Souped-up MOS-face with two MOSFETs = no sound.

[fuzzmonger]

My main gigging fuzz is a tarted-up AMZ MOS-face (voltage starve control, BC550 in Q1, foot-bypassable BMP tone stack and switchable schottky diode clipping/taming on the output). Here's the basic schematic, for those unfamiliar:

http://www.muzique.com/schem/fuzzface4.gif

Looking to get even more Oomph out of it, I tried putting a MOSFET in Q1 as well as Q2 and the sound was.. gone - silence. I tried several 2N7000s and BS170s, tried every plausible pinout configuration (despite it being a standard CBE=DGS swap-out), checked the transistors in Q2 to make sure I wasn't blowing them due to the lack of diode protection, kept my voltage starve control at a full 9v, and still nothing.

The idea came from those Many Faces of Fuzz / Multi-face articles, showing some explicit MOS-on-MOS Action.

http://www.home-wrecker.com/multiface.html

As there's bugger-all difference between that Multi-face and my AMZ MOS-face, I can't see what's going wrong. Any ideas, chaps?

-Fuzzmonger

[R.G.]

I suspect it's a bias issue. Those two do differ in one place - that being the arrangement of resistors on the emitter/source of Q2. The second has a trimmer for setting the DC voltage on Q2 source, and has the gain control AC coupled through a cap. In the first one, the DC source resistor is fixed.

That means that you can trim in the DC voltage at the source of Q2 to properly bias Q1 in the second version, but have to take pot luck on the modified MOS Face. MOSFETs have minimum gate-source on voltages of 1.6 - 3V, so I suspect that when you put in a MOSFET for Q1, it's not getting enough Vgs to turn on.

Converting the Q2 source resistor to a trimmer and gain pot as in the second version should clean this up.