Understanding the 22k to ground on the Mosrite Fuzzrite

[mordechai]

It seems that some versions of the Fuzzrite did not include this resistor going to ground, but the ones that did have the gnarly (yet beautiful) "pinched" bite to the tone.  I am guessing, then, that decreasing this resistor value would soften the pinch a bit while increasing it would do the opposite, am I correct?

Also -- a bit OT -- I have a few Tungsram AC125s (some rated around 60-80 Hfe, and others at about 110-120 Hfe).  These are the correct gains, I think, for the Q2 and Q1 values respectively.  Is there another transistor type that anyone here has found especially nice in either of these positions?  I like the bite of the AC125 in a Tonebender Mk II but have never tried them in a circuit like this, and I do have a few Mullard OC76s waiting to be used...

[allesz]

Hallo, I just breadboarded some fuzzrite alike circuits last night and to me the 22k to ground is essential to balance the output from the second tranny with the one from the single (and first) transistor. Without it the circuit loose some of his voices.
Since I was going on changing transistors, caps and resistors (to find the holy grail of course) I just put a 50K trimmer in place of that resistor and have been very happy: you can mod the hell out of the thing and always get a nice balance between the sound of the unit.

[mordechai]

I was thinking about that, actually.  I found a good vero layout for a regular 22K resistor, but I have been adapting it to allow for a 25K trimpot.  How does it sound when turned up closer to 50K?  I imagine it must get really, really sharp and snarly!  I might try it with that if you think the upper range (past 25K or so) is usable tone-wise.

[Gargaman]

I've built this one with no 22k resistor at all. I used the General Guitar Gadgets wich seems to have some errors, beside the 22k resistor missing. I've searched the web for information on this resistor, but didn't came to a definitive position. Is the 22k resistor goes from Depth lug 3 or Depth lug 1???
Mordechai, can you tell what layout you used? Any schematic with the correct position of the 22k?
Many thanks!

[blackieNYC]

I preferred it without the 22k.  To improve the balance of the panning control between the 1st and 2nd transistor stages, I put a 100KR after the collector output cap of Q2.  My panning control is 500KB.  The ends of the pot are louder, but the best sounds are throughout the range of the pot, excluding the two extreme ends.  I tried a 100k pot there and lost a little variety. 
That dip in volume is probably due to the attenuation introduced by my 500 k pot, but also - the two signals you are blending/panning there are out of phase.  I need to turn up the output a bit, but all the good sounds are between 9 o'clock and 3o'clock.
I love this control.  It's not fuzz level, it's not tone, or drive.  I think of it as a "color" control.
I have also added a (bypassable) Ge boost in front of it, to push it into sickness.  Very nice.  Perhaps I should go back and revisit the 22k one last time.  I'm boxing this thing up.

[Electron Tornado]

I built a fuzzrite a while ago. I did a bit of research at the time and found that the 22k resistor may not be found on photos of original fuzzrite circuit boards. However, it can be found wired off the circuit board.

I liked the sound both with and without that resistor, so I put it on a switch.

[Mark Hammer]

Hi Mark,

So, this diagram is your friend.  I believe what you are referring to is called R8 in this diagram.  And, as you can also see, sometimes it is used, and sometimes not.  When in place, it seems to do a few things.  First, it essentially "tunes" the output of Q2 so that C5/R8 becomes a highpass filter.  Think of it as half of a BMP tone control.

Of course, in the Shin-Ei FY-2, there is no need for that resistor, since the Fuzz control is followed by an explicit midscoop network, whereas in the case of the Fuzzrite, there's not much else between the transistor outputs and the volume pot, save for the panning control between Q1 and Q2 output.  I suppose one doesn't absolutely need R8 for Q2 to sound buzzy, but it helps to nail down what Q2's output will be.

The other thing R8 does is tame the output level of Q2 a bit.  Looking at the diagram, if we move the Fuzz wiper all the way "up", R8 is effectively placed in parallel with the output pot.

I suppose it might be interesting to make the R8 resistance toggled.  Make it a 1M "default" value to mimic something close to open circuit, and use a 3-way SPDT toggle to put either none or one of two different values of parallel resistance, to yield something close to 22k and 100k.  So, you mimic a Fuzzrite with no R8, one with a 22k resistor, and one with something in between.

Note that C4 impacts on your low end.  Lifting it will bring back some bottom.

A sweet year to you...and a fuzzy one too.

[blackieNYC]

Cool!

[boogietone]

I have one of these at home and will take a look inside.

[Gargaman]

Very nice information, I nearly understand how it works! I got this breadboarded and now I'm getting close to a good sounding fuzzrite.
So, the Depth control balances the output between Q1 and Q2, right?
If the 22k resistor to ground (together with tthe 0.0022 cap) acts like a highpass filter, changing the value of resistor might give different frequency response, right?

My panning control is 500KB.  The ends of the pot are louder, but the best sounds are throughout the range of the pot, excluding the two extreme ends.  I tried a 100k pot there and lost a little variety. 

I got the same situation, but I preffered a lower pot value than the dip in volume. Another bad thing is that I got a poor sound when the Depth is minimal.

I preferred it without the 22k.  To improve the balance of the panning control between the 1st and 2nd transistor stages, I put a 100KR after the collector output cap of Q2.

Do you mean a 100k instead of the 22k? I found the best sound with a 100k resistor from Depth lug 3 to ground, and 6.8nF out caps.

Another question: should i tweak this circuit based on my transistor hfe? How exactly could I do that?

[blackieNYC]

the 100k is in series, between the output of Q2 and its end of the blend pot.
the 22k, is just removed.

[Mark Hammer]

If a person was going to do that (and I think is it a good idea), consider swapping the 100k fixed resistor for a 2k2, in series with a 50k pot used in SWTC format.  That is, the output cap of Q2 goes to a 2k2 resistor, then to one outside lug of a 50k pot (log OR linear I suppose).  The other outside lug of that pot goes to the Q2 end of the Fuzz/Balance pot.  The wiper of the new added pot goes to  a .01uf cap to ground.  That will provide a gentle treble cut on the fuzzier side, with highest rolloff point at 7.2khz, and lowest rolloff point at 304hz.  Having a Q2-only tone control will add more flexibility, permitting one to blent in some growl from the fuzzier Q2 with some clearer tone of Q1, as well as pretty well stock sounds (minus any piercing fizz).

[blackieNYC]

Oooh, tone control for the fuzzier side.  I was just about to box this up. 

[Mark Hammer]

I'm not one to get obsessed with having 3-band EQ for fuzzes.  But if a person has an amplifier and guitar set for a brighter, clearer tone, and then you step on a fuzz switch, one shouldn't be overwhelmed by the treble content.  Good to be able to dial things back a bit just so that not only the volume level is appropriate, but the shrillness as well.