Weird voltages on a twin face Ge side

K

kurtis1

Member
I built this twin fuzz face from pedal pcb. The silicone side works fine but on the Ge side I can't get less than 8.2v on Q2.


The voltages are wierd all over the Ge side of the circuit. I've tried a different set of ac128 transistors and I get the same results. The voltages after R1 should be 10x what they are. What would cause this? Is C1 somehow dumping voltage to ground? Is that possible? I've attached my voltage readings, a gut shot and a diagram from electrosmash dictating the correct voltages.

Thanks


https://drive.google.com/file/d/11HeCZtQq7UNQSgBajusg15dBdL9xz1_A/view?usp=drivesdk


https://drive.google.com/file/d/11JzXTKd427Umf7C3R2YzHwNdkFP4bMte/view?usp=drivesdk


https://drive.google.com/file/d/11HwKKZ5r85hbMWDtrI2pKyV40K4kqqJr/view?usp=drivesdk
 
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music6000 said:
Can you test the Transistors you just pulled to give us those readings?
Click to expand...
Just tested these 2

20210426_210927.jpg

drive.google.com drive.google.com

https://drive.google.com/file/d/11yzMbZLuy32Drm9aY7x96STigBjtUq-7/view?usp=drivesdk

I've also tried and tested they other "medium gain" set and the "low gain" set I purchased from them. Both sets read like :
Medium2
Iceo=0.55m
Ices=0.16

Medium 1
Iceo=0.36m
Ices=0.12m

Low 2
Iceo=0.58m
Ices=0.16 m

Low 1
Iceo=0.39m
Ices=0.11m

Even the low set takes 16k resistance on a trim pot to bias to -5v
 
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Chuck D. Bones said:
The R4 measurement confirms that PNP1 is way too leaky for this circuit.

Leakage is not "good" or "bad." Some circuits rely on a certain amount of leakage to function.
Click to expand...
I tried to get some readings with my low gain ac128. I still only get - 0.25v on pnp2 emitter. Electrosmash dictated it should be -0.5v

When I test R4 with the method you mentioned earlier I get -0.2v

And it still takes 16k to bias it... But I guess it's better than the 72k it takes to bias the high gain transistors I've been dealing with.
 
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Chuck D. Bones said:
Did you use the transistor you call "Low 1" as PNP1?
Click to expand...
Yes, in all cases #1 was always used and tested in PNP1.

The "low" matched set seems to give me the voltages closest to what electro smash dictates. But they're still only half the voltage that they should be and can't even be biased unless I use a much larger trim pot than the pedal pcb build doc specifies.
 
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Electrosmash is a good website, but you should treat their biasing recommendations as suggestions, not hard and fast rules. Fuzz Face biasing is a matter of personal preference.

I'm betting that once you get some decent transistors, your Twin Face will work as it should. In most Germanium-based pedals, leakage is much more important than HFE. In a Fuzz Face, the lower the leakage, the better.

Here's a typical Russian P28. These would work great.

P28 test data.jpg
 
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Chuck D. Bones said:
Electrosmash is a good website, but you should treat their biasing recommendations as suggestions, not hard and fast rules. Fuzz Face biasing is a matter of personal preference.

I'm betting that once you get some decent transistors, your Twin Face will work as it should. In most Germanium-based pedals, leakage is much more important than HFE. In a Fuzz Face, the lower the leakage, the better.

Here's a typical Russian P28. These would work great.

View attachment 11331
Click to expand...
Awesome, thanks for the help. I'm checking with banzai music to see if they can send out some AC128's that arn't so leaky and if not maybe ship out some "work alike" transistors. If they cant then I'll try to source some from pedal hacker if small bear doesn't get any in soon.
 
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After Banzai screwed you once, you're going back for more?

I don't usually recommend buying transistors on eBay, but that's where I got the one in the pic. Paid less than $2 ea including shipping for 60 transistors, a mix of P28 and MP38A. They came from Ukraine, so shipping took a few weeks.
 
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Chuck D. Bones said:
After Banzai screwed you once, you're going back for more?

I don't usually recommend buying transistors on eBay, but that's where I got the one in the pic. Paid less than $2 ea including shipping for 60 transistors, a mix of P28 and MP38A. They came from Ukraine, so shipping took a few weeks.
Click to expand...
Yeah, good point about banzai... Maybe I'll try doing some research on ebay then. I'll Look for someone with some good feedback, at 2$ a piece I don't really care if I have to throw-away 3/4 of them.
 
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I checked on eBay to see if the guy I bought from in 2019 is still selling Germanium, but he has disappeared. There are plenty of other sellers in Russia selling the same or similar devices. The MP38As were all leaky (<500uA) and medium gain (55 to 75) and are all usable in one pedal circuit or another. The P28s were all slightly lower gain (50 to 70) and very low leakage (<50uA). It's always a crapshoot on eBay, but they do protect the buyer if the product turns out to be crap.
 
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Chuck D. Bones said:
I checked on eBay to see if the guy I bought from in 2019 is still selling Germanium, but he has disappeared. There are plenty of other sellers in Russia selling the same or similar devices. The MP38As were all leaky (<500uA) and medium gain (55 to 75) and are all usable in one pedal circuit or another. The P28s were all slightly lower gain (50 to 70) and very low leakage (<50uA). It's always a crapshoot on eBay, but they do protect the buyer if the product turns out to be crap.
Click to expand...
Thanks chuck, I took your advice and ordered 20 p28's from ebay. They where like 18$ with shipping. I'm still going to try to get ahold of some AC128. Looks like pedal hacker is the only reliable source currently.
 
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music6000 said:
Did you end up getting this sorted ?
Click to expand...

I'm still waiting for the p28's to arrive in the mail but tracking says they've been in Canada for 5 days (just not where in Canada) so I should have them fairly soon.

Also I ordered some real ac128's from pedal hacker but they still haven't arrived yet.

I tested all My leaky Motorola ac128's from banzai and used the least leaky ones (still in the 300-600 range) and they arn't exactly "matched" and the pedal does sound pretty good. But I'm using a 17k resister to bias it down to 5.2v.
This setup actualy sounds usable, it's just a little quiet and I don't end up with a ton of fuzz. So I'm 99% sure that my issue was bad transistors.

When I get my new transistors, I'll post back here and let you know how it goes.
Thanks again for all your help!
 
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