The FZ-1A/Fuzz-A-Tort

[smallbearelec]

Yes, Small Bear Can do slightly leaky germanium!

I got around to breadboarding these circuits so that I could offer matched transistors for them. A growly, bearish Thanks! to Gez Paton for doping out the recipe and posting it years ago. It was nice to discover that Q1 and Q3 use devices that we would normally sell in assortments or remainders. So a PNP set is available on the Stock List, and I put together a few notes to assist those who don't want to wait for a full project:

http://www.smallbearelec.com/Projects/FZ-1A/FZ-1A.htm

To anticipate a question: I hope to be be able to offer an NPN version shortly. It should not be too hard, since much NPN germanium is either too low gain or too leaky for other uses. Add one "good" device for Q2, and we may have an interesting-sounding, germanium, non-Fuzz Face that plays well with other pedals on negative-ground boards.

Regards
SD

[stringsthings]

very nice!

i appreciate the circuit explanation, also .... at first glance, i was wondering if the circuit would run on a single 1.5 volt battery, and then the explanation confirmed my wonderings! .... PNP circuits are quite interesting ....

[theramin]

Hi there totally new to this incredible forum

I am in the middle of my first ever project which happens to be the FZ-1A/FUZZ A TORT. Got the schematic from an old popular mechanics scan.

A friend of mine is very good with electronics to say the least but we are still unable to get a proper sound. We are still working through it. All the
connections are checking out except just getting hum and no fuzz sound.

for transistors I found some rare original 2n2613's as the schematic suggests. New in the box. Checked out the measurements and they work.

Are you aware of any pitfalls or have any suggestions on known bugs on this unit? Any tips on it? Still at the breadboard stage. I find 1.5 volts quite low but what do I know?

Thanks for any help on this. Cheers, Theramin

[trjones1]

Can someone explain exactly why that circuit likes or tolerates leaky/low gain transistors?  What's the secret?

[smallbearelec]

getting hum and no fuzz sound...transistors original 2n2613's...Checked out the measurements and they work.

Do you mean that you confirmed gain and leakage? Have you taken voltage measurements at C B and E? How do they compare withe what I saw:

http://www.smallbearelec.com/Projects/FZ-1A/FZ-1A.htm

I have made the thing to work with both PNP and NPN devices.

Are you aware of any pitfalls or have any suggestions on known bugs on this unit? Any tips on it? Still at the breadboard stage. I find 1.5 volts quite low but what do I know?

Note that a 2N2613 might leak too much to be good for Q1. Gez Paton recommends that the Emitter be at .4 to .6 Volts, and you have some control over this if you tweak R3. Best is to find a  low-leakage part with gain 60-80. For the rest, you may have to tweak R8, R9 and R10. See my response to the next post.

[smallbearelec]

Can someone explain exactly why that circuit likes or tolerates leaky/low gain transistors?

I'm speaking partly from recall of a discussion I had with R. G. Keen when I started experimenting with this, and partly from some educated guesses.

The people who designed this thing knew that relatively high leakage was normal for commonly available germanium parts. They set up the bias accordingly so that (I am guessing here) the device would have half a chance of being temp-stable. I think that's why they powered it from 1.5 volts rather than 9; as the supply voltage goes up, leakage current increases. At some point, the circuit would run away too easily.

That's the "why" of the design choice. Functionally, (I'm recalling the discussion with R. G.) both Q2 and Q3 depend on leakage current flowing from the Collector into the Base to turn them "on". Q2 also gets a fixed bias from R4, and the Fuzz pot sets the actual operating point. Q3 is turned on entirely by leakage current. You can prove this by trying a low-leakage, Fuzz Face-type device there; it will sound very gated. If Q3 leaks enough to forward-bias it, its operating point can be adjusted by tweaking resistor R9.

[PRR]

> why that circuit likes or tolerates leaky/low gain transistors?

Q1 and Q3 have _NO_ bias current into the base. Note: no resistor (or other trickery) from power rail to base.

A perfect transistor would then conduct NO current and pass NO signal.

To modern minds, this won't work.

Ah, but Ge transistors leak. Good ones a little. "Experimenter grade", and the odd lots found behind old warehouses, often leak a LOT. Enough to "work" without any explicit or designed bias.

Build one (assuming 1.5V supply), look for over 0.1V and under 1.3V at Q1 emitter, Q3 collector.

If you are stuck with "good" transistors.... well, buy some "bad" parts from Small Bear. If he ever runs out, you can fake it with 100K-1Meg resistor from power rail to base, aiming for Q1 emitter and Q3 collector to be "between the rails", not slammed to zero or to 1.5V.

Q2 bias is adjustable. R4 forces Q2 "on". Turn R6 and Q2 goes "off". In-between there is "optimum" bias and also a range of "forked-up" biases which is where the fun is. The exact setting of R6 will vary with Q2 leakage; it may not tolerate VERY leaky or VERY low-gain devices.

[theramin]

For Q1 what would be a good/better transistor then the 2n2613? Thank you for all this help

[smallbearelec]

For Q1 what would be a good/better transistor

The type number Does Not Matter. You want a PNP device with a leakage of 100 microamps or so at 9 Volts C to E and gain of 60 to 80. Test a bunch and find one that fits those specs.

[theramin]

Thanks. the fuzz a tort is powered in the schematic at 1.5 volts. Should this be disregarded and should we up the power to 9 volts or is that just for testing the Q1?

[smallbearelec]

The gains and leakages are measured at 9 Volts, but the circuit is powered at 1.5.