Piggyback Si transistors to simulate Ger ???

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.

Re: Piggyback Si transistors to simulate Ger ???

Postby ezugaru » 03 Dec 2012, 03:40

you should test some different values and play around with them. Since in this matter there is also the "like" factor, a no sound like an "original" would be better for your taste, or am i wrong?
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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 04 Dec 2012, 21:47

Just to confirm that the concept works fine, i did a little experiment today:
experiment.jpg


In the following, "resistance" is the measured resistance of the potentiometer between the two transistors emitters
The "primary transistor" is the one with all three legs used, hence the "secondary transistor" is the other one ;)

i tried to get a hFE of 110 with a handful of BC550's with hFE ranging from 180 to 250, and got the following results:

- A hFE of 110 was in all five cases acquired with a resistance between 20KΩ and 40KΩ
- The needed resistance varied significantly depending on hFE for the primary transistor, and not so much for the secondary transistor.
- Sweeping the resistance from 0Ω to 100KΩ, using a 100K lin pot, seemed to affect hFE in a somewhat linear manner. For a primary transistor with hFE ~180, the sweep resulted in hFE of the circuit going from ~4 to ~140
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Re: Piggyback Si transistors to simulate Ger ???

Postby atreidesheir » 05 Dec 2012, 01:24

That is really encouraging.
What is Sherwood Forest pedal pirates? Sound like our kinds of people. :)
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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 05 Dec 2012, 05:50

atreidesheir wrote:That is really encouraging.
What is Sherwood Forest pedal pirates? Sound like our kinds of people. :)

viewtopic.php?f=7&t=7714&p=87900&hilit=Sherwood+Forest+Pedal+Pirates#p87900 ;)
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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 30 Jan 2013, 22:55

Hmm.. It seems like whenever i haven't posted in this thread for a while, it dies out.
Tonight, i'm gonna build a couple of those, and get a tonebender build started!
GE-sim_BC550.png
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Re: Piggyback Si transistors to simulate Ger ???

Postby Cub » 04 Feb 2013, 13:23

Splendid ! :applause:
Do keep us posted. :thumbsup
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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 05 Feb 2013, 06:44

I tried to build a NPN Fuzz Face a couple of nights ago - without much luck. Perhaps it's the layout?
I built two emulators from a set of 2N3904's and a set of BC550's and hFE was easily controllable. Using my multimeter i trimmed one to 70hFE and the other to 120hFE, and built them into HBO's supposedly working NPN Fuzz Face vero layout.
Result: Almost no sound. I managed to get some sputtering decaying notes across, but nothing pretty.

Step 1 from here is trying the emulators in a NPN rangemaster build, just to verify they're actually working.
Step 2 is creating a new FF board layout from Madbeans Hipster schematic. I trust that schem is verified :)

I remember Mictester writing something about a diode or resistor from base to ground to simulate leakage, which should help turn modern non-leaking transistors on, but i can't remember where. That might be important as well as emulating leakage has got to be part of the GE-sim circuitry, right? :D
Help someone?

Any other great tips on biasing, or verified schematics of NPN fuzzes are greatly appreciated. I'm determined to get those pups to make some real noise! :)
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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 06 Feb 2013, 23:58

Alright, here's a couple of questions for the clever guys...

In mictesters buzzaround-alike he throws in a 1M resistor from base to v+ to make low leakage transistors turn on by simulating leakage. Is that what we're missing here? I don't suppose modern SI transistors are leaky as **** like the old GE types?

How about the reverse biased diode from base to emitter? Does it have any significant effect on the sound, or does it just stabilize the temperature sensitivity of GE transistors?
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Re: Piggyback Si transistors to simulate Ger ???

Postby Cub » 07 Feb 2013, 09:10

I wish my skills were at a level where I could contribute to this research. :( Germ sims have so much potential.
You're doing great work, grrrunge! Hats off to you. :applause:
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Re: Piggyback Si transistors to simulate Ger ???

Postby beedotman » 07 Feb 2013, 09:59

grrrunge wrote:How about the reverse biased diode from base to emitter? Does it have any significant effect on the sound, or does it just stabilize the temperature sensitivity of GE transistors?

Here's recent discussion about this:
http://freestompboxes.org/viewtopic.php?f=10&t=21670
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Re: Piggyback Si transistors to simulate Ger ???

Postby induction » 07 Feb 2013, 18:39

I built an NPN fuzz-face with piggybacked 2N3904's. I chose my gains in advance, and used a trimmer to dial in the necessary piggyback resistance on an external piggyback tester. Then I measured the resulting resistance and used fixed resistors on the build. I breadboarded it with a few different gains for the piggybacks (90 & 110, 200 & 400, 300 & 600, and several more based on info from various threads on this forum and the other one) using different tranny types, and ultimately chose the lower gain, classic fuzz face values because I liked the tone a little better, and they gave tons of available fuzz but clean up better with the guitar volume knob than the higher gains do. I chose the Miller caps by ear on the breadboard. The pedal sounds fantastic, but I can't comment on the similarity to Ge because I've never had a Ge fuzz face.

I included some variations on the Fuller mods:
1. A variable resistor in series with the input (Smooth knob). This reduces the fuzz, but gives a different clean-up contour than reducing the fuzz knob. (The fuzz can be adjusted with the fuzz knob, the smooth knob, and the guitar volume knob, but they each affect the tone differently. The smooth knob is very useful for adjusting for different pickups, but also gives you some different options for how the tone changes when you adjust the guitar volume.)
2. An external bias knob that adjusts the voltage on the collector of the second piggyback between 2.8 and 6.8 V. Low bias gives a sputtery, square-wave type fuzz, high bias gives a harder-edge distortion, right in the middle gives a spongy fuzz sound with great clean-up.

Here's my schem (I labeled lug 1 on every pot for orientation purposes):
javelina_schematic_v1.png


my vero:
javelina_vero_v1.png


and my build:



Results:
On my tele, I have 50's les paul wiring for the tone knob, and I can use the guitar volume knob to control the fuzz without otherwise changing the tone very much. On my strat, I have a treble bypass cap for the volume knob, which gives a much brighter sound when I roll back the volume knob to reduce the fuzz. On other dirt pedals, this can sometimes sound too thin, but with the fuzz face it gives a whole new sound. On the bridge pickup, this is a bright, brash sound that reminds me of The Clash, and I haven't been able to get it any other way. On the neck pickup, with the volume knob down enough for the sound to be almost clean, it's bright yet round. It sounds like a jazz tone with an edge. I'm surprised at how much variety I can get with this pedal. I'd always assumed fuzz faces were more limited.

With a battery, the pedal is very quiet at all settings. With an adapter, it hums more than my other dirt pedals. For some reason, the hum jumps up a lot when the guitar volume is rolled all the way off. I haven't figured out how to solve that yet, so I just use a battery.
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Re: Piggyback Si transistors to simulate Ger ???

Postby induction » 08 Feb 2013, 16:04

grrrunge wrote:I tried to build a NPN Fuzz Face a couple of nights ago - without much luck. Perhaps it's the layout?
I built two emulators from a set of 2N3904's and a set of BC550's and hFE was easily controllable. Using my multimeter i trimmed one to 70hFE and the other to 120hFE, and built them into HBO's supposedly working NPN Fuzz Face vero layout.
Result: Almost no sound. I managed to get some sputtering decaying notes across, but nothing pretty.

Step 1 from here is trying the emulators in a NPN rangemaster build, just to verify they're actually working.
Step 2 is creating a new FF board layout from Madbeans Hipster schematic. I trust that schem is verified :)

I remember Mictester writing something about a diode or resistor from base to ground to simulate leakage, which should help turn modern non-leaking transistors on, but i can't remember where. That might be important as well as emulating leakage has got to be part of the GE-sim circuitry, right? :D
Help someone?

Any other great tips on biasing, or verified schematics of NPN fuzzes are greatly appreciated. I'm determined to get those pups to make some real noise! :)



What you describe sounds like a mis-biased transistor (Harald does really good work, so I suspect the layout is fine, but I haven't tried it). I forgot to mention that the 2.7k resistor in my build was chosen by trial and error to give me the bias range I wanted on the collector of the Q3/Q4 piggyback. If you're using this layout, then adjust VR1 until you get the bias you want (a 10k trimmer should be enough, but if not, try a bigger one). You want something near 4.5V for classic fuzz face sounds, but you can adjust it to taste or use a pot in series with a fixed resistor (like I did) to give you a range of useable sounds.

If that doesn't work, try my layout, or try IvIark's, which has onboard trimmers for dialing in the piggyback gains.
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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 08 Feb 2013, 17:05

induction wrote:What you describe sounds like a mis-biased transistor (Harald does really good work, so I suspect the layout is fine, but I haven't tried it).

Sounds plausible :) I've built several of Haralds layouts, and I've never had any trouble with those. I suppose it's just the experimental nature of the project, that got me wondering :)

induction wrote: I forgot to mention that the 2.7k resistor in my build was chosen by trial and error to give me the bias range I wanted on the collector of the Q3/Q4 piggyback.

Good to know :) thanks!

induction wrote:If you're using this layout, then adjust VR1 until you get the bias you want (a 10k trimmer should be enough, but if not, try a bigger one). You want something near 4.5V for classic fuzz face sounds, but you can adjust it to taste or use a pot in series with a fixed resistor (like I did) to give you a range of useable sounds.

Yep! That's the one i was using :) My first thought was something along the lines of "I bet HBO has a good layout somewhere - let me just use that for starts..."

induction wrote:If that doesn't work, try my layout, or try IvIark's, which has onboard trimmers for dialing in the piggyback gains.

Yup, that's the one he posted in this thread as well. I just want to play around with this in a more modular way, to "see what all the fuzz is about" :)
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Re: Piggyback Si transistors to simulate Ger ???

Postby induction » 19 Feb 2013, 17:03

induction wrote:With a battery, the pedal is very quiet at all settings. With an adapter, it hums more than my other dirt pedals. For some reason, the hum jumps up a lot when the guitar volume is rolled all the way off. I haven't figured out how to solve that yet, so I just use a battery.


Update:
With an unregulated 9V adapter, the increased hum at zero on the guitar volume goes away with a small resistor in series with the input (I just turned up the Smooth knob a tiny bit, so I'm guessing about 100 ohms should do the trick), but otherwise the adapter hum is unaffected.

Last night I breadboarded a regulated and filtered 9V DC supply (using an unregulated 12V adapter, a 7809, 22 uF on the source voltage, 22 uF and 100 uF on the output voltage). The Javelina is as quiet with this supply as it is with a battery, at all settings.

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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 19 Feb 2013, 18:43

induction wrote:
induction wrote:With a battery, the pedal is very quiet at all settings. With an adapter, it hums more than my other dirt pedals. For some reason, the hum jumps up a lot when the guitar volume is rolled all the way off. I haven't figured out how to solve that yet, so I just use a battery.


Update:
With an unregulated 9V adapter, the increased hum at zero on the guitar volume goes away with a small resistor in series with the input (I just turned up the Smooth knob a tiny bit, so I'm guessing about 100 ohms should do the trick), but otherwise the adapter hum is unaffected.

Last night I breadboarded a regulated and filtered 9V DC supply (using an unregulated 12V adapter, a 7809, 22 uF on the source voltage, 22 uF and 100 uF on the output voltage). The Javelina is as quiet with this supply as it is with a battery, at all settings.


Sweet! That's good to know. I actually do that with the supply rail on all the boards i design myself. There's nothing worse than an excessively noisy drive-pedal!

EDIT* oops! I misunderstood you there. What i usually do is send V+ through a 100R resistor, and then a 100nF and a 100µF from there to ground. Tried that?
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Re: Piggyback Si transistors to simulate Ger ???

Postby induction » 19 Feb 2013, 19:32

In this pedal I just have a 47uF from V+ to ground, but when I breadboarded it I tried the 100R series resistor and a 100nF cap to ground as well. I omitted them in the build because they didn't seem to make any difference to the hum, and the resistor seemed to affect the tone a little bit (this was purely subjective and I could have been imagining it). I'm guessing that the regulator is doing most of the magic in this case because filtering alone was not enough.
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Re: Piggyback Si transistors to simulate Ger ???

Postby atreidesheir » 24 Apr 2013, 22:56

Could I get a similar effect with a 2n3440 low gain (@50hfe) silicon transistor with a 1n34a tied to the base and collector to simulate leakage?
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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 25 Apr 2013, 19:47

atreidesheir wrote:Could I get a similar effect with a 2n3440 low gain (@50hfe) silicon transistor with a 1n34a tied to the base and collector to simulate leakage?


That was discussed a couple of pages ago: viewtopic.php?f=44&t=2674&start=20#p194144
That turned out to be correct in concept, wrong in detail. I forgot the internal rbe of the transistor junction. What a bipolar transistor base-emitter junction does that a diode junction does not is to have an equivalent nonlinear resistor arising from the transistor action. This internal resistor causes the working junction to have a higher voltage at the same current as the dummy junction; looked at the other way, the dummy junction eats more current than the working one at the same voltage.
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Re: Piggyback Si transistors to simulate Ger ???

Postby Cub » 26 Apr 2013, 08:07

Hey grrrunge, have you made a breakthrough in this already ?
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Re: Piggyback Si transistors to simulate Ger ???

Postby grrrunge » 26 Apr 2013, 09:07

Nope, I've been building overdrives and pedalboard interfaces for everyone but myself lately. Sadly that meant no time for experimentation :(
Once I get back on it, i promise my findings are going straight up here!
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