Adjustable hFEs in this FuzzFace (yay!)

[brett]

Hi.

After ages fiddling with the Miss Piggy idea of altering hFE of modern Si transistors down to those of old Ge ones, I was getting frustrated messing with the circuit and measuring hFE.  Then  :idea: .  Why not put a little resistor at the base of the transistors.  Voltage across the resistor tells how much base current, which tells how much hFE (as long as collector current is known - hFE=Ic/Ib).  Too easy!  I wish I'd thought of it 3 months ago. :roll:

Anyway, here's the new Miss Piggy with tunable hFEs.  In fact, I'm going to call her Miss Flexible (besides, the "Miss Piggy" thing has gotta be infringing someone's copyright, and I don't want to wind up in court with the muppet-makers.)

cheers

[jimmy]

thats bloody incredible!!

[Gringo]

Well, out of excuses to build this one  :twisted:

Thanks a lot for sharing the results of all the hard work!!!

[lvs]

Thanks Brett for investing your time into this. I was working in the same direction and this is a big help to me.

Lucas

[RedHouse]

This weekend I had a chance to spend considerable time perfboard experimenting, in detail, with this piggy-back idea on a "jig" I made up some time ago. The "jig" has trim-pots for every resistor in the FF circuit  and multiple caps for C1 and C2 (2.2, .1, .068, .022, .0033) and sockets for adding bypass caps across the collectors and from collector to gnd.

Using this "jig" makes it painless to dial-in transistor bias and tune the FF circuit, and eliminates any need for trim-pots on the circuit board. It allows tuning of the circuit "by ear", in real time while playing thru it.

Anyway here's the values I've settled on, using four 2N5087 transistors (hfe around 350) in the piggy-back mode:

R1= 33k
R2= 47k
R3= 150k
R5= 2k
R4= 10k
C1 and C2= .068
Q2 bias set at= 4.8v

I've added a 100pF cap across the collectors, and a 56pF from Q1's collector to the juction of R3/R5 (which is ground on my negative-ground version of the FF circuit).

I've chosen a 16k resistor where you have you base resistor "X1" and  no base resistor on Q2, and I've added a textbook 680 ohm resistor on the emitter of Q1 bypassed with a 33uF cap.

Now it sounds like a very well behaved Fuzz Face.

In the past when I built Si versions of the FF circuit they never sounded quite right to me ...until now... this piggy-back idea seems just perfect in this application.

Great work Brett, and thanks for sharing the idea.

[Nasse]

Could not resist myself from posting this pic of real Miss Piggy

Miss Flexible looks even more sexier

[gez]

Are those silicone or Ge implants?!

[javacody]

I'm pretty sure that's all natural pig t**s.  

[brett]

If the collector current for Q3 is similar in different fuzzfaces/Miss Flexibles, then a particular hFE can be dialled by measuring the voltage drop across resistor Y.  But is the collector current similar in different circuits?  I can't see why it isn't just dependent on the base current and hFE.  Maybe RG or Mark or one of the experts could help?

I need to re-check, but I thought my Miss Flexible measured 4.5V/3.5k = about 1.5mA collector current.  The resistance seems way low for Rc in a fuzzface (8.5k being the old standard).  Any ideas?

[RedHouse]

(bump)

RG, care to take a swing at validation?