Silicon FF

bonehead1972 · October 14, 2019, 02:47:39 AM

Hi geeks..
My first attempt to build a silicon FF. Transistors are low gain bc303 (q1 hfe78, q2 hfe112)The voltages are right but there is something wrong with the sound.. there is some scratching/sputering on top of the fuzz going into ugly decay of the notes. It disapeared as soon as I put 100pf cap across the q1 c/b. Tried some higher gain bc178 and bc109 with the same result. Germanium sounds right though...what am I missing?

Kipper4 · October 14, 2019, 03:00:22 AM

Mis biasing.

bonehead1972 · October 14, 2019, 05:34:29 AM

Thanks...

antonis · October 14, 2019, 05:36:17 AM

I should agree with Rich, but placing 100pF NFB cap on Q1 should heal high-pitch osclillation..
(I don't know if that's what you mean by "scratching/sputering"..)

bonehead1972 · October 14, 2019, 09:50:10 AM

The bias looks kind of ok...
Q1
C-1.5
B-0.6
E-0

Q2
C-4.65
B-1.5
E-0.9
Battery 9.35V
All parts with stock FF values except q2 collector resistor which is 4k7 which is good in my book..
Looks like it should sound fine without the q1 c/b cap.. but it does not

R.G. · October 14, 2019, 12:32:37 PM

Silicon is to germanium as a rusty pocketknife is to a freshly stropped straight razor.

bonehead1972 · October 14, 2019, 02:08:40 PM

Thanks to all of you guys!

R.G. · October 14, 2019, 07:35:51 PM

Quote from: R.G. on October 14, 2019, 12:32:37 PM
Silicon is to germanium as a rusty pocketknife is to a freshly stropped straight razor.

Actually, I got my own simile wrong.

I meant to say that silicon is to germanium as a freshly stropped straight razor is to a rusty pocketknife.

Sheesh. I can't even get my own stuff right!

bonehead1972 · October 15, 2019, 06:18:53 AM

Thanks for the tips

jgenet99 · October 15, 2019, 03:25:44 PM

Hey,

I've had great luck with a silicon fuzz face. Off the top of my head it looks like biasing issues, I think on the first transistor. According to electrosmash, the germanium biases the first at .7 on the collector, the schematic below is for a silicon and it is 1.35, so I think you've got something there. This is a link for a schematic for what is supposed to be a Jimi Hendrix Dunlop fuzzface, and they bias the second transistor much higher (you might also want to take a look at the fulltone '70 schematic... it's an ironic mess, as they both claim to be the 'authentic silicon fuzzface' and then also mod it extensively).

If you can post any pictures of the circuit, it might help diagnose the problem.

http://revolutiondeux.blogspot.com/2012/02/dunlop-jh-f1-jimi-hendrix-fuzz-face.html

Also double check two things from the 'stock' values... when the silicon was issued, they switched to NPN (which means you will need to flip the capacitors (it would be possible that the capacitor on the fuzz control was flipped, I am not sure what that would do, but it is there to increase gain)) and one of the resistor values. Honestly, you can make it work with a very wide range of parts, so it's hard to know how much that will affect it. If you have trimpots, I would definitely replace the collector resistors so that you can bias it. Let us know what you come up with!

Electric Warrior · October 15, 2019, 05:45:43 PM

Quote from: jgenet99 on October 15, 2019, 03:25:44 PM
Hey,

I've had great luck with a silicon fuzz face. Off the top of my head it looks like biasing issues, I think on the first transistor. According to electrosmash, the germanium biases the first at .7 on the collector, the schematic below is for a silicon and it is 1.35, so I think you've got something there.

.7 is pretty much the upper limit for germanium Fuzz Faces. They tend to have lower voltages on q1c.

In a silicon Fuzz Face, the voltages on both collectors mostly depend on Q1's hfe. Between 1.2 and 1.3V is typical for the high gain transistors they used (BC108C etc). Lower gain transistors have higher voltages here (they used BC183KA for a while), but 1.5V is quite a lot. Personally I'd use higher gain devices so it will bias right.

I don't think this is a bias problem, though. Silicon Fuzz Faces are prone to oscillating. It's hard to avoid and may depend on the grounding scheme, the circuit board layout or the wiring. The cap between q1 c and b is a good solution.

jgenet99 · October 15, 2019, 05:53:33 PM

It doesn't sound like oscillating is being described at all. The ugly decay sounds like a biasing issue.

bonehead1972 · October 16, 2019, 02:31:42 AM

Its
still on breadboard spagetti wiring.
That's why I am writing here:) voltages looked good right off..(I thought 1.5V on Q1C was a bit high but not so wrong) and the thing sounded misbiased...I will try to rebias with trimpots.
Thanks to the info in diystompboxes I have built 5-6 ge fuzzes so far and they sound very good!
Cheers

Electric Warrior · October 16, 2019, 01:10:34 PM

Quote from: jgenet99 on October 15, 2019, 05:53:33 PM
It doesn't sound like oscillating is being described at all. The ugly decay sounds like a biasing issue.

Oscillation can cause an ugly decay as well. As it can be fixed with a cap, I think oscillation sounds more likely.

bonehead1972 · October 16, 2019, 04:10:10 PM

yes..with a cap across q1 c/b it sounded nice and cleaned up with the guitar vol knob exceptionally well /vol and fuzz at max- fixed resistors/,
may be some higher gain si transistors would be easier to bias but i dont want to give up those bc303 low gain, they sound good i think..

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