Fuzz Face - Clipping in Q1 and Q2

[Jaicen_solo]

This evening, I read through R.G's technology of the Fuzz Face document, looking specifically for references to the effect of different transistors when I noticed this line:

Fuzz Faces naturally tend to bias with only about half a volt on the collector of the first transistor, so there is a lot of room upwards. The input stage will first hit mushy saturation on one polarity of signal and then if driven hard enough, hit cutoff on the other polarity. The Fuzz Face has asymetrical clipping designed into it!

I've done a lot of reading about using silicon Q1 transistors for FF's, because Q2 is 'the clipping' transistor. Re-reading the article made me think that actually, a lot of the character of the distortion is derived from the first transistor's asymmetric clipping. The hybrid works so well because the second transistor is adding (subtracting?) character with it's more limited frequency response.

So, I figured that I'd scope it out and see wether this holds true. I recently finished a boutique NPN '69 pedal from the GGG layout, so I popped my scope on that using a 2N3904 and an AC127, biased at 4.5v and sounding great (if a little wooly).
As it turns out, Q1 is adding an awful lot of distortion to the sound. It clips heavily asymmetrically with very little signal, just as R.G says. The waveform is heavily clipped at the bottom, with rounded peaks. Scoping out Q2 shows that this stage is pretty much just squaring off the waveform.
I just thought i'd share my observations, I wonder if anyone else has any thoughts?

[brett]

RG is a v smart fellow about these things.  Yes, fuzzfaces are mostly about Q1.
You might consider Ge transistors - they are more asymetric (the base of Q1 is 0.3V from ground, whereas Si is 0.7V from ground).

[mac]

I like FF Q1 distortion so much that I built a kind of booster + soft distortion unit using just the first stage.
With a small input cap, and the neck HB, the clean chn of my amp screams, a kind of creamy hard jazz sound
I'm thinking of adding Ge switcheable clipping diodes, ie, Electra dist.

One thing I noticed with the transistors I have is that power germs clean up better than small signal ones. I think this has something to do with gain of power devices falling faster at low input currents than small signal ones. Opinions?

mac

[Jaicen_solo]

When you say power Germ's, which ones are you referring to?
I would think that it may have something to do with the amount of gain available, and the way the transistor responds to input signals.
I would speculate that a power transistor is not really designed to handle small signals, it's less sensitive if you will. Also, the gain of a particular device is not linear, so a power transistor might produce more gain for large signal swings than for smaller signals.

Anyway, it looks like you really do need to have two Ge devices to get the Ge sound. Having said that, I really like the hybrid sound!

[mac]

By power I mean ACxxx, NKT275, 2SD72K, 2SD352, etc, most about 1w max.

mac

[Jaicen_solo]

According to my data sheet, which is a contemporary list of AC transistors and their properties, only two are listed as having more than 1w dissipation (AC180K & AC181K). They're all listed as small signal transistors.

[brett]

Hi
a few things might be getting confused here.
Power devices usually have lower hFE, which means that thet distort less due to less gain and also reduce the input signal due to lower input impedance. 
They usually have a stronger "knee" in the Ic vs hFE curve, so that gain is less at low hFE, and that has 2 separate effects: at low Ic the average gain is lower, and the gain varies dynamically a lot (the signal makes the transistor move up and down the Ic-hFE knee in time with the Vin).
Most Ge devices are fairly "bent" in terms of Ic-hFE, but old power devices are extremely bent and basically don't function at lowish Ic (uA).
Higher power devices also have high capacitance, so they might roll off high frequencies.  In any case the low input impedance (a result of low hFE) will rob lots of highs from the guitar pickups. 
IMO an ideal Ge transistor for a single stage or Q1 is a russian GT308b.  They were made in the early and mid 1980s and there are still quite a lot around.  The ones I have measured have excellent hFE (average 51 at 1mA, range=30 to 80), very low average noise (25uA) and almost none noisey or dead (1 in 100).  Otherwise, many Japanese devices (2SB) are excellent.
cheers

[mac]

I have to find my notes; long ago I measured the gain of 2n388, AC121, 2sd72 and 2sb172 at different base currents scaling RG method.  2sb175 gain curve is more flat than 2n388 and AC121, 2sd72 respectively.
IIRC AC121 and 2sd72 gain at ib=1ua are 1.5 - 2x lower than at 10ua.
I think that when Vin is low, ie you lower the guitar volume, AC121, 2sd72 or similar are less responsive than 2n388 and 2sb172.

Brett,
You are right, I have tons of japanese 2sbxxx, being my favourite Matsushita 2sb172,175,176 family, and Hitachi 2sb77.
Toshiba 2sb56/54 are great too, low leakage which reduces thermal runaway, but a too bright to my ears.

mac

[WGTP]

http://web.archive.org/web/20050513200126/www.geocities.com/tpe123/folkurban/fuzz/fuzzyfaces.html

Might want to consider a Mosfet or 2. 

[Electric Warrior]

Newmarket datasheet? http://www.datasheetarchive.com/pdf-datasheets/Databooks-5/Document62841.html