Fuzzface and Miss Piggy "flobbiness" linked to hFE

[brett]

Hi.

With the Miss Piggy circuit I have the ability to vary the hFE of the Q1 and Q2 positions in a fuzzface type circuit.  The Miss Piggy schematic is here.http://members.optusnet.com.au/~jethro.dog/mainpic5.jpg  Recently I built a couple of Miss Piggies hFE in Q1/2 of about 50 and Q3/4 about 110.  But listening to these pedals in direct comparison to an easyface was quite disappointing.  They suffered from a muffled bass and mid-range.  Some people also find their Ge fuzzfaces "flobby" in comparison to Si fuzzfaces, Axis fuzzes, etc.  I *think* I know something about this.

One thing that I was kind of aware of was that the hFE of Q1 is proportional to the input impedance of fuzzface circuits.  With hFE of 50, I was concerned this might be too low.  So I measured the input impedance - about 8 or 9 kohms. :shock:   So I increased the hFE of Q1/2 (by changing the piggyback resistor to 22k) to about 120.  The sound cleaned up A LOT (maybe too much?).  The input impedance was now about 15kohms.  (Also, it seemed to become a receiver of radio, etc interference)

So what did I learn?  
That the hFE of Q1 in a fuzzface or Miss Piggy circuit dramatically affects tone via its effect on input impedance.
That when the hFE is 50, the input impedance is less than 10kohms, which is too low.
Increasing the hFE cleans out the "flobbiness", but the input impedance can become too high and the tonal characteristics change.
It might be tricky to work out how big the piggyback resistor needs to be to get the hFE of piggybacked Si transistors into just the right hFE/input impedance zone.  10k was too small, 22k was too large for Q1/2 in the circuit I was using.

[javacody]

brett, it sounds like about 15K might be perfect. Have you gotten the ranges that RG suggests in his fuzz face article? If memory serves, Q1 would be 70 and Q2 would be 90?

[WGTP]

I found using a 100 to 250K pot on the input to vary impedance to be quite useful in tuning the FF.  Or the cap blend circuit also works nice.  

[MarkDonMel]

Would someone define flobbiness for me?

Thanks, Matt.

[brett]

Hi.  First, what is a "flobby" fuzzface?.  It's one that has weak treble and slightly "muffled" midrange.  It ranges from mild to serious.

RE:

I found using a 100 to 250K pot on the input to vary impedance to be quite useful

I assume that means using it as a voltage divider, connected to ground, rather than a variable series resistor on the input.  If so, it's the same setup as in the easyface (ie a 250k pot). This was my "benchmark" in terms of treble and clarity.

Anyway, I'm gonna try a different approach to the Miss Piggy and the tone-hFE adjustment issues.  The idea will be to make the hFE of Q1/2 and Q2/3 measureable and adjustable with a DMM.  I *think* this will work ok.  It'll take me a week to get to it and prototype it.  Unfortuantely, I owe a couple of tubescreamers to friends, so I gotta do them first.

cheers

[WGTP]

No, just as a resistance on the input.  The higher the input impedance, the less wooley it is.  Or, just try a 47k resistor at the input, or a smaller input cap.  

[brett]

Thanks for the info WGTP.  So you've used some quite large input resistance.  I tried 10k today, and that didn't do much (no too surprising).  I'll try 47k tomorrow.  

But I did manage to prototype a new Miss Piggy circuit with easily-adjusted hFE.  I'll draw up the schematic and post it in a new thread tomorrow.  It's very neat, even if I say so myself.

[WGTP]

A dual 10/50K pot with the 2 pairs wired in reverse might be a nice external pot so you could vary the ratio between the 2 from the outside of the box.  

[MarkDonMel]

flobbiness???