Another fuzz face thread! got a quick question about variation in sound....

[OCTAF1SH]

Hi guys, I know, another thread about fuzz faces....even I am bored of reading them now!

Have just built 2 fuzzfaces, using the GGG late 60s schematic, one as a gift for a mate.  They are my first pedal builds, but I have built a few valve amps, and am working on a long term project all valve theremin too.

Here is the schematic I've used:
http://www.mediafire.com/?qf5t8ncua7f1d1r

As my first pedals, I have little experience in effect circuits, and modification thereof.

I have used AC128s, and made them +ve ground. One FF has matched AC128s with an Hfe of 111-120, and the second pedal has a different matched pair, with an Hfe of 101-110. I got the Ge trannies from Banzai, and so was not equipped with SBE's suggested resistor values.

The first pedal I have left all as per schematic (with the 111-120 AC128s), and it sounds pretty good, although a little harsher than the sound I had in my head, and I cannot adjust the trimpot down to 4.5v, I can only get down to around 5.1v.  The second pedal (built as a gift) I took the advice from someone on this forum (sorry, can't find the post any more) and adjusted the value of R5 (retaining the 10k trimpot) from 1k, to 8.2k (the advice said anything from 5-10k - I thought this may reduce the voltage to Q2, and both enable me to test a lower range with the trimpot, and soften the distortion a bit). Having just finished building, I went to bias the transistors, and could only get the voltage down to 5.4v, measured at Q2's collector, and on plugging it in, all be it only a low output from my amp, it sounds even more harsh and flabby. Now for the questions.....!

What causes the sound of a fuzz face to be harsh and fart like? In my (quite possibly incorrect) assumptions it would be too high a voltage being driven through the circuit, but although Q2's collector voltage is higher than my other pedal, it is not what I would consider too high. (BTW, I am someone who would rather use my ears to bias a pedal, than to say 4.5v at Q2 produces the ideal sound, but as these are my 1st pedals, obviously, I want to try the 'golden number', and see why so many clever people like it so much, then adjust to my preference if necessary)

Also, can anyone tell me how the Hfe  of a Ge transistor affects the signal and consequently, the tone of the effect. My clearly faulty understanding is that a higher Hfe transistor has a higher output, which would make the Q2 C voltage higher therefore a more overdirven signal, but the pedal with the higher Hfe is the one which sounds better....( this is all assuming the farty sound IS caused by too high a signal voltage, not too small - please correct me if i'm wrong)

And also, surely the increased value of the R5 resistor in the second pedal, should have reduced the input voltage to Q2, and therefore output from the component. Could this cause my problem?


Sorry for the garbled post guys, just trying to give you as much info as poss, while not really understanding what causes the problem I am experiencing. Just hope it makes sense to you!!

Many thanks for any replies,

Wil.l

[brett]

Hi

What causes the sound of a fuzz face to be harsh and fart like? In my (quite possibly incorrect) assumptions it would be too high a voltage being driven through the circuit, but although Q2's collector voltage is higher than my other pedal, it is not what I would consider too high.

Farting/stuttering is usually caused by a transistor turning on and off.  The signal itself is turning the transistor on and off instead of the bias voltage.  Therefore, I'd say that your farty FF is mis-biased.  Check the base on Q1.  It should be 0.3V (+/- 0.3V) below the emitter voltage (the supply).  Q2 should also have a base-emitter difference of 0.3V.
The voltage on the collector of Q2 isn't critical.  Half the supply +/- 1 volt is fine.
cheers

[OCTAF1SH]

Thanks brett,

This is all beginning to make sense to me, slowly but surely!

Thanks for the prompt reply, as soon as I get in later i'll break out the dmm again and check out your suggestion.

Thanks again brett, i'll let you know how I get on.

Wil.

[OCTAF1SH]

Solved...kind of!

bizarrely, when I rechecked Q2's collector voltage, it read 2.3v......clearly a glitch with my DMM last night, either user error, or errant reading. re set the trim pot to 4.5 and sounding sweet, anything from soft Gilmore fuzz to a jack white muff-ish (bit less sharp clipping) noise - perfect for me!

out of interest, when checking the voltage difference from base/emitter, do i check both in reference to each other, or ground? had (obviously) some funny readings, but before I rechecked them, I discovered the collector voltage from Q2, reset that, tested pedal, and was pleased, so closed it up and didn't go any further.

Thanks so much for your reply Brett, even though it turned out (i think) to be a different issue, it helped me further understand this and other circuits - which is what we're all here for!

take care,

Wil.