Tweak Fuzz

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jayp5150:
I couldn't find much here searching...

I'm working on a Tweak Fuzz for a friend (modding a factory one).

Found the schem here: http://analogguru.an.ohost.de/193/schematics/SeymourDuncan_TweakFuzz.gif

One thing to point out first is that Q1 and Q2 are swapped on the board. SD marked Q1 as Q2, and vice versa; so the schematic actually has the tranny types flipped.

Q1 (as we know it in the schem) is actually the 2XD, which I can't really find any info on... marked as a "4401 ?" on the schem, and Q2 is the 1AM, which should be a 3904-alike.

The issue I'm having is getting the bias down to 4.5V on Q2. I've actully bumped R4 to 2k at this point, and have a 10k trim on R5. This, maxed out, still has me at 5.25v. that seems odd to me. Should I have that much resistance to bias a FF?

So, I'm wondering if I should just junk what's in there and start swapping trannies? Once that SMD tranny is off the board, it's pretty much gonna be dead lol. OR, just up the resistance til it biases and use my ears?

The other question is that if Q1 IS a 4401, that doesn't make sense in the traditional FF circuit, as they hover around an hfe of 250, and the 3904 would be around 150...

Just looking for insights from some folk who are smarter than I lol

Thanks!

Gus:
Use this circuit with the stock transistors, keep the switchable input cap section.  FWIW the schematic has been here for years and it was up before the axis face(the lowpass, 1K and .01 cap) and Jack emitter resistor at Q1 that does not do as much with a 33K as a 10K collector resistor for the first transistor.
http://www.diystompboxes.com/pedals/gusFuzzFace.gif

Why are you modding it anyway?  What was wrong with it?

petemoore:
  My buddy bought one. Dog...I looked and checked, decided it'd actually be hard to top the FF in this one, I guess I've been through the FF mods enough.
  Maybe put a booster in front...but I think getting the right' pickups and amp for it'd be where I'd start.
  There are plenty of FF derivatives which will go where the basic FF with all the whistles like this one won't go, but they're OT really and best drawn from extensive researching [probably something like bias starve or whatever else is seen around with a Face in it [I guess the feedback resistor has to stay in the 2 transistor circuit to call it a FF, I suppose a cap or pot added here might still let it qualify?] to find the ballpark sound type desired, populated on a breadboard or something like the tweekfuzz [the capacitor switch voicings are pretty slick and should cover most of the gamut of TB/FF/other FF sounds.
  To improve'' on the cap value selections for the voicing switch or put some new twist on the FF beyond the capability of the TF...I would suggest actually warrants the 'other routes' as compared to a re-mod of the already quite modded tweekfuzz.
  Guitar or amp change of course will reveal Tweek sounds beyond the scope of the guitar/amp now used, beyond that a new circuit board seems like a small addendum to either find the new Fuzz or find the Tweek that the TF is somehow missing.

jayp5150:
Well, it was actually inoperable when he sent it too me (gain pot wires had been sheared due to a loose pot and what seems to be some serious twisting lol)

The only complaint he had was that it lost sustain at the fatter settings, which I think was due to the high bias voltage stock.

I actually threw a 4.7k in r4 a few minutes ago, and with the 10k trim, got it down to 4.5v. It's super loud now, I might swap in a 15k trim and move r4 back to 1k

Honestly, it doesn't really need anthing else.

One of my faves is to put the 100k feedback resistor on a switch to go between that and a 1n914. That tames it down and seems to add more touch sensitivity.

Now that it's biased properly, it's actually very nice. I was just thrown by the high resistance needed to get it there.

PRR:
> marked as a "4401 ?"

2N4401 is a very standard large cheap NPN Si transistor. Really more than this circuit needs. Neither transistor is fussy. "Si NPN with hFE over 100." The circuit is locked-in by NFB, as long as hFE is not "small".

> start swapping trannies?

As long as they are not -dead-, or amazingly weak, tranny swap make no difference. hFE from 150 to 800 will all work very nearly the same. This rig does what the resistors tell it to do.

> getting the bias down to 4.5V on Q2.

P2 should be just about 2K.

What is it now? Has it been replaced?

NFB from Q2 E to Q1 B sets up about 1V across Q2 emitter resistor, here P2. If this is 2K, we flow 1V/2K or 0.5mA through Q2.

Q2 collector voltage is this current, times R4+R5, down from 9V.

If R4+R5 is about 9K, Q2 collector sits at 4.5V.

> I've actully bumped R4 to 2k

Changing R4 is not much change.... R4 is a very small part of R4+R5. If you can't change Q2 emitter resistor (because it is a pot you don't want to change), then fiddle R5 from 8K to 5K or 10K. If P2 is way-off from 2K, do what you need to do.

> It's super loud now

With Q2 collector near 4.5V, it can put out almost 3V of signal at R5. This is way more than we want. The designer added R4 so he could tap-down and get a fraction of this. The 8200:470 ratio will give about 0.15V signal on R4, which is a good ballpark. If you have increased R4 toward 4.7K to get a 4.5V bias, the ratio 8.2K:4K will give maybe 2V of output, which IS "super loud".

Check what P2 really is. You can make it work with 1K or 5K, but you will have to make R5 about 4 times higher to get near 4.5V at the collector. Then make R4 about 7X-20X smaller than R5, to get a "reasonable" maximum output level (depending what you call "reasonable", and what your amp likes).

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