60s Style Fuzz box Build

[jmusser]

I used the schematic off of http://www.muzique.com/schem/sixties.gif   I believe I have been caught up in this one now, for a good 5 hours. First of all, it didn't work, and I went through the schematic 3 times, and it was wired exactly like it was written. I started taking voltage checks, and everything was basically dead from Q2 forward. I had run the transistor part numbers, and found that the SK3020 was a general purpose NPN, like a 2N3904 for Q1 and Q3. SK3005 comes out to an PNP germanium. I was kind of interested to see what the germanium would sound like in Q2 position. After it wouldn't light off after checking it through, I replaced Q2 with a 2N3906, just in case I had biasing problems. Still no go. I got to looking at the schematic, and something didn't look right. Q2, had the emitter facing the 9v rail, and I didn't recall seeing that before. I pulled it out turned the emitter towards ground, plugged it back in.... and was in business, but...it sounded lie crap! I played around with it, using several different transistors, with just a little bit of difference, until I finally put an MPSA13 Darlington in Q3. Yeah Baby! I ended up with a 2N3904 in Q1, 2N2907 in Q2, and the MPSA13 in Q3 position. The 2N2907 is supposed to have a lower hfe (minimum of 50) than the 2N3906, and helped define the fuzz a lot better to my ears. One thing I definitely didn't come up with, was a 60s Fuzz. I'd say we're into 70s Classic Rock territory now. This thing has a real nice fuzz, and a beautiful crunch. Set up with the Darlington, you can back off on the guitar volume, and the amp's gain. It gives it the "rolling boiling" break up. Right now, I have no intention of trying a germanium in Q2 position.I tied a couple in there, and it made the circuit die, so I figure it would need biasing work in there, and she sounds real nice now. If anyone has had any luck actually making a 60's sounding pedal out of this, I'd like to know about it. With the guitar volume and amp gain all the way up, you're close to 80s Heavy Metal territory. Set up with the gain at half, and the guitar volume at about 7, it had a great growl and bite to it. Set up with other than the Darlington, things had to be really cranked, and even then you were just amplifying crap.

[jimbeaux]

This Fuzz Circuit is actually from an RCA Hobby Circuit Manual (1970). It was just called "Circuit No. 43 - Fuzz Box". The manual consisted of projects promoting RCA's "new" SK line of transistors.

I checked the muzique.com schematic/parts against the ones in the manual, and everything is the same.

Thanks for the build report.

- Jimbeaux

[The Tone God]

This Fuzz Circuit is actually from an RCA Hobby Circuit Manual (1970). It was just called "Circuit No. 43 - Fuzz Box". The manual consisted of projects promoting RCA's "new" SK line of transistors.

The scary part for me is that I have that book somewhere still.

Andrew

[Stevo]

I have tried this circuit many times and asked Jack too is it right ,,    The only way I got anything out of it is like you say turn the pnp transistor around...but really no gain or real type fuzz to speak of.. There must be something wrong with the design?...All you did was turn the pnp around and add a darlington as the last transistor?....Looks like it should work but I could not get much of anything out of it...tried it quite a few times thinking I was doing something wrong too ....Maybe someone will chime in here and see something I dont??

[jmusser]

It's nice to know where it came from. I figured from the "SK" series of transistors, that the design had been around for awhile. Basically yes, that's all I did, was turn the PNP around, and add the Darlington to Q3. I'd had this schematic in a drawer for probably 4 years, and I went back last night to see if I had written down something wrong. It had printed out cutting everything after Q3, so I'd drawn that out by hand, and figured I had the values wrong. This is a dandy circuit with the Darlington in it. It's kind of a mystery for it to be drawn wrong. You are right. Without the Darlington, it's a horribly anemic circuit. If you still have that circuit around or whatever, add in the Darlington and see how you like it.

[Paul Perry (Frostwave)]

I don't think there was a mistake in the original circuit (which I have checked from the book with the one on the link above). Q2 and Q3 form a perfectly normal pnp/npn direct coupled amplifier pair, except that (according to the original article) Q2 is biased to almost cutoff (thus producing extreme asymmetric clipping). The idea is to adjust the 5K pot to give a signal level of about 1 volt, which means you will need a hot signal to get results.
turning the pnp upside down, will just give you a pnp wiht less (much less!) gain, and replacing Q3 with a Darlington maybe compensates.
Probably there was either not enough input signal when the circuit was first built, or else the bias on Q2 was out.

[jmusser]

It would be interesting to hear it how it was intended to sound. I guess if anyone has ever made it work as shown, I'd sure like to know about it. It really does have nice tone set up the way it is now. I think probably the lower gain from Q2 being turned around, and then jacking it up with the Darlington, helped smooth out everything. I will play around with the transistor set in correctly, and a 3904 back in Q3 to see if I can get anything. I never had anything close to a volt anywhere on that transistor, so I need to play with that pot, and see if I can get it to work. Interesting......

[Stevo]

I ve gotten nothing with the scheme as is.... Change the second trans(  I know it acts as a diode I think that way but it will at least give you sound)... Interested to see why you cant get anything as it is?...Someone must know

[jmusser]

OK, I have done some checking on this circuit. I change everything back the way it was intended, with Q2 emitter towards the 9V rail. I had a 3904 in Q1 and 3, and a 3906 in Q2. If the 5K is turned clear off, I get a badly gated tone bender sort of fuzz, that is very low in volume. I thought maybe it could used  boosted, so I boosted it with the Titan. I really like the thin Tone Bendery type tone while it's there (getting more into the 60s), but it farts out fairly quickly. With Q2 reversed, I get instant tone (not great), but at least it has volume and doesn't fart out. The voltages are as follows (with the transistors per the schematic) Q1 C=8.8, B=4.0, E=3.4. Q2 E=8.8, B=8.1, C=8.4. The Collector voltage on Q2 drops with how hard you attack the strings, and goes down with the strum to 4.1, and then back up again to battery voltage. Q3 C=.1, B=.7, and E=0. If Q2 is turned around backwards, the Q3 Collector voltage goes up to 3.3 while strumming. I don't know exactly what I'm looking at as far the voltages are concerned, but it appears that Q1 is the only one that's really doing anything. Q2 appears mostly shorted, and Q3 mostly open. It would be great if someone got hold of this circuit and whipped it into shape that knows what they're looking at, because the 60s tone is there, just good an buried. I absolutely can't believe this ever worked as written. I feel that the resistor values were written down wrong or something. I guess someone can set me straight on my assumptions, but the voltages don't lie as far as what the intended circuit is doing. I honestly believe, that the Darlington has so much gain, that it would amplify a gnat fart to sound like a volcano, otherwise it wouldn't sound as good as it does the way I eventually set it up.

[Stevo]

 

Who can shed some light on this cool looking circuit ...I have been wanting to know for years why it does not work

[MartyMart]

Has anyone tried some trimpots on this ?
A 20k where the 15k is and a 10 where the 2.7 is ...?
Possibly 100k's to Q1/Q2 also....just a thought

Marty.

[jmusser]

I'm surprised R.G. hasn't taken this circuit under his wing, and whipped it into shape like he has so many others. I figure that the original circuit was so geared to the attributes of the "SK" transistors, that it's going to be real finicky with what it will naturally bias in the circuit. I figured I basically bullied it into submission by what I did to it, and I figure there's at least a decent effect lurking at the heart of this circuit as designed. I could hear a tone in there, especially when I pushed it with the booster, that's worth slavaging. When I hit the strings hard, and got the other transistors amplify, it's plenty loud, but that lasted just long enough to do it.

[armando123]

hi,about the 60's style fzz box,i had the same problem... now,in 2023 ,downloaded the rca sk manual and the rca hobby pdf,i saw that the second transistor q2 is in fact a germanium type transistor ,hfe 90,the others are silicon 125 hfe,so now its time to try....

[mac]

A germ at Q2 makes the pnp-npn stage unstable. Think Fuzz Face.
You can try a low gain Silicon like MPSA92, or simply a 2N3906.
Anyway, Ge or Si, you will need trimpots at Q2 base and/or at Q3 collector.

Q2 collector bias at about 0.7V so it clips one side of the signal sooner than the other.
Q3 bias highly depends on Q2.
IMHO this is the circuit's mojo...

mac