Ideal HFE's for BC108C's

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
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peter25
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Post by peter25 »

I pretty much got all my parts to built a '68 fuzz with BC108C's
The sound I'm looking for can be heard on these clips:
The first one is an actual ( very successful) rebuilt of Jimi's red fuzz face:
https://www.youtube.com/user/fuzzfaceex ... rPKmWT3tG8

This is an original BC108C fuzz from 68:
(don't mind the wah, the fuzz can be heard just fine^^)

So what do you guys think about these fuzzez? And especcially the hfe's

Any comment will be greatly appreciated!

Thx

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Post by Hides-His-Eyes »

They're so consistently high I wouldn't worry. The difference between 500 and 550 is not the same as the difference between 70 and 120 obviously!
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Post by sinner »

I've sorted the lowest posibble range from my stash, and that sounded the best to me. I don't remember what was that exactly, but it was around 130-250. I've shifted about 300pcs to have 5 sets close to this :mrgreen: This is why I prefer bc183l, 150-200hfe range is more common

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peter25
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Post by peter25 »

Thx for the replies.

My guess is I'm gonna like the lower ones also more... they have a more Germanium kind of gain I guess...
I thought I was overdoing it with the 30 BC108C's I got... Well maybe I'm lucky :lol:

Is this assumption right that the lower ones sound more Germanium like?
Not that I want Germanium sound, but I'm not aiming for too much hi gain fuzz

I love that clip in my first post where the guy plays a SG through a FF. At 2:20 you can see the inner parts of the fuzz.

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Post by RnFR »

if you want low gain bc108s, the bc108c is not the way to go. the "c" designation is for the highest gain range for 108s and 109s. bc108a's would be a better way to go. sorting a bunch of high gain transistors for low gain just because they were in the original fuzz face seems kind of like a pointless excercise when you can get the same exact transistors presorted from the factory.
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Post by peter25 »

Hm this makes sense.... I know what sound I like, but don't know if this is hi gain or not. Would you describe these clips more likely as BC108C's or BC108A's?
Or is the sound maybe very close to each other?

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Post by MicMicMan »

for me, the fuzz on the second clip doesn't sound like a "low gain" one. The first neither. Hfes might be higher than 200 for both of them.
Anyway this is quite hard to figure this out with a wah plugged in front of the fuzz, as the behaviour of the fuzz will heavily depend on the device it's plugged-in. Buffer / true bypass ? What output impedance ? Which guitar ? Your fuzz face tone will heavily change according to that...
Adding a pot (let's say : 20k ?) used as a variable resistor right at the input of your fuzz face might help you getting the sound you want to quite easily and quite independently from the transistors you pick.
Also, you can have a glance at Jack Orman's very simple mod : http://www.muzique.com/lab/yaff.htm

PS : BC108a are probably a good choice for a fuzz.

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Post by peter25 »

If these units are considered Hi Gain, I'm on the right track I guess.
Also I think I will add a pot, if I can control the sound more that way. I have a 25k trimmer pot which I'll install.

Thx!

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Post by devastator »

I put two bc108"C" in a fuzz face style circuit I made , the transistors (Q1 actually) gave a violent oscillation....

We could certainly use them but you've to had some caps to tame highs and maybe put a small resistor at Q1's emetter to reduce gain.

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peter25
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Post by peter25 »

Just measured the hfe's of all the bc108c's I've bought. Just 30 of them.
Their values all are between 600 and 850 hfe. These should be no higher then 800 right?
Reading messages here I kinda hoped they were not as high. Maybe some around 200 or so.
I wonder if I did something wrong measuring. I used a DMM which has a built in hfe meter.
Another question, could I break something while installing these hi hfe trannies?

thx.

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Post by RnFR »

that is just the manufacturers average spec. it could definitely go off that.

again, if you want lower gain transistors, you have to buy lower gain transistors! BC108C's are high gain- buy BC108A's if you want low gain 108s. or just buy some 2369's like everyone else.

no you can't break anything. but a circuit meant for a certain transistor with a certain Hfe may not bias properly.
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peter25
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Post by peter25 »

Thanks for the reply. The part about breaking stuff was the most important for me. The hi gain issue can be dealt with afterwards.
Maybe stated by micmicmanbut would a bias pot do the trick as for making the biasing more easy?
Like in this mojo face.
http://www.forrestwhitesides.com/node/59
You have to scroll down a bit.
The 8.2k has been replaced with a 25k pot.

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Post by peter25 »

Just an update.
Bought some bc108A's to see the difference between the C's and the A's.
Basically the C's have an Hfe ranging from 600~800.
The A's have a Hfe of all about 160. Highest 162, lowest 157... in a pick form 30 pcs.
Still waiting for some resistors. Postage has been on strike over here, which can be a bit annoying.
Will post some clips and pics ofcourse.

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Post by DrNomis »

peter25 wrote:Just an update.
Bought some bc108A's to see the difference between the C's and the A's.
Basically the C's have an Hfe ranging from 600~800.
The A's have a Hfe of all about 160. Highest 162, lowest 157... in a pick form 30 pcs.
Still waiting for some resistors. Postage has been on strike over here, which can be a bit annoying.
Will post some clips and pics ofcourse.


I built myself a Fuzz Face using a couple of unmarked Germanium transistors,one had a gain of 105,and I used it for the Q1,and the other had a gain of 119,I used it for the Q2,they sounded very good,i got a nice smooth bluesy sounding fuzz from them,the Q1 collector voltage was about .5V,the Q2 collector voltage was 5.21 V..... :)
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

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Post by peter25 »

Just finished my first try at attempting to build a fuzz face. Used 2 trannies both 161 hfe. I honestly can't believe how good it sounds...
I really never expected to get this close immediately to the sound I wanted. I haven't even tried different trannies or a bias pot yet and I can hear some hendrix woodstock tones.
Yup I'm a happy man atm! :D Sunday I'll go practice and try this thing out for real. I hope to get some good clips also.
Only thing that's not perfect is that there's a bit too much fuzz and not enough face... So I'll try some different combinations in trannies. If you guys have any suggestions I'll sure aprreciate it.
The bass and hi notes are just perfect for me. No ice pick or over muddiness. The fuzz can be a bit overwhelming when set at full though.

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peter25
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Post by peter25 »

Early Pics.
Image
Image

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Post by HTH »

I needed a fuzz for a few songs in my live set, so in the summer I made a silicon fuzz face using BC108s - it oscillated and needed some caps to keep it under control, plus the fuzz was a bit too much; kind of mushed out when used at a gig.

I changed the transistors to BC183Ls and like the tone much more - they're MUCH less susceptible to oscillation and the tone is more focused due to having less gain. I've used 2N3904s for fuzz faces before and they sound fine if you want a lower gain fuzz closer to what you'd get from a Germanium fuzz face.

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peter25
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Post by peter25 »

Yeah I agree. But I absolutely love the fuzz now.
When the fuzz is maxed it does oscillate, but just a hair under max it's ok.
I exchanged the 8.2k for a 25k pot. This is an awesome trick I learned from http://www.forrestwhitesides.com/node/59 .
I probably will exchange this pot for a 15k or maybe even 10k. The 25k pot can only be used half, after that it goes out of bias. So it's a bit harder to bias atm.
But the sound. I can't believe it. Yesterday I played with my strat through a plexi clone and it's awesome.
Compared to my other fuzzez it's at least as good. As for a woodstock Hendrix type of fuzz it's my best. The sustain seems endless.
The feedback I get from my amp seems to be better also. I can get that Foxy Lady feedback now.
Another great thing is it obviously colors your tone, but when I turn down the volume of the guitar and you turn on the effect there's a smooth transistion instead of a single point where you can hear the effect being turned on. None of my other fuzzez have that and is usefull going from a rhythm to a lead tone. I promiss I'll make some clips next time we go practice.
My friend forgot to take his recording equipment last time. We'll probably go in a week again.
For a first build I'm more than satisfied. I'll definately go do another one in a few months or so. Probably a MKII tonebender, which is next on my list of all time classics. After that maybe a more difficult circuit. But for that I'm going to need some basic knowledge of electronics I guess...

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Post by peter25 »

Oh forgot to mention. I ended up using the BC108C's with about 560 hfe. Didn't sound better, just different. Very much more gain instead of smooth fuzz.

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Post by DrNomis »

I took the lower gain Ge transistors out of my fuzz face build,and put the higher gain transistors in,the Q1 transistor has a gain of 139,and the Q2 transistor has a gain of 148,the output capacitor is a 630 V .047 u7 Greencap,anyway,I tried it out and found that I could get a nice smooth fuzzy sound with the fuzz control set to the 12 O clock position,nice for blues,maxing out the fuzz control gave a very thick bassy fuzztone with smooth sustain..... :)

But one thing I noticed with this pedal.....very dynamic.....you can vary the amount of fuzz just by varying your picking.... :)

I changed Q1's collector resistor from 33k to 22k,Q2 collector voltage was -4.7 V,just .2V shy of -4.5V,clipping was pretty symmetrical.... :)


You could probably try BC108a transistors with those gains if you can find some..... :)

Also try BC548a transistors,which are same as BC108a,but in a black plastic encapsulation style.... :)
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

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