Tycobrahe Octavia

1. Tycobrahe Octavia Because I Like This Sort of Thing

2. No really, what is it? • It is an Octave/Fuzz pedal for electric guitar. • It uses three BJTs along the way. • It was first made in the early ‘70s.

3. The Circuit

4. Things To Consider • First of all, this is a “positive ground” circuit. Basically this means that most of the voltages are of lower potential than the ground. • A few of the parts were not available through normal means, so I substituted. It worked out, so I’m satisfied. • For simplicity, I modeled the output of an electric guitar as a .1 V sin wave. This isn’t completely correct, as there are harmonics thrown in there initially, but it illustrates the point well.

5. The first transistor utilizes the collector voltage off the third transistor for its emitter voltage. The input signal goes into the base. The First Transistor

6. What does this do? • The current that goes from the first transistor’s collector to the base of the second transistor depends on the potentiometer. • The spikes have different slopes to them… this will be important later on.

7. For 1k resistance due to potentiometer. For .1k resistance due to potentiometer.

8. The Second Transistor • This transistor is there for current amplification. • PNP (2N3906)

9. The Third Transistor • PNP (2N3906) • Due to the amplification from the second transistor, this transistor flips in and out of saturation. • The result is a clipped signal. • It also amplifies the signal a little.

10. Clipping The result of the third transistor is that the output current is clipped. It makes fuzz. Fuzz is good in this case, as this pedal is supposed to provide fuzz. The following is the –1 x the current from the collector of Q3 when the potentiometer caused .5k ohms of resistance:

11. This is the resulting voltage on the primary side of the transformer for a .5k potentiometer value: Transistor Output Result Now that all is said and done with the transistors, we have most of the ac component being able to pass the capacitor and go to the transformer.

12. And Now For The Rectification... That last, clipped signal will now go through this transformer setup: The transformer steps it down 1:3, and the diodes then rectify the signal, taking off the .7V off the voltages.

13. The following is the resulting voltage of the rectification with the .5k ohm pot resistance:

14. This is after going through a little volume control. There is actually a potentiometer instead of the two resistors in series, but it doesn’t affect the circuit other than amplitude. The Output

15. Fourier Breakdown The fourier analysis shows that there is actually more of the second harmonic (the octave) present than the first.

16. What About The Potentiometer? For 1k Ohms Here are the resulting output graphs for two other values of the pot:

17. For .1k Ohms

18. Results • As you can see, a greater resistance due to the potentiometer causes higher octave amplification. • A low resistance causes that steep signal we saw earlier to blend into itself, distort, and the octave element is not significant. • It was constructed, and after some finagling, it worked. • Before actually soldering the circuit I should work on the biasing and optimize it for the desired affects. • A true bypassing switch should be added, because otherwise this of little use and a pain to use. That Is All