Fuzz Face - any risk of frying a transistor with bias pot?

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Fuzz Face - any risk of frying a transistor with bias pot?

Postby Tone-Analyst » 07 Sep 2014, 05:14

I decided to change the 8.2k collector resistor of Q2 for a 10k outboard pot. I like how it becomes spitty at lower bias voltages.
My question was if it was dangerous to fry a resistor if that pot went to extreme settings. I suppose the starving side is safe but on the other hand if the pot rotates to a 0k value will the transistor be damaged? If yes then I could put a 1k resistor in series to prevent that?
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Re: Fuzz Face - any risk of frying a transistor with bias po

Postby Nocentelli » 07 Sep 2014, 09:24

Yes, a limiting resistor is the safest approach: You're never going to want a collector resistance of less than 1k so that value will work fine. If you're breadboarding it, you can play around with the bias points until you find a useful range, and replace the 1k with whatever you find to be the lowest useable resistance so your bias control has useful settings all across it's range.
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Re: Fuzz Face - any risk of frying a transistor with bias po

Postby Tone-Analyst » 09 Sep 2014, 20:31

Ok thx! I sort of guessed that this was a "dangerous" spot but it may have been there for a few seconds. If the pedal still works and sound normal can I assume they were not damaged?

Another thing, with the recommended 8.2k on Q2 collector I only get 1.1 volts. To get a decent 4.5v the resistance on my pot is 4.3k. While on Q1 the total opposite, I got 2.0v which is twice what it should be with a 33k resistor. Isn't it odd? I thought that using identical transistors that they would have the same trend, both too high or too low.

I'm using old Si transistors that I pulled from early 70's electronics, they looked cool with rounded metal helmets :)
Matsushita 2SC645 (C) if it means anything..

Also I read that adjusting the 8.2k on Q2 collector was not the best way to bias, because it altered the sound of the circuit. That the best way is to adjust the 1k at the emitter, of course being a pot it's a bit more trouble.. any truth?

Another thing that bothers me is that my gain pot does nothing until the last bit, soubds like i got a log pot there instead of linear right? I bought it as a linear but there's no marking on it..
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Re: Fuzz Face - any risk of frying a transistor with bias po

Postby Tone-Analyst » 10 Sep 2014, 00:03

Here's a video of the pedal: https://www.youtube.com/watch?v=hZlk-E-Ntr4
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Re: Fuzz Face - any risk of frying a transistor with bias po

Postby astrobass » 11 Sep 2014, 03:17

2SC645 is rated for an Hfe of 100. Which is right in the range of where people often like Fuzz Face transistors to fall most times. Sometimes a higher gain second transistor is good. But by luck the transistors you scavenged are a good fit for that circuit.
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Re: Fuzz Face - any risk of frying a transistor with bias po

Postby DrNomis » 16 Sep 2014, 02:29

Tone-Analyst wrote:Ok thx! I sort of guessed that this was a "dangerous" spot but it may have been there for a few seconds. If the pedal still works and sound normal can I assume they were not damaged?

Another thing, with the recommended 8.2k on Q2 collector I only get 1.1 volts. To get a decent 4.5v the resistance on my pot is 4.3k. While on Q1 the total opposite, I got 2.0v which is twice what it should be with a 33k resistor. Isn't it odd? I thought that using identical transistors that they would have the same trend, both too high or too low.

I'm using old Si transistors that I pulled from early 70's electronics, they looked cool with rounded metal helmets :)
Matsushita 2SC645 (C) if it means anything..

Also I read that adjusting the 8.2k on Q2 collector was not the best way to bias, because it altered the sound of the circuit. That the best way is to adjust the 1k at the emitter, of course being a pot it's a bit more trouble.. any truth?

Another thing that bothers me is that my gain pot does nothing until the last bit, soubds like i got a log pot there instead of linear right? I bought it as a linear but there's no marking on it..



With Si transistors and a stock 33k Q1 collector resistor, you should get about 1.2V on Q1's collector when your breadboarded circuit is biased right, you can also check to see if each transistor has .6V across it's B-E junction with a multimeter set to read DC volts, I have a Jim Dunlop JD-F2 Fuzz Face and the gain pot behaves the same way as you're describing so it's actually normal, I think some original Fuzz Faces probably used a reverse-log pot for the Fuzz control..... :thumbsup


The reason why Q1's collector voltage tends to be about 1.2V in a correctly biased Si transistor Fuzz Face is because the Q1 collector voltage effectively biases both Q1 and Q2 (since Q2 is DC-Coupled to Q1's collector), Q1's collector-voltage will therefore be equal to the two B-E voltage-drops of Q1 and Q2 added together, sorry if I'm getting a bit technical here.... :thumbsup
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