# Simulcast Transistor - which one for OC71 ?



## lcipher3 (Jan 28, 2020)

I got my bag of resistors from overseas - measured with my transistor "tester".
I'll definitely socket and play with them, but what has everyone used before?
These are measurements I got:

*TG5S = OC71 Germanium PNP   * 
*ID    hFE    Vf*
#1    75    146
#2    64    143
#3    77    147
#4    31    238
#5    63    138
#6    38    263
#7    44    125
#8    42    126
#9    41    122
#10    40    122


*ASY37S = AC128 - Germanium PNP* 
*ID    hFE    Vf*
#1    175    199
#2    75      273
#3    133    183
#4    67      253


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## Nostradoomus (Jan 28, 2020)

I’d use the ones around 70-80 but try the 133 just for fun.


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## benny_profane (Jan 28, 2020)

When you say "tester," what does that mean? Unless you're working with a meter that is specifically tuned for Ge devices (e.g., a Peak Atlas), those numbers aren't telling you much.

Reference this article to see how to get reliable gain and leakage readings from Ge transistors.


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## lcipher3 (Jan 28, 2020)

Yeah - I've done that method before.  Will have to peform it again.  Just using my cheapo tester:


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## benny_profane (Jan 28, 2020)

Yeah, that’ll work with silicon just fine, but you won’t know what it’s telling you about germanium.


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## lcipher3 (Jan 29, 2020)

benny_profane said:


> Yeah, that’ll work with silicon just fine, but you won’t know what it’s telling you about germanium.



Thanks - I'll use the method you linked.  Sounds like 70-130 is the way to go


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## benny_profane (Jan 29, 2020)

lcipher3 said:


> Thanks - I'll use the method you linked.  Sounds like 70-130 is the way to go


Yeah, that sounds about right. You’ve got a bunch, though. Try them out and see what works for you. I think as low as 60 would still be totally fine.


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## Chuck D. Bones (Jan 29, 2020)

HFE, even if measured correctly, is only half of the story with Ge transistors. You definitely need to know the leakage. If the leakage is too high, Q2 won't bias correctly, no matter what the HFE is. From the Simulcast schematic, it looks like the circuit is pretty insensitive to Q2's HFE. I'm guessing that as long as they aren't too leaky, most of your transistors will sound about the same. Maybe people who have built a Simulcast can report the collector voltage on Q2. That will provide some guidance on picking transistors.

My $18 Chinese tester measures Vbe, leakage (Iceo & Ices) HFE and reports the collector current used for the HFE measurement.  I have confirmed its HFE measurements with my curve tracer.


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## benny_profane (Jan 29, 2020)

Chuck D. Bones said:


> HFE, even if measured correctly, is only half of the story with Ge transistors. You definitely need to know the leakage. If the leakage is too high, Q2 won't bias correctly, no matter what the HFE is. From the Simulcast schematic, it looks like the circuit is pretty insensitive to Q2's HFE. I'm guessing that as long as they aren't too leaky, most of your transistors will sound about the same. Maybe people who have built a Simulcast can report the collector voltage on Q2. That will provide some guidance on picking transistors.
> 
> My $18 Chinese tester measures Vbe, leakage (Iceo & Ices) HFE and reports the collector current used for the HFE measurement.  I have confirmed its HFE measurements with my curve tracer.


Spot on in re leakage being a huge factor there. I should’ve made that clearer.

The issue with the leakage readings from those units is that, unless you know the test circuit, it might not be more than a ballpark figure. For this application, that’s probably fine, though.


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## farwest1 (Mar 22, 2020)

What will be the difference in sound from high leakage to low leakage transistors? 

Do the high leakage ones sound more velcroey and sputtery? Or is there some other difference?


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## Chuck D. Bones (Mar 22, 2020)

Depends on the circuit.  Some pedals, like the various Tone Benders, depend on leakage to set the bias on one or more of the Ge transistors.  If the leakage is too low, no sounds gets thru. With too much leakage you get fuzz, but no velcro rip.  Somewhere in the middle, you get that familiar gating, ripping velcro, torn speaker sound.  Other pedals, like the Sunface (Sunflower), have variable bias so you can dial in just the right amount of gating & sputter.  You still need to install a transistor that's in the ballpark, but that circuit can accommodate a range of transistor leakages.  The Hoof (Ungula) only works right with low leakage Ge transistors.  That's why it's a good idea to install sockets for the Ge transistors.  You'll be able to pick the ones that give you the sound you want.  Vintage Ge transistors are all over the map.  Unless you are buying tested devices that are matched to your circuit, like Small Bear sells, you are going to have to procure a bunch of transistors and audition them in your pedal.  If you buy them off of eBay, then you're gambling they will even work, let alone work in your pedal.


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