Re: Voodoolab Superfuzz Diy

Posted by reddesert on Jul 23, 2017; 10:33am
URL: http://guitar-fx-layouts.238.s1.nabble.com/Voodoolab-Superfuzz-Diy-tp39180p39209.html

Two Superfuzz schematics are here: http://freestompboxes.org/viewtopic.php?f=7&t=449&hilit=voodoo+superfuzz and
http://freestompboxes.org/viewtopic.php?f=7&t=449&p=254952&hilit=+voodoo+superfuzz#p254952
They disagree on the orientation of the PNP 2N2907 which might be part of the problem.

The transistor voltages also look odd to me.

If you look at the resistors above and to the left of the MPSA18 transistor, there is a path for DC voltage from V+ to ground through R1-R2-R3-R4, 18K - 560K - 560K - 150K.  When the MPSA18 is not conducting, this sets the voltages of both the base of the MPSA18 and the collector of 2N2907. It biases the MPSA18 - the base of a NPN transistor needs to be held above the emitter by roughly a P-N voltage drop (about 0.7 V) or it won't do anything.  The voltage at the base of the MPSA18 should be 9V * 150 / (150 + 560 + 560 + 18) , so about 1.0 V.  The voltage at the collector of the 2N2907 would be 9V * (150 + 560 + 560) / (150 + 560 + 560 + 18) = 8.9 V.  

However, when the MPSA18 is conducting, it should provide a low-resistance path that pulls these voltages down. In the extreme case that the MPSA18 pulled a lot of current through it, the path from V+ to ground is essentially R1 - R5 - MPSA18, and the MPSA18 is effectively zero resistance.  So then the R1 - R5 junction is at 9V * 18 / (18 + 18) = 4.5 V.  And the base of the MPSA18 is at 4.5 V * 150 / (150 + 560 +560) = 0.5 V.

So that might explain why you're getting the base of the MPSA18 at 0.5 V, but it wouldn't explain why the collector of 2N2907 is less than 4.5 V, unless the 2N2907 is itself sucking the voltage down.

To summarize, I'm a bit skeptical of both the vero layout orientation of 2N2907, and that the transistors are getting the right bias voltages.

[Edit: I meant the resistors above and to the left of MPSA18 on the schematic, not the vero.]