first Germanium experience, I am confused

allesz · Post by **allesz** » 25 Mar 2011, 14:23

Yesterday I passed some time on the breadboard, because recently I found some germanium transistors: ac187, ac184, ac181 and a couple of other types I don't remember now. My first germanium transistors.
I wanted to breadboard just a simple booster... just to try and ear what the fuss is about.

I had no success, they seem very difficult to bias; silicon transistors has always been very forgiving for my mistakes.
As usual I started kidding and finished with a strange schem: a 1uF cap on the base (the input), a 1uF cap on the collector (output), emitter earthed, and a 100k pot in series with the battery + as a variable resistor.
I got some strange but interesting sounds, sputtery fuzzes, almost synty sounds, some good overdrive (muff fuzz?), and some clean boost too.
The really strange fact is that it did not matter how I turned the devices, npn pnp... I always got a sound, sometimes bad and sometimes not.
Are my transistors gone? Is this what is called "leakage"?; Can someone explain this phenomenon to me?

DrNomis · Post by **DrNomis** » 25 Mar 2011, 15:22

allesz wrote:Yesterday I passed some time on the breadboard, because recently I found some germanium transistors: ac187, ac184, ac181 and a couple of other types I don't remember now. My first germanium transistors.
I wanted to breadboard just a simple booster... just to try and ear what the fuss is about.

I had no success, they seem very difficult to bias; silicon transistors has always been very forgiving for my mistakes.
As usual I started kidding and finished with a strange schem: a 1uF cap on the base (the input), a 1uF cap on the collector (output), emitter earthed, and a 100k pot in series with the battery + as a variable resistor.
I got some strange but interesting sounds, sputtery fuzzes, almost synty sounds, some good overdrive (muff fuzz?), and some clean boost too.
The really strange fact is that it did not matter how I turned the devices, npn pnp... I always got a sound, sometimes bad and sometimes not.
Are my transistors gone? Is this what is called "leakage"?; Can someone explain this phenomenon to me?

Germanium transistors are finicky creatures, they are very temperature sensitive, and sometimes they go leaky, when you think you've got them operating right they'll drift alot, it's something that always happens with them.....

Leakage is where the transistor is conducting current when it shouldn't be, here's an analogy, a water tap is turned on and water is flowing into the basin, inside the tap is a rubber seal that is supposed to stop the flow of water, but it happens to be a bit old and worn, so, you try to turn the tap off but water drips out even though the tap is off, the worn seal is not able to do it's job effectively....

When you're trying to turn a leaky transistor off, the leakage prevents you from doing so....

A good way to try and stabilize your transistor booster is to try putting a small value resistor from the emitter to circuit ground, rather than just connecting the emitter to ground, this sets up a negative feedback system that opposes what the transistor tries to do....

A good value to try is anything from 100 Ohms to 1k....

Hope that helps....

allesz · Post by **allesz** » 25 Mar 2011, 15:56

Thank you, I suspected it.
Anyway the thing that I really find strange is the fact that the trannies could be reversed and they continue to work.
At one point, with a ac187 that was giving strange but good sounds I considered to box the thing, but an answer came to my mind: will it last?

RnFR · Post by **RnFR** » 25 Mar 2011, 18:07

you aren't biasing the transistor. you have to use a bias arrangement to get the transistor to turn on. as of right now, the only thing biasing the transistor is the leakage. try a proven design like the rangemaster and see how it works.

allesz · Post by **allesz** » 28 Mar 2011, 16:14

I suspected it: only the leakage is biasing the thing! Tanks a lot.
As a first germanium experience I was tring something as the rangemaster or the lpb1, but the results were not so good: I barely had unity gain and getting more was almost difficult.
I got two ac187 that have pretty high hfe: 250 and over 300 three ac181 that go to about 200 hfe; last I have three ac184 with hfe around 200: they seem to high and so I suspect my trannie are too leaky.

By the way I had an empty old box and components (from a homemade overdrive that got rebuilt in a better box) where I put the circuit attached below; I also put a dpdt switch that can switch between emitters and collectors... It gives very interesting sounds. Ok, it is not a fuzz factory but I think it gives some useable sound (and some horryd noises too

. Maybe it can be a good way to use too leaky germanium transistors.

RnFR · Post by **RnFR** » 28 Mar 2011, 18:26

yeah, you are missing one or two resistors in your schematic. you could bias it a few different ways. google BJT amplifier for more info. and yes, Ge transistors with that high of gain are probably leaky. try RG Keen's method of testing.

allesz · Post by **allesz** » 29 Mar 2011, 14:17

I would miss some resistor for sure without leakage.
I put a socket for the transistor, and if I try to put a silicon trannie on it won't work (because this design lacks bias resistor, or resistors, on the base).
This sort of "self biasing" is quite interesting for me... and it sounds good: this morning I had an hour to test the sound of the unit (with an ac181 on) with my small vht special 6 and I was impressed.
If I find a not leaky germanium transistor, and put it on, it won't work right?

RnFR · Post by **RnFR** » 30 Mar 2011, 00:15

correct, it shouldn't bias. modman did this same thing a ways back. I think it's on his blog.

allesz · Post by **allesz** » 30 Mar 2011, 15:15

Ok, this is a good thing.... if I find a transistor that is not working on the effect it will mean that I got a good one.
By the way what is modman's blog? I would like to read some reports about this new (for me) experience.

DrNomis · Post by **DrNomis** » 30 Mar 2011, 15:53

If you find a non-leaky Germanium transistor, and put it in the circuit the correct way around(emitter to ground), it should work as normal, but, if you put it in the other way (collector to ground), it should not work at all, or the gain should be almost non-existent....

allesz · Post by **allesz** » 30 Mar 2011, 16:11

To tell you the truth I planned to put this project in the ready to do section; that's because the sound you get are really interesting: I have a friend that plays blues, only electric guitar and voice, and likes dirty tones; I made him listen to the effect ad now he wants me to build one for him.
I mean, using a good transistor for this would be a waste... but considering that bad germaniums are more than the good one it can have some real life application: connected in the right way you get a simple preamp booster (the volume is circa the same as unity gain) but when you reverse the thing you can get some mild distorted and fuzzy sounds, pretty lo fi, that imparts coolness to everything you play.
By the way I think that the pots I used are too big: probably I could use a 1k for the base and 22K for the collector.

DrNomis · Post by **DrNomis** » 30 Mar 2011, 16:18

allesz wrote:To tell you the truth I planned to put this project in the ready to do section; that's because the sound you get are really interesting: I have a friend that plays blues, only electric guitar and voice, and likes dirty tones; I made him listen to the effect ad now he wants me to build one for him.
I mean, using a good transistor for this would be a waste... but considering that bad germaniums are more than the good one it can have some real life application: connected in the right way you get a simple preamp booster (the volume is circa the same as unity gain) but when you reverse the thing you can get some mild distorted and fuzzy sounds, pretty lo fi, that imparts coolness to everything you play.
By the way I think that the pots I used are too big: probably I could use a 1k for the base and 22K for the collector.

Yeah, I reckon a 1k for the base and 22k for the collector are worth a try....

allesz · Post by **allesz** » 04 Apr 2011, 17:36

I tried 1k and for the emitter and 10k for the collector and the effects is more easy to tune up; since the collector pot is smaller it is easier to get the right sound, but I feel that some extreme splatty and strange sounds are almost lost (for me it is better this way, but it depends what your needs are)
By the way, the 2,2 K pot I had was actually 4,5K at his maximum; the 1k pot gets around 1,5k at max (from 100% to 50% of tolerance

).

Thanks to RnFR I discovered modman's blog, he did the same around four years ago: http://modman.blogdrive.com/archive/37.html

His project is a little different, considered his goals seemed just to make a simple booster, germanium fun seems the same.

allesz · Post by **allesz** » 04 May 2011, 14:44

In the end I came up with something like the range master (see the schematic 2) minus the bias network; The effect works but it gives just a little boost (useable volume goes from 3 - 4 o'clock to max).
Right now I plan to change something to get some more gain (see schematic 3) and to be able to get some strange (boostfactory?) sound with jus one knob....

By the way I tried to put a bias network on the base, like the original rangemaster, but it seems that in my trannies leakage is so bad that nothing different happens

allesz · Post by **allesz** » 04 May 2011, 14:47

ok I forgot to upload the examples

DrNomis · Post by **DrNomis** » 05 May 2011, 09:28

To get more gain out of the transistor, you can put a 1uF/16V capacitor in parallel with the 100 Ohm emitter resistor....

allesz · Post by **allesz** » 05 May 2011, 17:06

I plan to try this thing tonite.
By the way I am using the schematic below: In the end I feel that the 100K pot gives a little more range.
Thank you Dr. Normis. I really appreciate your input and help.
allesz

deltafred · Post by **deltafred** » 05 May 2011, 18:19

allesz

You are still not biasing it, you need a small voltage on the base to bring it into it's working region. IIRC you can either put a high value resistor from the collector to the base or a 2 resistors from the base, one to the +ve, one to ground (a potential divider across the supplies). Again they need to be reasonably high values as they directly affect the input impedance.

Best thing is to Google it or find a circuit to start from, Wikipedia has this on it http://en.wikipedia.org/wiki/Bipolar_transistor_biasing

Unfortunately there is a lot of maths involved if you want to do it properly.

Sorry I'm a bit rusty on all this, it's a long time since I last did any.

deltafred · Post by **deltafred** » 05 May 2011, 19:11

allesz

I've just done a bit of searching and come up with this

https://www.freestompboxes.org/viewtopic.php?f=7&t=4178

A nice simple one germanium transistor circuit, the D.A.M. Red Rooster. It has the potential divider biasing.

allesz · Post by **allesz** » 09 May 2011, 16:31

Thanks a lot for your help...
Right now I think I will stick with the last schemo + 1uF cap on the emitter just as Dr Normis said.
The transistor seems to bias by itself (that's what confused me, because I never had previous experiences with germanium transistors).
I found similar use for too leaky germanium trannies adviced in another site (aron nelson's ? I don't remember

) and they suggested the use of bad trannies to build a buffer.
Leakysnob is somewhat a buffer, or a low gain booster, that don't give a lot of gain really... but if you put it in your chain the sound gets good (so good you don't want to turn it off actually).