b is about the same as the collector current of Q2a. In other words it's just a current mirror(!!!!)

Interesting. That is a valid way to look at it. I think that by not connecting the collector of the "dummy", it's forcing the current mirror to be quite non-perfect. I wonder if the mirror gets more perfect - and the effective gain drops to nearer one - if you connect the dummy transistor's collector to its base. That gives you the classic simplistic current mirror circuit.

Hmmm... you could connect up a number of "active" devices in parallel, with one "dummy" device connected collector to base, then base and emitter parallel with the active devices, and have a more-than-unity current mirror, a common thing in IC circuitry.

I'll have to think some more about that. Thanks!

QuoteThat gives you the classic simplistic current mirror circuit.

Yes. No problem there provided the transistors are matched.  If you use a diode there is some mismatch.  The mismatch isn't a "problem" just it makes thing harder to pin down precisely.

As far as playing around with this goes, IMHO, putting a resistor in series with base of the unconnected transistor seem to be the best thing to try.  Adding the resistor gives am increasing HFe vs. current curve which generates even order harmonics in a *reliable* manner.  In the past I've spent some time searching for transistors with this characteristic only to find different batches an brands of transistors don't maintain the characteristic.

I suggest a switch with resistances in the 1k to 22k region for the series resistor and a second switch contact which switches different collector resistors in so the Q2 collector voltage is at 4.8V for each and every base resistor option.  This should give you a controllable smoothness FF.  2N3904's should be fine for the job.  The transistor with the unconnected collector can be a transistor or diode. For consistency of results I'd start with a 2N3904 from the same batch, but sticking in other transistors and diodes will still work- I think with the switch the different tones will be close to these options anyway.

Hmm. I'm not sure I agree with the naysayers. I have yet to try this with perfectly matched transistors, and the only transistors I have tried are bc109's in the range of 490 to 530, but the gains I'm getting are much higher than 1 to 3. I would consider gains of 8 to 12 (which I am seeing) quite a bit higher than what was mentioned. Also, this setup almost nails the smooth germanium sound to my ears, but I'm no self proclaimed expert, so take it as you will. There may be subtle differences, but this is the best fuzz I've found (not saying much, again, not an expert) yet. It smacks the dunlop reissue (with ge transistors no less) silly.

I guess I'm one of the pathetic few who would rather play guitar and drop in a couple of cheap and common transistors that retain their sound no matter the temperature than sort through a couple of dozen germanium transistors to find the "magic pair" which only keep the magic sound within a very finite temperature range. But hey, thats just me. For one, I'm not going to try to dimish this discovery, nor try to steal the credit from RG and Brett, who at least deserve a good clap on the back for waking a few of us up to some much better options than we had before. Good Job RG and Brett! Even if some others would dimish your contributions, I appreciate this wonderful discovery which is new to me and has significantly improved the fun factor of my playing!

QuoteEven if some others would dimish your contributions, I appreciate this wonderful discovery which is new to me and has significantly improved the fun factor of my playing!

I don't think anyone's out to diminish contributions by anyone.  GFR's site has been up some time and I know there are threads in the old archives discussing lowering the gain of Q2 for a FF-  in particular putting diodes, resistor and voltage dividers on the base of Q2 - the threads occured around 2001 and 2002.  I believe shortly after that Joe Davisson brought out his Obsidian transistor version which is a FF with a divider at the input of Q2.

New, or old, this technique is new to me [I used to try it wayy wayy back with a Tweek-O or a FF but never really discovered anything interesting...at that DIY Stage I was mostly interested it 'keeping' it working].
  The fact is, that it does in fact allow absolutely wonderful Fuzz Face results [with whatever Q's you Choose], and can be used as yet another option to get desired results.
  I got from point A to Point B...with this "new" [to me] FF Q Gain selection technique...And I 'feel' like I know more about what a "FF" [what does a FF sound like?] sounds like.
  Most important of all of this is the way the darned ckt sounds with these low gain 'PiggyQ's'. By far the most fantastic and flexible [well of course you know I'm mixing ckt's and use an amp...lol] sound pallette to date...it all became new again too...the sound just draws you in and time really flies when You're Having Fun...there's just something about the way a FF with low'er' Gian Q's ... really quite amazing and hard to describe the touchy feels, the attackk envelopeS, harmonic structure, and general 'workableness' of it.
 SOme us may choose to go for the Low Gain FF sound and Mix that with Ckt Fragments LIke a high gain transistor gain stage, a Booster, A tone control [with OA drive] etc. ...TONS OF TONES to be had messin' there...

QuoteGFR's site has been up some time and I know there are threads in the old archives discussing lowering the gain of Q2 for a FF- in particular putting diodes, resistor and voltage dividers on the base of Q2 - the threads occured around 2001 and 2002.

That my be true, but did anyone ever come up with the very simple idea of simply using another tranny? It doesn't sound like it to me. I don't need to mess around with all that other stuff (allthough, if you can provide me a link, it would make interesting reading for sure) I can just chunk another tranny in there. That is the ultimate simplicity and a beautiful thing for sure. Something, that even a relative newcomer like myself can easily do and have great results. It is also now indexed in such a way that newcomers can easily find it. Two very important points that should be remembered. Of course, new ideas in hindsight seem simple, but no one else put it together in just this way, which to me, is significant, and the credit of the discovery goes with the people who did so. Just my 2 pfennigs.  :)

new ideas in hindsight seem simple....  heh, nobody's really figured out why it works yet so it's not that simple  :P

btw for whoever it was thinking about piggyback transistors used in power amps, that's used a lot with matched FETs.  you can even get pre-packaged stuff like this - a 'HexFET'  is six piggybacked FETs in the same three-leg package.  with FETs it basically raises input impedance, lowers output impedance and power handling, and i saw a mathematical proof of why it works one time but forget the principles now.

it's totally different from how bipolars work though.  bugger aye!

To reply to R.G.'s question about why I used an Si and Ge diode
in series-
Simple,  It gave me the closest match to the diode drop of the piggy-back transistors I was "removing".
By the way the transistors I used in Miss Piggy were all 2N5088's
this could be why I didn't like the tone.
Who knows?
I gotta set this aside,  but I'm curious to hear what the rest
have to say about this deal.
Thanks, Ed R.

also of interest, i liked the sound of my piggybacked fuzz face when i cranked the gains of the transistors up to 90 and 110, and they also became far easier to bias.  

with low gains the bias on Q2 goes from 1V up to 2V and then jumps to 7V with increasing resistance on the collector of Q2, whereas with the higher gains the bias went up steadily and let me dial in 4.5V a lot easier.

is there a reason for specifying 1.2V on Q1?

1.2V is simply what I've measured on Q1's collector when I've trimmed for a good sound in various Si fuzzfaces and an Si Hornet.  So it's just a guide.  Other values probably sound as good or better...

Hello Ed, I've tried 2N5088s and 2N5089s and could not have a really good sound with them  :?
The bad thing is that when I was trying this things I was doing it like crazy at the breadboard and have not put the right attention or enougth care, OK that was just a crazy mess  :shock:  After had the 2n3906s sounding good, I thought the 2N5088s would be even better, but we may still have some weird factor somewhere the keep given us unpredictable results, at least for me...

How are you measuring HFEs of the piggy backed combos so easy?

Kleber AG
PS: I must say that with the easy face and 2N3906s I really had a GREAT sounding fuzz with a little dirty from my JCM800 amp.

I know I'm chiming in a bit late here, but I have a comment. I have a "Well Tuned Ge Fuzzface" and I don't like it much. I also have one that I built(not as well tuned :P ) that to me sounds much better. Maybe I just don't have the right rig, or maybe I just don't like the basic sound of the fuzzface, but lately the only sound I've been comfortable with is my double mosfet booster or my modded peppermill OD into an already cranked tube amp(JCM900 50watt).
I like a tight sort of OD or distortion and a FF just won't do it for me. Believe me though, I've tried.

Regards

RDV

QuoteHow are you measuring HFEs of the piggy backed combos so easy?

I used my DMM. It has an hfe tester for transistors. I just slipped em both in at the same time.

RDV said

QuoteI like a tight sort of OD or distortion and a FF just won't do it for me. Believe me though, I've tried.

I can strongly relate to your problem.  Neither my Ge FF nor my Si-Ge Easyface were ever as tight and mud-free as I wanted (certainly not as tight as my tubescreamer or my Vox 1901).  And Si in a  FF seemed to make it "congested".  Hence, to find a better fuzzface I tried RG's suggestion of piggybacking transistors.

I think that RDV (and others) can make an important contribution here.  Who better to test the new circuit than people who never really liked the old circuit?  

You may like to try the Miss Piggy circuit (above).  Instead of the 2x 1k resistors shown, I suggest using up to 10k emitter resistors in each gain stage.  It seems that PN100 or 2N3904 transistors are best (of those tested so far).  Good luck.

Quote from: RDVI know I'm chiming in a bit late here, but I have a comment. I have a "Well Tuned Ge Fuzzface" and I don't like it much. I also have one that I built(not as well tuned :P ) that to me sounds much better. Maybe I just don't have the right rig, or maybe I just don't like the basic sound of the fuzzface, but lately the only sound I've been comfortable with is my double mosfet booster or my modded peppermill OD into an already cranked tube amp(JCM900 50watt).
I like a tight sort of OD or distortion and a FF just won't do it for me. Believe me though, I've tried.

This is a good example of how tough it is to discuss various distortion/OD/ Fuzz circuits and what sounds *good*. If you are using an already cranked JCM900, your idea of what sounds *good* is going to differ from someone using say a 15 watt SS practice amp or maybe a Fender Twin.  Are you trying to sound like Slayer, Blink 182, Mudhoney,  SRV, Jimi, the Allman Bros. , Carlos Sanatana or early Iggy and the Stooges? Personally, I found the DIY Ge FF I made to be a revelation, but lately I haven't been bringing it to gigs, as I prefer the sound of the *Marshall* channel of my Rivera nailed with a mosfet boost. I used a vintage TS9 for over a decade, but now the sound of it just makes me cringe. Some people even like the sound of a Boogie Triple Rectumfryer. To me it sounds the way burnt plastic smells, but some people like that smell.

Kerry M

Here's the one of the older threads on this topic from the archives. You will probably have to log-in to read it.  The thread was titled "altering transistor gain"

http://diystompboxes.com/sboxforum/viewtopic.php?p=39549&highlight=resistor+series+diode#39549

Putting that aside, you can add a simple control which lets you vary the tone between the modded and unmodded.  Wire Q2 as usual then on the base put a 10k pot in series with a 1k then put that in series with a diode, transistor BE (B = anode E =cathode), or a transistor with the B and C connected together (B+C = anode E =cathode).  Wire-up as:  "anode" goes to the base of Q2, "cathode" goes to the 1k resistor then through the 10k pot then that goes to the emitter of Q2.   Choose the collector resistor of Q2 so the collector voltage of Q2 is roughly centred around 4.8V over the extreme pot positions.

Quote from: idlefactiona 'HexFET'  is six piggybacked FETs in the same three-leg package.  

errr, I don't think so, it's the hexagonal grid structure. As seen here:
http://www.irf.com/technical-info/an937/an-937p2.htm

Quote from: bwanasonic
Are you trying to sound like Slayer, Blink 182, Mudhoney,  SRV, Jimi, the Allman Bros. , Carlos Sanatana or early Iggy and the Stooges?
Kerry M

My favorite sound is the big, massive "Live @ Leeds" tone, but alas, I have no Hiwatt CP103. I'm getting pretty close with my P-90 equipped Les Paul & the boosted Marshall.

Shakin' all over

RDV

To clarify my previous post, I meant to use 1 pot of a dual gang pot for each of the resistors at the transistor junctions, but wired in reverse.  Sounds like a 5K or 10K would work and by varying the nob, the hFE between the Q's would change and you could go from having the Q1 low and Q2 high, to having the Q2 low and Q1 high,  I'm thinking this would change the character somewhat.

Another option would be to wire them the same and vary the hFE up or down with both Q's at the same time.

Not sure which would be the most useful.   8)

What would be really cool is to put a couple of meters on the pedal to display hfe of Q1 and Q2. How would you do that? Also, a switch to switch out the second tranny for each pair.

javacody - if there's some way to use an opamp to mathematically 'divide' voltages, you could measure the voltage between each side of a resistor in series with the base and collector of each Q and do a divide-by operation on the two voltage drops, and scale the result,  to find the Hfe.

that would sure be a good use for a VU meter  :P  have you seen joe gagan's nitroburner, with the VU meter for bias?  hahahaha

don't forget that bias moves when you disconnect the extra Q's though.  :)

What are you having great success with as far as transistors and 'cross emitter' resistors in you're Miss Piggy or Piggiebacked FF's?
  Im having real good time with a 2k2 between the 104 hfe Ge and 3906 in Q1 and a Ge hfe 148 only in Q2.
  It is actually quite low gain, about 2/3's Fuzzy and Rocks most of the  stages I put after or before it, as well as being just the Blues Face by itself !!!

Darren,
   You would need several 3 displays then, maybe those little led level displays?

haha, yeah you could do that!!!  *grin*

but they'd look cooler if you ran your signal into a three-way crossover and then had three led displays giving you a spectrum analyser  :P

woah, a spectrum analyser pedal!  that would rule.  absolutely no functionality at all, all it does is load your signal down *giggle*

Guys, im trying to make a Fuzz factory and was wondering if I could Piggyback the PNP transistors the same you do with NPN


The resistor goes from emiter to emiter?

Hi
It's just the same.  Add a transistor, leave the collector unattached and put a resistor between the emitters.  I generally like low resistances (say 2.2 k to 4.7k) and low gain, but other people like higher resistance and more gain (say 10k).
cheers
PS was piggybacking done 5 years ago?  I haven't had time to go back and really sort it out.  I guess the subsequent popularity of low hFE 2N2369A made piggybacking kinda redundant.

Thanks Brett!!

Its like I got a bit late on the topic :D

Anyway I gonna give this a chance. Don't know if were I live (Argentina) I could find the

2N2369A
so it seems it's time for second chance on piggybacking

regards!

I used trimpots instead of fixed resistors to set the exact hfe. You can check in a DMM or set by ear in the working circuit.