Maestro FZ-1 Fuzz Tone analysis/questions

[fuzzy645]

So I'm thinking my next project will be to make myself a Meastro FZ-1 clone.  I noticed RG Keen's schematic posted (see image below).  At this point, I'm just trying to understand the basics of the circuit.

1.  Why the 3 transistors, Q1, Q2, and Q3?  Does each one amplify the signal generated by the one before it to generate distortion/fuzz?  What is particularly puzzling me is the emitter for both Q2 and Q3 are grounded.

2.  Are those 2N270 transsistors germanium NPNs?  Either way, is there a source to get these, or a suitable replacement for a clone?

3.  What is the purpose of R10 in parallel with the volume pot P2 on the output?  

4.  What is the purpose of the 100K resistor in series with the input cap C1?

5. Is this is a postive ground or negative ground pedal?

In general, if some  can talk through the parts of this circuit I'm sure many will learn from it.

Thanks!!

[smallbearelec]

Hi--

You'll find a lot of your questions answered in this article:
http://www.smallbearelec.com/Projects/FuzzE-One/FuzzE-One.htm
but not necessarily in the order or way that you asked, so I will re-cap a little below.

1.  Why the 3 transistors, Q1, Q2, and Q3?  Does each one amplify the signal generated by the one before it to generate distortion/fuzz?

Re Q2 and Q3, yes. Q1 doesn't amplify; it's an emitter follower that provides a high input impedance for a guitar.

What is particularly puzzling me is the emitter for both Q2 and Q3 are grounded.

It's not unusual to see a common-emitter stage biased this way. Other people may have more to tell you here.

2.  Are those 2N270 transistors germanium NPNs?  Either way, is there a source to get these, or a suitable replacement for a clone?

They are PNP. Once a number of people who know more than I figured out how the thing really works, it became clear that the pedal could easily, and better!, be done NPN, and with relatively cheap parts. Leaky NPN germanium is common!

3.  What is the purpose of R10 in parallel with the volume pot P2 on the output?

I had not seen this resistor in other schems. It clearly changes the taper of the pot. I am not sure how. I found that 50K Audio works well there.  

4.  What is the purpose of the 100K resistor in series with the input cap C1?

Input load resistance for the guitar.

5. Is this is a postive ground or negative ground pedal?

As shown positive-ground, not friendly to pedal boards. This was one of several problems that I fixed in doing the Fuzz E-One.

Happy Construction!

[fuzzy645]

2.  Are those 2N270 transistors germanium NPNs?  Either way, is there a source to get these, or a suitable replacement for a clone?

They are PNP. Once a number of people who know more than I figured out how the thing really works, it became clear that the pedal could easily, and better!, be done NPN, and with relatively cheap parts. Leaky NPN germanium is common!

Thanks Small Bear!  I will get the parts from you.  A friend of mine says he has has a single old 2N270 transistor available for me.  If that is the case, I will need 2 more PNPs to match up with this.  Do you sell anything I can use that will match up with the single 2N270 I can get, or do I need to somehow locate an old pair of genuine 2N270's? 

Thanks! 

[fuzzy645]

If it is best for me to go with a "matched set" then what is the best readily available PNP replacement for a 2N270 that you sell?

[smallbearelec]

If it is best for me to go with a "matched set" then what is the best readily available PNP replacement for a 2N270 that you sell?

The test fixtures we have are set up for NPN negative-ground. Matched NPN sets are available, and also I just posted some loose 2N2430 that are prime candidates if you want to sort your own. If you really want PNP devices that are leaky enough for the FZ-1/1A, other posters here probably have some in their "discards" bins that they'll sell you. But I suggest again: Do it NPN/negative-ground, and include the voltage regulation. You'll avoid dual power-supply issues, and the pedal will play nicely with all of your other gear.

[PRR]

> emitter for both Q2 and Q3 are grounded.

Google "grounded emitter"

http://en.wikipedia.org/wiki/Common_emitter

> Are those ... NPNs?

Google "NPN PNP"



> Is this is a postive ground or negative ground pedal?

Hey, R.G.! You didn't put a "ground" symbol on this drawing... deliberate?

Generally in guitar pedals, the "body" of the in/out jacks is "ground". I have added that to this snip and I'll let you decide which side of the battery goes to ground.



> What is the purpose of R10 in parallel with the volume pot P2 on the output?

There are two versions, -1 and -1A. Draw it twice with the values noted in the table.

-1: 56K fixed parallel with 500K pot... acts like 50.3K
-1A: 50K pot alone

Either way, the last stage and C4 are loaded in 50K. I suspect the design goal was 50K, but in the -1 production they had lots of 500K pots on hand.

C4 0.0033u against 50K (or 50.3K) gives a 1KHz low-cut. Evidently the designer wanted to lose the lows so the highs would stand-out, "scream".

> changes the taper of the pot

No. To change taper you load the wiper.

[fuzzy645]

Thanks all for your responses!

So Q1 seems like a kind of input buffer then, if its main job is enforce a high input impedance.

In looking at Q2 and Q3, it seems as if this is possibly a type of Darlington pair as seen in the image below, since the  collector of Q2 connects into the base of Q3, although in this case there is a 20uf cap (C3) between them, so then I suppose that breaks the pair.. 



Another question:  which are the bypass caps in the Maestro circuit?   If I try to label the caps, it seems like

C1 = input coupling cap
C4 = output coupling cap
C2 = is this a bypass cap?
C3 = is this a bypass cap?

Also, about resistors I am assuming R4, R7 and R9 are all used to bias the transistors.

[PRR]

> many will learn from it.

No; or "many" will learn poorly.

The original plan NEEDS "bad" transistors.

Today we can only get "very good" transistors (unless we shop at specialists like SmallBear who stocks all types). If you want to learn, learn "good" transistors.

If built with good transistors, Q1 simply won't pass signal, and Q2 won't work without the later -1A's R4. Q3 is dubious though if Q1 and Q2 actually do their job, Q3 will distort almost anything above dead silence.

Back when this was new (and today at SmallBear), "bad" transistors were readily available, as "rejects" from more demanding systems.

Even so, we surmise that Maestro actually ran out of the really bad/leaky transistors, had to add R4 to get their new lot of pretty-leaky transistors to work.

Unless you can go back and live in 1961 again, or can make a living servicing genuine 1961 gear, I do not think there is much to "learn" from this circuit. It was specifically adapted for a Very Different time-and-place, when good transistors were $20, reject transistors were $3, and gasoline was $0.25/gal.

It is "stone knife and bear-skin" technology. Many of our ancestors sliced bears with flint knives. However the skills are little use today. I happen to have lots of stones, and sometimes bears wander through my woods, but even so I use steel knives and dacron jacket, and am glad of it.

While SmallBear stocks several grades of "bad/leaky" transistors, and "understands" the transistor issues as well as anybody, leave that to him and the few others who work or did-work with reject Germanium. He'll sell you a 3-pack selected to work in his adaptation of the original circuit. Don't spend a lot of time trying to save a little by trying "equivalent" Ge transistors: Maestro selected their _reject_ transistors for this product, they are not "Prime" 2N270/2N2613.

> possibly a type of Darlington pair

An interesting insight. The DC connection is not Darlington. The AC/audio action is Darlington-like.... but the essence of Sidney Darlington's invention is the utter simplicity, without all the R3 C2 R5 etc busy-work between.

I didn't mention in the other thread, but: I can not think of any "Germanium Darlington". Why? I am speculating that the leakiness of Germanium is bad enough with one transistor. With two the leakiness is multiplied. Even with "good Germanium" the leakage becomes hard to manage, eats-up all the advantage. Selecting for lower gain with low leakage to make "Darlingtons" is chancy... you may not find many in a bucket-full.

So what's with SmallBear? Well, for 50+ years re-sellers in America Europe and Asia have been going through barrels of old-stock rejects pulling out the not-so-bad ones. Now that we are at the bottom of the barrels, he's more likely to find the odd combination of very-low gain with low leakage. Apparently his stash of NPN Germanium (itself rare; most Ge was made as PNP) includes enough with the right combination of parameters to be worth sorting-out and selling for the explicit purpose of "making Darlingtons" which act fairly similar to "good" single transistors. This may be the new "stone knives" technology, but what else is he gonna do with the barrel-bottom leftovers?

[LucifersTrip]

I didn't mention in the other thread, but: I can not think of any "Germanium Darlington".

I actually just finished tweaking this...and it's great....no noise or hiss. Knowing how the leakage is multiplied, I chose Q1 & Q2 with very low leakage:



also,


and variants:
http://fuzzcentral.ssguitar.com/3knob.php

[LucifersTrip]

In looking at Q2 and Q3, it seems as if this is possibly a type of Darlington pair as seen in the image below, since the  collector of Q2 connects into the base of Q3

In a Darlington pair the collector goes into the collector

In a Sziklai pair the collector goes into the base

[guitar413]

I got one of these from a friends friend to repair. The input and wire are missing. Does anyone know the size shielded wire the input is? I'd like to get as close to orginal as possible