Leakage (Germanium transistor)

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Leakage (Germanium transistor)

Postby polifemo » 10 Dec 2013, 21:45

As germanium transistors are getting more and more difficult to obtain, I know that I in a near future will try to emulate the sound/behaviour of Germanuim Fuzzes using silicon trannies (Impossible, I know - a silicon transistor will never be a germanuim one - but one must try ;-)

I've found this explanation to the phenomena of leakage:
"... Transistor leakage is the leakage current across the reverse biased collector-base junction. This current then crosses the base-emitter, which, of course, causes a current from collector to emitter equal to the leakage current times the Hfe of the transistor, just as any current from base to emitter does..."

So my first question is:
- Do you find this to be correct?

My second, and most important question is:
- How do we best fake this when using a silicon transistor?

I've tried "piggybacking" trannies.
I've tried connecting large resistors from collector to base, but there's no sign of any leakage, just a dramatic drop of gain.
I've also connected small pF caps between collector and base in order to smooth out the high end, and this actually works, but it doesn't help the "leakage issue"..

Is there anything else one can do?

(Would love to build/bias a SI Tone Bender Mk1, and I know that some people have done this with great result!)
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Re: Leakage (Germanium transistor)

Postby DrNomis » 10 Dec 2013, 22:30

polifemo wrote:As germanium transistors are getting more and more difficult to obtain, I know that I in a near future will try to emulate the sound/behaviour of Germanuim Fuzzes using silicon trannies (Impossible, I know - a silicon transistor will never be a germanuim one - but one must try ;-)

I've found this explanation to the phenomena of leakage:
"... Transistor leakage is the leakage current across the reverse biased collector-base junction. This current then crosses the base-emitter, which, of course, causes a current from collector to emitter equal to the leakage current times the Hfe of the transistor, just as any current from base to emitter does..."

So my first question is:
- Do you find this to be correct?

My second, and most important question is:
- How do we best fake this when using a silicon transistor?

I've tried "piggybacking" trannies.
I've tried connecting large resistors from collector to base, but there's no sign of any leakage, just a dramatic drop of gain.
I've also connected small pF caps between collector and base in order to smooth out the high end, and this actually works, but it doesn't help the "leakage issue"..

Is there anything else one can do?

(Would love to build/bias a SI Tone Bender Mk1, and I know that some people have done this with great result!)




There's probably not much else you could do to fake the leakage present in Germanium Transistors, I guess you could do some experimenting.... :thumbsup

I think the explanation of what leakage is sounds like it's correct, but I also think that leakage in Germanium Transistors has alot to do with the inherent electrical properties of Germanium itself, (this may sound a bit technical) basically it has to do with how tightly or loosely the electrons are held in one of the shells surrounding the nucleus of a Germanium atom, this particular shell we're interested in is what's called the "Valence Shell" and determines the electrical properties of the Germanium atom, as it happens, the electrons in the valence shell of a Germanium atom aren't held in it as tightly as they are in a Silicon atom, because of this the electrons need less energy to get them to leave their place in the Valence shell, the energy could be in the form of electrical, or, thermal energy, which is why Germanium is so temperature sensitive, my understanding of it anyway........ :thumbsup
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Re: Leakage (Germanium transistor)

Postby polifemo » 11 Dec 2013, 06:11

Thanks! You're always very helpful :D

Have you tried building a SI Tone Bender Mk1 yourself?
The circuit is dependent on leakage in order to work, and it seems really difficult to get it to bias using SI trannies...
As I wrote in my earlier post I know that it's been done ( http://stompboxes.co.uk/forum/viewtopic ... ilicon+Mk1 ) and I've heard sound clips of such a pedal (sound clips that I can't find at the moment...) and it sounded fine, with only just a hint of the raspy silicon character in the high end.

Would be cool to be able to build a pedal like this!
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Re: Leakage (Germanium transistor)

Postby Cub » 11 Dec 2013, 09:24

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Re: Leakage (Germanium transistor)

Postby polifemo » 11 Dec 2013, 12:56

Thanks.

I've read this one, as well as trying out the "piggy backing thing already.

Yes one can reduce hfe this way, but I've got plenty of low/medium gain SI trannies that corresponds to the hfe of the germanium ones, and I don't find that this method simulates leakage.
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Re: Leakage (Germanium transistor)

Postby DrNomis » 11 Dec 2013, 13:01

polifemo wrote:Thanks! You're always very helpful :D

Have you tried building a SI Tone Bender Mk1 yourself?
The circuit is dependent on leakage in order to work, and it seems really difficult to get it to bias using SI trannies...
As I wrote in my earlier post I know that it's been done ( http://stompboxes.co.uk/forum/viewtopic ... ilicon+Mk1 ) and I've heard sound clips of such a pedal (sound clips that I can't find at the moment...) and it sounded fine, with only just a hint of the raspy silicon character in the high end.

Would be cool to be able to build a pedal like this!



I have been thinking of building a Mk 1 using some Silicon Transistors, the original Mk 1 circuit was originally designed to exploit the leakage that Germanium Transistors have, it's used as the biasing system for two of the Germanium transistors (the ones which have a single resistor connected to to their base terminal with the other end of the resistor connected to circuit ground), basically the leakage acts like a resistor connected from the base of the transistor to the supply, for the Silicon Transistor version you need to add a resistor from the base to the + supply (assuming you're using NPN transistors), if you just have a resistor going from the base to circuit ground then the transistor will not turn on and you won't get any signal output...... :thumbsup

Note that the tone will sound harsher, or sharper compared to the original Germanium Transistor version, but you can try putting a 100pF cap across each transistors Collector-Base junction to soften the sound.... :thumbsup
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Re: Leakage (Germanium transistor)

Postby polifemo » 11 Dec 2013, 21:21

DrNomis wrote:
I have been thinking of building a Mk 1 using some Silicon Transistors, the original Mk 1 circuit was originally designed to exploit the leakage that Germanium Transistors have, it's used as the biasing system for two of the Germanium transistors (the ones which have a single resistor connected to to their base terminal with the other end of the resistor connected to circuit ground), basically the leakage acts like a resistor connected from the base of the transistor to the supply, for the Silicon Transistor version you need to add a resistor from the base to the + supply (assuming you're using NPN transistors), if you just have a resistor going from the base to circuit ground then the transistor will not turn on and you won't get any signal output...... :thumbsup

Note that the tone will sound harsher, or sharper compared to the original Germanium Transistor version, but you can try putting a 100pF cap across each transistors Collector-Base junction to soften the sound.... :thumbsup


Yeah, please build an Mk1, and then tell me how to do it :applause:

Are you referring to Q2 and Q3 when you speak about "the ones which have a single resistor connected to to their base terminal with the other end of the resistor connected to circuit ground" ?

If I just understand which transistors you're referring to. I can at least try out different resistor values in order to (hopefully) get the circuit producing some sound!

I still however think that you should build that Mk1 :lol:

Thanks again!
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Re: Leakage (Germanium transistor)

Postby DrNomis » 11 Dec 2013, 22:50

polifemo wrote:
DrNomis wrote:
I have been thinking of building a Mk 1 using some Silicon Transistors, the original Mk 1 circuit was originally designed to exploit the leakage that Germanium Transistors have, it's used as the biasing system for two of the Germanium transistors (the ones which have a single resistor connected to to their base terminal with the other end of the resistor connected to circuit ground), basically the leakage acts like a resistor connected from the base of the transistor to the supply, for the Silicon Transistor version you need to add a resistor from the base to the + supply (assuming you're using NPN transistors), if you just have a resistor going from the base to circuit ground then the transistor will not turn on and you won't get any signal output...... :thumbsup

Note that the tone will sound harsher, or sharper compared to the original Germanium Transistor version, but you can try putting a 100pF cap across each transistors Collector-Base junction to soften the sound.... :thumbsup


Yeah, please build an Mk1, and then tell me how to do it :applause:

Are you referring to Q2 and Q3 when you speak about "the ones which have a single resistor connected to to their base terminal with the other end of the resistor connected to circuit ground" ?

If I just understand which transistors you're referring to. I can at least try out different resistor values in order to (hopefully) get the circuit producing some sound!

I still however think that you should build that Mk1 :lol:

Thanks again!



Come to think of it,I might actually try breadboarding the circuit over the Christmas holidays and I'll see if I can optimize it to work with Silicon transistors, Looking at the original Tonebender Mk 1 schematic I've posted, the transistors I'm referring to are Q1 and Q3, Q1 has a 1M resistor going from the transistor's base to circuit ground and Q3 has an 8k2 resistor going from the base to circuit ground, to get the circuit working with Silicon transistors an extra resistor from the base to the supply voltage needs to be added to Q1 and Q3, I'm not sure what values the resistors need to be though so we'd need to experiment with different resistor values, I think it is quite do-able though......... :thumbsup


Just had a thought, the two extra resistors needed could be trimpots so you could tweak the biasing to where you get a tone that you like, or, make them half of a dual-gang pot which could be mounted on the enclosure with the other control pots, this will give you more control over the tone of the circuit.... :thumbsup
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Re: Leakage (Germanium transistor)

Postby DrNomis » 11 Dec 2013, 23:10

Here's a Silicon Transistor Mk 1 circuit I just found will doing some googling, you don't necessarily have to use the ones specified, you could easily use pretty much any NPN transistor, I'm going to try using three BC549c Transistors..... :thumbsup
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Re: Leakage (Germanium transistor)

Postby blackbunny » 12 Dec 2013, 05:08

DrNomis wrote:Here's a Silicon Transistor Mk 1 circuit I just found will doing some googling, you don't necessarily have to use the ones specified, you could easily use pretty much any NPN transistor, I'm going to try using three BC549c Transistors..... :thumbsup


Hey Simon, the Mk 1 circuit is supposed to work best with low gain transistors. Using BC549C's in your Mk 1 might result in a really farty sound due to the HFE range of 400-800!

Should be an interesting experiment anyway - give it a try and let us know what it sounds like. :twisted:
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Re: Leakage (Germanium transistor)

Postby DrNomis » 12 Dec 2013, 05:19

blackbunny wrote:
DrNomis wrote:Here's a Silicon Transistor Mk 1 circuit I just found will doing some googling, you don't necessarily have to use the ones specified, you could easily use pretty much any NPN transistor, I'm going to try using three BC549c Transistors..... :thumbsup


Hey Simon, the Mk 1 circuit is supposed to work best with low gain transistors. Using BC549C's in your Mk 1 might result in a really farty sound due to the HFE range of 400-800!

Should be an interesting experiment anyway - give it a try and let us know what it sounds like. :twisted:



Will do mate.... :thumbsup
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Re: Leakage (Germanium transistor)

Postby polifemo » 12 Dec 2013, 06:05

Thanks a lot :thumbsup

Planning some "Fuzzin'" over Christmas and I'll most definitely will give all this a try 8)

Edit: Regarding R2 on the Silicon Mk1 schematic: Is 4M7 really needed there?
I always thought that 2M2 was big enough for a job like this.(Will try two of these in series as I don't have anything larger than that)
Just asking out of curiosity
:D
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Re: Leakage (Germanium transistor)

Postby DrNomis » 12 Dec 2013, 07:17

polifemo wrote:Thanks a lot :thumbsup

Planning some "Fuzzin'" over Christmas and I'll most definitely will give all this a try 8)

Edit: Regarding R2 on the Silicon Mk1 schematic: Is 4M7 really needed there?
I always thought that 2M2 was big enough for a job like this.(Will try two of these in series as I don't have anything larger than that)
Just asking out of curiosity
:D





R2 is probably that big to ensure that the input impedance is high enough, but I can't see why you couldn't just substitute a 2M2 resistor for R2, the other resistor (R3) may need tweaking though...... :thumbsup
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Re: Leakage (Germanium transistor)

Postby polifemo » 12 Dec 2013, 09:02

Found the thread over at DIY, and there was a second, updated, schematic uploaded:

Image

Sorry about the size of the schematic...

The thread can be found here: http://www.diystompboxes.com/smfforum/i ... ;topicseen
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