Help my bias this circuit so it works correctly

[saxtim]

I've built a Supa Sustain from GGG.

The schematic can be found here:

http://www.generalguitargadgets.com/v2/diagrams/cssussc.gif

When I got this effect working, it distorts the signal significantly (even with modest sensitivity settings).  I traced the signal through and it's ok after the first transistor, but distorts heavily on the second.

JD suggested in a previous thread that with the right biasing this distortion can be reduced or removed.  

I have no idea about how to adjust the transistors biasing in this circuit - I imagine it's a matter of tweaking resistor values, perhaps with trim pots?  Anyone care to explain where to start and what method to use?

Thanks
tim

[Igor]

I'm not exactly an expert here but I'll tell youu what I know:)

I just had an almost identical problem with a Boss DS-1. It distorted like crazy on the second transistor. It WAS misbiased but in addition to that it provided TOO MUCH GAIN. Maybe this could be your problem too.

Try using a small pot (or a trimmer like you suggested), let's say 10K instead of the emitter resistor of the transistor you want to check. Then plug in your guitar, turn the guitar volume all the way up and start playing. Start adjusting the pot(or trim) to increase the resistance until you get a good sound from the guitar. When I hit the strings hard, it sounded disgusting, like everything's gonna fall apart:) This worked like a charm!  

You should also check the bias, of course. I don't know enough to suggest some values of the top of my head now, but you can also use a trimmer to adjust the resistance on the collector and base resistors.

I hope this helps.

Igor

[Igor]

Just a correction:)

I'm not sure how the BASE resistor affects the bias, the advice I got for my problem was to adjust the COLLECTOR resistor, until I get half of my supply voltage on the collector, so that the transistor could have the same swing in both ways and be in it's linear region.