Fuzz Face mods

[nonost]

 Hi. I wanted to ask you about a few mods I happen to see here and there and that I'm not sure what they're doing. I hope experienced builders could help!

I) Little cap across B-C What does it do? Treble roll-off? Reduces noise? Does it affect gain? I've read many things...It looks that it is a more silicon common practice

II) Resistor from E to GND I've come across this mod a few times. If I'm not wrong it does change the gain of the transistor. (?)

III) 470r resistor Idem. Increase/reduce gain? Does it have any drawbacks or collateral damage?

I'm pretty used to hfe and leakage stuff, as well as biasing and input/output caps. And now I'd like to tweak a bit more this easy&crazy tiny circuit.

Cheers!

[antonis]

I) Prevents high pitch oscillations (reduces gain at high frequencies due to negative feedback action)
II) More for bias stability issues (Emitter resistor serves as negative feedback item both for beta and V_BE variations)
III) It reduces Output gain (and Volume) by a factor of 470R/(470R + Collector resistor) and also reduces Output impedance (but in minor degree 'cause it's mainly affected by the feedbck network)

[nonost]

Ey Antonis, many thanks!

I) Does it affects audible range? Or is it like really above hearing?
II) I guess this is great for q1 transistors with a bit of leakage
III)Increasing to, let's say, 820r. Reduces gain? I thought it was the other way around

Cheers!

[antonis]

I) Only in case of NFB cap big enogh value (you may consider it as a Miller cap). Otherwise, no audible difference with and without it.
II) Quite right. In the presence of Emitter resistor, leakage current levels up Emitter voltage resulting into lowering V_BE hence lowering total Collector current..
III) The "other way round" you thought is in compliance with what I said..
(the term "factor" might confused you a bit 'cause here that factor is in reverse proportion to output..)
By increasing 470R value to 820R you increase Q2 stage gain 'cause you increase total Collector resistance and "offsets" upwards Collector to Output voltage dividing effects..
(signal coming out of Collector, through Collector resistor, is splitted to Power supply (AC ground) via 470R resistor and output GND via OUT pot).

[Steben]

caps from base to collector are sometimes or used to be popular tricks to make a silicon stage sound more like "germanium capacitance". In the end it's all about less treble

[pinkjimiphoton]

Ey Antonis, many thanks!

I) Does it affects audible range? Or is it like really above hearing?
II) I guess this is great for q1 transistors with a bit of leakage
III)Increasing to, let's say, 820r. Reduces gain? I thought it was the other way around

Cheers!

you can use those as snubbers to reduce parasitics, or even make a "fake"variable cap and use it to control treble content without killing the mids

its less useful for leakage... a ge transistor reverse beta from the q will work better.... its really more about the gain in the case of fuzz. a directly grounded emitter will give the transistor max gain,  so adding a resistor there softens the possible gain. i use it in a bunch of projects to make silicon sound more like ge... search schizoid face and skydog fuzz here, they both use different combos of this kinda trickery

a bigger resistor tends to make for a louder fuzzface, you can go to 1k or higher if you want, but after a point it really doesn't improve the tone any.

[nonost]

 Thanks guys!

So I) & II) are little tricks more appropriate for silicon units I would say. While III) could be useful for too low gain germanium devices.

Is there a way to make a low leakage transistor a leaky one? Let's say a Russian leakes 70uA (which is very common), and I'd want to turn it into a 250uA, any tricks for it?

Cheers!

[Steben]

Can you explain (again) why you would need leakage? No stash of transistors? One could try adding a large R from collector to base.... it is how leakage bias circuits are transformed to silicon.

[pinkjimiphoton]

or a ge diode in paralell with the e and b

[nonost]

Hi Steven. I want to experiment. I have a hint that these low leakage Q1 transistors would sound better with a bit of leakage. Just a bit more.

Wasn't the diode across E-B to reduce leakage?

[pinkjimiphoton]

no, the diode is used to INCREASE leakage, kinda... its for temp stability more than tone, the idea that the ge diode will leak and compensate for the leaking transistor.

[Mark Hammer]

Begin here:  http://www.geofex.com/article_folders/fuzzface/fffram.htm