Trouble Biasing a (GGG) 3 Knob Tonebender

[afrogoose]

Hey Everyone,

I built a 3 knob tonebender from the GGG PCB.  It works okay but there isn't as much fuzz as there should be and the fuzz is pretty gated.  I'm finding that I can't get the transistor at Q3 biased anywhere near 4.5v, even if I jumper the biasing resistor and run the trimpot all the way open.  At the most I can get it to about 2v.  To make things stranger, when I turn the transistors upside down (switch around the collector and emitter) I have no trouble biasing the fuzz at 4.5v, but while the pedal makes sounds, it doesn't distort at all.  Is it possible I've wired + and - backwards, because the build instructions didn't include LED or True Bypass and it's very possible I've mixed that up (although someone was kind enough to send me a link in a previous post)  I 'm really hoping someone has some wise words of advice for me!!!

Thanks!!
A

Links:

http://www.generalguitargadgets.com/diagrams/tb3bb_lo.gif?phpMyAdmin=4a28f86a515b7883e7bc35a68d4e7b6d

Here's the diagram I used to add the LED and TB switch

http://www.generalguitargadgets.com/diagrams/switch_lo_3pdt_tb_dcj_pnp_gi.gif

[mac]

Because of the trimmer between Q3 base and collector, are you using Ge or Si transistors?

You won't get 4.5v at the collector of Q3 because there is a voltage divider formed by the two 10k resistors, and from the junction of both there is a 18k going to the collector of Q3. Since there is some current going to the 18k you will get 3.8v aprox at the junction of the 10k's, and a good value for Q2 collector is around 1.5v - 3v, say 2v is fine.

The reverse Ge diode, which is used to control to some extent Ge temperature run away, can cause some good trnasistors to bias and others to sound like crap. This is beacuse even in reverse mode there is some leakage current that can vary from diode to diode.

If you don't have much fuzz check if the trimmer is not near zero, if the bypass cap at the 3k3 resistor is well connected, the dirt pot.

http://www.diystompboxes.com/smfforum/index.php?topic=64677.msg512877#msg512877
http://www.diystompboxes.com/smfforum/index.php?topic=72481.0

mac

[Solidhex]

Yo

  Yeah I was confused when I built the same circuit. I assumed q3 needed to be at -4.5v but this is a different circuit than the Fuzz Face. I just biased mine by ear. Its sounds like you might need to try some different transistors unless there is a problem with your build.

--Brad

[afrogoose]

Hey guys thank you so much for the info. Special thanks to Mac for that link to your previous posting. I was pretty confused by reading some other posts where some people described being able to easily bias their TB at 4.5v while others were unable.  Hopefully this thread will make this more definitive for future seekers.

I bought my transistors matched from Small Bear. Both the transistors and the diode are ge.  Unforunately, I switched out the diode when I was trying to get it biased to 4.5v and now I can't find it. Right now, it biases just shy of 2v all the way open (with a 1n34) and after reading macs fimding about the interdependence of the diode and Q3, Im wondering if the one from Small Bear wouldn't be a better choice?

[mac]

If you have it biased at 2v it should sound fine. The diode has nothing to do with the distortion.
What about Q1,2 voltages? It should be near vcc/2, although I prefer 3.8v - 4v.

mac