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Old 12th April 2012, 02:50 PM   #31
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pot measures at 1,003K (and says 1K on it). I turned it all the way off and remeasured Q2, results:

b- 2,18 (i thought i'd include this as it also came in slightly higher) c- 1,52 and e- 1,51

rail (both at input jack and 330 resistor) comes in at 9,16
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Old 12th April 2012, 03:02 PM   #32
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Just rechecked every resistor and the 33K resistor on Q1-c / Q2-b seems to be a 3.3K one. Damn! Can't believe i didn't think to do this before (or that I actually placed the wrong value in there). But does the 2,12/2,18 reading here seem too high? and could that be drawing the current that Q2 collector should be seeing? I'll have to change it to a 33K one and check back. I'll have to order new ones so if you think that my choice of transistors would prefer a different value in it's place give me a shout and i'll order some of those as well.
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Old 12th April 2012, 04:18 PM   #33
Simon B is offline Simon B  United Kingdom
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Quote:
pot measures at 1,003K (and says 1K on it)

33K resistor on Q1-c / Q2-b seems to be a 3.3K one


Both of these will prevent it working.

Always a good idea to check components with a meter before soldering in. Especially resistors and pots, are they're so easy, and so prone to confusion as to their values!

Have to admit that I'm now a bit baffled how the voltages around Q2 are so low! Where's the current apparently flowing through the 8k2 resistor going to?

Eugene, may I suggest, given the small component count of this circuit, that you have another go at building it? All new bits, checked carefully etc. Your soldering will probably be quicker and better a second time round too.

Wrt transistors:

BC108, without the C at the end is listed as having gain (hfe) in the range 110 to 800. Popular folklore has it that the ideal values of gain for Q1 and Q2 are 70 and 115, respectively, but you don't want to get too hung up on that.

BC107s, gain listed as being 110 to 450 would be worth a try.

At the moment, your accidental use of 3k3 instead of 33k in the base feed to Q2 means that there's loads too much current flowing into what is probably a high gain transistor at Q2.


As you've got a meter, do think about testing transistors for gain, as detailed by RG Keen / Geofex, at the page I linked too earlier.
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Old 12th April 2012, 05:39 PM   #34
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Yeah, I'll probably build another one later on, but i feel a need to get this one working first (all other values checked out, and voltages seem even across all joints so i still think it's worth a try). I will, as i said, order some different resistors (around 33K) for both Q1 and Q2 collectors and give a go at biasing this thing. what voltages should I be looking for on Q1-c?
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Old 12th April 2012, 06:49 PM   #35
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Eugene .......1,003K....dont you mean 1,003 ohms

Suggest you dont alter the resistor values from those shown on the schematic......just put the correct values in and get it working first .

If you put 33K in the collector of Q2 also you will be back to square one .
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Old 12th April 2012, 07:37 PM   #36
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I mean 1K and a little tolerance
I thought I might test some different values in the position if i knew what voltages to look for and try to get as close to the mark as possible... thought I might as well get it right before soldering it in place, but i would obviously start with a 33K and check the voltages.
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Old 12th April 2012, 07:54 PM   #37
shanx is offline shanx  Canada
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Hi Eugene,
Here is some web site discussion on NPN Si version with some typical voltages. At Q1 collector: 30K at collector will probably be alright, but if you still have too high a DC voltage there just try & add more resistance at Q1 collector. Then adjust Q2 collector resistor to get around 4.5 VDC.
Sili-Face
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Old 13th April 2012, 01:00 AM   #38
Simon B is offline Simon B  United Kingdom
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I wouldn't try changing the circuit as per the runoffgroove schematic, at least not til you've got it working, though the point about getting the collector of Q2 around about halfway between the two power rails is a good one.

Good you feel like sticking on til you've got it working.

Don't stick anything like 30k in the collector of Q2 - if anything, you'll want less than 8k2, not more.

First of all get the Q1 collector resistor sorted - I'll mail you one if you want!

Also Eugene, please tell us, is the convention for decimal point/comma in Norway that 1,003k means 1003 ohms, or that it means 1003000 ohms - it varies country to country - I think in Italy for instance, it means 1003 ohms, but don't quote me on that.

Last edited by Simon B; 13th April 2012 at 01:05 AM.
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Old 13th April 2012, 01:02 PM   #39
shanx is offline shanx  Canada
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Simon is right, just FYI that link was to give you some typical voltages to look at for Si transistors. Q1 needs to be under control or you will drive the base too hard on Q2. Once that is done you should check Q2 collector at halfway point.
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Old 13th April 2012, 02:42 PM   #40
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Breadboarded the circuit earlier today for my own curiosity....results

BC107Bs

Q1.................. Q2
B 0.613V.......... B 1.330V
C 1.330V.......... C 3.18V (8K2 and 330R) , 5.03V (5K6 and 330R)
E 0.00V............ E 0.691V

mid point 8K2 and 330R 8.9V 5K6 and 330R 8.9V

BC109Cs

Q1............... Q2
B 0.594V....... B 1.283V
C 1.283V....... C 3.56V (8K2 and 330R) , 5.29V (5K6 and 330R)
E 0.00V......... E 0.652V

mid point 8K2 and 330R 8.9V 5K6 and 330R 8.9V

I put 27K resistor in series with the input to try and simulate a passive guitars output impedance and the output pot was set on max........AC results

fuzz pot on min..... gain = 1.16X
fuzz pot on max.... gain = 240X

both sets of trannies gave the same AC results

With the 8K2 collector resistor in circuit clipping started to occur on the negative first with an output of approx 65mV rms and the positive at 130mV rms .

With 8K2 replaced by 5K6 the output started to clip symmetrically at 140mV rms.

As i had done these measurements just thought i would post them .
What assymmetrical clipping does for the fuzz sound i dont know as i dont own a guitar and wether this matters when your are basically creating squarrish waves again i dont know . My only thought is youd probably get a louder and perhaps more controllable sound with the 5K6 and resulting
symetrical clipping/squaring/fuzzing.

PS rail volts 9.16V

Last edited by epicyclic; 13th April 2012 at 02:47 PM.
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