just got some germ trannies... fuzz face time!!!

[xianx]

hey everybody

i'm new here, although i've been reading for a while

well... i just opened up an old panasonic turntable that i bought some years ago at 10 bucks (mainly for the speakers, which rocked).

inside i found no less than seven () germanium transistor, all matsushita (to be expected) of the 2sa and 2sb variety.. one of them measures very high hfe, around 300, so i guess its just a leaky specimen.

every other is just about right for a fuzz face, from 80 to 140.

i'm about to begin building my long awaited fuzz face (along with an op-amp big muff) and a couple of questions arose when i tried and simulated the tonepad schematic.

the schematic can be found here:
http://www.tonepad.com/project.asp?id=38

1. the suggested 4.5V at the second transistor just doesn't appear with R5 being an 8k2 as i'm getting around 1V (negative, since it's postitive ground), but it does at 4k7. My simulation (done in QUCS) doesn't consider the germanium leakage current, could that be it? is the 8k2 the correct value?

2. should i just replace R5 with a 10k trim and adjust as needed with a voltmeter? or should i place a 10k pot to make quick changes on stage?

one problem i can think of with the pot is that trying to adjust bias voltage mid-set leaves me totally clueless as to what is the value i'm actually playing at :S

3. is there any other resistor that would realistically benefit from being replaced with a trim?

4. whats the origin of the much famed 4.5V bias voltage?

thx for reading!!!

[petemoore]

  Try 10k VR + 4k7 stop resistor.
  That's a 10kL pot > 4k7 = 4k7 - 14k7, smoothly spread, adjustable Variable Resistance.3.
  is there any other resistor that would realistically benefit from being replaced with a trim?
  Depends on what realistically is defined as, but yes, there are other resistors and other places without them.
  4. whats the origin of the much famed 4.5V bias voltage?
  AKA: 1/2v with 9v supply.
  AC signal output swings [+/- from idle bias point], assymetrically anyway in a fuzzface, 5 or more volts is also 'famous bias point'.
  Moving the bias point + or - from 1/2v of 9v [4.5vdc] allows more room on one side of the + or - signal swings than the other, before running into one of the power rails [9V and 0.0v].
  At 6v bias point, the output signal only has to swing 3V positive from bias before hitting the 9v rail, yet has room to swing nearly 6v in the other direction towards ground. [I should note these comments relate to a Neg Gnd. NPN FF though...just easier to discuss if Gnd. is 0 and the supply potential is called positive.
  You want a Positive Gnd / PNP Fuzzface schematic, with Gnd. assigned as Battery +.
  A million mods yep, and on face bias control knob is fine, can be 'fine tuned' [reduced in value range] to the chosen transistor so even if the knob gets twisted at least something can exit the box [ie if transistor blocks AC signal, box sounds broke when it just needs a knob twist].
  You can count the # of early FF's and TB's where the input and output cap values change [or some 1 or 2 other component value change]..the topology of the FF circuit is fixed, messing with the input and output cap values reveals most of what each and every one of those various FF's were like...what any self respecting FF owners Fuzzface should be like your Fuzzface should be...tweeked to the pickups/amp !
  The basic circuit, biased...has many faces. You might want a couple, by making the wire between the circuit board and bypass switch go through a switch with capacitors..shoot just splice another cap right into that wire...or socket the input cap [I used sockets and 'longlegged resistor' in FuzzfaceS perf-bread testboard...
  Then I tried all the 100+FF's...each went like this:
  Transistor swap = bias reset, gain adjust ?...Extensive testings.
  Capacitor swap = may be reliant on transistor gain, high gain tends to equate to more treble, for one thing.
  Both require guitar volume tests at 10, 9.4, 8.2....sometimes the "FF impedance thing'' happens suddenly on the pot...this too can be messed with if need be, the important thing is to remember to adjust the gain with the volume pot of the guitar when setting up the circuit...if you're into the 'inbetween tones' [ie FF cleans up w/guitar volume]..optional pregain control does the same thing but allows preset of max, I like it set to just under the FF thick-mud tone [guitar at 10]...ie turn the guitar ATW up= mud, back the pregain input control to where it's barely mud, leave set there for unmuddy guitar on 10 tone.

[clamup1]

Good luck and happy fuzzing. it took 6 tries for me to get it right. i did the roger mayer mods on mine http://www.geofex.com/article_folders/fuzzface/fffram.htm and im thinking about doing the Fuller mod next.

[xianx]

thx guys

i'm really excited to begin building these pair of babies!!!

pete: your explanation was very clear to me, i guess i should build it, stick a pot to control bias and just let my ears be the judge as to what sound i prefer.

clamup: it had been a while since i read that R.G. article on the fuzz face. It's such a nice piece of writing!!!
i appreciate it even more now that I'm actually thinking about specific devices and stuff.

wish me luck!!! I hope the first one works alright... at least i got a couple of spare trannies and i'll use sockets on them, just in case.

is it a good idea to cut out pieces of an IC socket i got laying around on the output capacitor or a particular resistor? it just seems like I could play around a bit more like that.