AC125 Germanium transistors

[teknoman2]

I just bought from ebay 150 of ac125VI trannies,
I know that are very good for making fuzz but are they good for treble boosters?

[ch1naski]

I've got a few of those, and I've not tried them in a treble booster, but I think they would probably be OK, if you can find some with an HFE range between 70-100

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[Gledison]

Hey mate! How are the measurements? Like hfe and leakage? Ive got some AC125 that have huuuuge leakage! I think that from 10 , 2 were ok!

[teknoman2]

Some are ok some are leaky.
Don't throw away a transistor that is above 300μΑ.
What is the conditions of measurements?
Do you use a atlas dca?

[Diamond]

How bad is leakage REALLY? I keep reading the same numbers and links, but is there someone with in depth knowledge that ACTUALLY knows and can explain instead of just quoting the same old websites and just saying "anything above xxx is bad, period"?

Is there someone who actually measured the leakage in good sounding germanium fuzzes and compared them to transistors with lower or higher leakage?

Finally, I read somewhere, I believe it was on FSB, that some verion(s) of the Tonebender actually require some leakage to work. Is this true?

[teknoman2]

The frequency respone of transistor and the hfe difference is important not the leakage to my ears,
I believe on Q3 on mkIII you can put a leaky transistor.

[twin1965]

Just finished building a Maestro FZ-1. Tried several transistors and the best sounding ones were the leaky ones.

This type of circuit needs leaky transistors in the Q2 and Q3 positions.

I take the advice given on ge transistor leakage but I also use my ears to decide whether or not I like the sound of them in a particular circuit.

Don't discard anything that's not within the 'acceptable' range.

There's a lot of circuits form the '60's that you could probably use them in.





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[DutchMF]

How bad is leakage REALLY? I keep reading the same numbers and links, but is there someone with in depth knowledge that ACTUALLY knows and can explain instead of just quoting the same old websites and just saying "anything above xxx is bad, period"?

Is there someone who actually measured the leakage in good sounding germanium fuzzes and compared them to transistors with lower or higher leakage?

Finally, I read somewhere, I believe it was on FSB, that some verion(s) of the Tonebender actually require some leakage to work. Is this true?

I'm calling Jakefuzz and Chromesphere..... Those guys have used/measured/sold so many transistors, if anyone knows, it's them. If they're not onto this thread tomorrow, I'll PM them..... 

Paul

[chromesphere]

Hey Paul.
Heres my special brand of understanding of all this germanium transistor mythos.  Probably full of holes?  Im no electronic engineer.  But here goes.
With the ac125 be careful which bin your buying from.  There are 3.  IV V and VI.  You have NO IDEA how long it took me to work that out!  I got a hold of the datasheet and can confirm these hfes: 
IV - 50-100 hfe
V - 75-150 hfe
VI - 100-150 hfe
With the ac125 the leakage is proportional.  So for say, a 50 hfe transistor you might get 80-120 leakage.  With 100 hfe, might be 200 leakage.  Some sellers show a photo of a V or IV bag, but send you something else.  I'm looking at you minifux...
So, if you get a VI bag, there all going to be very high hfe and leakage.  when you get around the 140 hfe mark, you start getting up to the upper limit.  Some will be 300-400ua which I reckon is fine.  Some will be over.
So, why cant you use over 300-400ua leakage?  Again, just my take on this.  Leakage doubles with every 10 degrees increase in temperature.  I take that as celius.  If so, at 20 degree (room temp), a 400ua leakage transistors is going to be 800ua at 30 degrees (hot room?).  40 degress its going to be 1.6ma.  That's getting up there.  40 degrees is getting pretty hot as well though, but its possible I guess.  My biggest concern would be run away leakage, I don't think you would get it at 400ua, but I usually try and keep them below 350 anyway (the ones I sell).  Run away leakage is where the current through the transistor heats it up, creating more leakage, more current, more heat.  Until the evitable.
What about tone with a leaky transistor?  I cant tell any difference.  Maybe a fuzz face or tone bender will misbias with a really leaky transistor in the hot sun?  One of the circuit designers around here could probably confirm that but i'm not 100%.  Otherwise they sound fine to me.  I have read a number of times, q3 in tonebender-esque circuits need leakage.  q1 and 2 doesn't matter so much.  I have to say, my Russian fuzz pedals do sound a bit different.  Not mind blowing though and I wouldn't spend too much time worrying about that.
One last thing about the ac125.  My sources (that's right I have 'sources') tell me the ac125 is in short supply.  I've got more then I will ever need so if you want some get them before they are gone.
Paul

[teknoman2]

Listen to Paul I ve been watching his channel and posts for a long time,
I agree with Paul 100% try to avoid minifux1 everything he sells is a waste of time and money.
I have bought some nkt 275 and they were non usable, after several mails I took refund of my money and I bought 150 ac125 VI bag from orpheus2005.
They are great to my ears.
I measured them with 15-18 celcius and the most of them had reasonable leakage.
Orpheus2005 is very good and repsonds to every mail in time.
Also he runs the LCI electronics ebay store.

I bought also 1000 1n4148 and they were great Vf was 400-420mV and they aren't chinese.
Try from orpheus2005 and you will not regret

[DutchMF]

Thanks for the explanation, Paul. It was very easy to understand, and it's really refreshing to hear from someone who has handled and measured enough transistors to be able to give some real-world info. I (and I think most of us around here) don't care if you're an electronics engineer or not, but you are what I call an 'expert through eperience', which is just as good (or better!!) in my book. Thanks mate!!

Paul

[JakeFuzz]

Three Paul's in one thread!     

My experience with AC125's is limited and I would defer to Paul ( ) for the best info. I did get a bag of AC125's a while back and they were all fairly high gain and leaky. They do sound good though. I've mainly put them in tonebenders with good results.

I have found leakage is important when building these classic circuits. I built a MKI a while ago that would only take 600uA+ leakage in the last position or it just wouldn't bias up right. On the other hand lower leakage seems to work well in fuzz faces. Typically the collector current is designed around about 1mA for BJT stages (using the FF as an example; that is about 0.55 mA with a 4.5v collector bias and an 8.2K collector resistor). Using Paul's example if we swing up by 400uA we are now biased @ 7.38v on the collector which is still in the active region but it probably wont sound quite right. Even higher and you might just hit saturation. I think you can design around higher leakage devices but it is the large variations in current which will make your quiescent point a moving target. 

[chromesphere]

Nice explanation Paul!  I usually add a bias control on q3 of tonebenders.  Im pretty sure that negates the temperature variance? I also just like the added versatility of being able to misbias it I usually find with a 50k pot on q3 collector I can get it to saturate and starve and everything in between (using ac125's with 300-400ua max leak on q3). Might not be possible with low leakage Russians?  You'd have a lot more experience with tonebenders then I though Paul so your thoughts are most welcome.
Paul

[ch1naski]

This thread will be of great help. I've got plenty of ac125's lying around. Thanks, all pauls.

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[chromesphere]

Forgot to mention, there are 2 ac125's i've seen (possible more revisions).  The older type is more of a cylinder shape then the newer 'top hat' style.  No idea what those ones measure like but thought i would just mention it.