diode function in the TB3

[mordechai]

If I understand correctly, the diode in the Tonebender 3 is employed to help stabilize Q3 against the effects of temperature change.  I wonder if it has a tonal affect, however.  Would the addition of this sort of diode to a Tonebender Mk 2 circuit affect that circuit's characteristic sound or gain characteristic?

[mordechai]

Anybody?  Bueller???  Diode function in the Tonebender Mark 3?? 

[Derringer]

skim through Marcelo's thread here:

http://www.diystompboxes.com/smfforum/index.php?topic=84315.0

he claims that the diode in his simplified circuit "does something, very subtle."

But yeah, the way I understand it, in the original Ge design the diode is there mostly to stabilize that transistor under temperature variations.

[mordechai]

Thanks Derringer.  This is very helpful and I appreciate it!

[PRR]

> diode in the Tonebender 3 is employed to help stabilize Q3 against the effects of temperature

?? If we are looking at a plan like this, there is no "Q3" and the diode's temperature effects are negligible.

It DOES change the clipping action. Without it, Q2 bas may rise to only ~~1V but can be kicked-down to -6V or so. The asymmetry will charge-down C4 and C7, changing the bias on Q2. With that diode, clipping at Q2 base is more symmetric, "different".

> Would the addition of this sort of diode

Might be quicker to try than to ponder.

[Johan]

here is the 3knob tonebender
http://fuzzcentral.ssguitar.com/3knob/3knobschematic.gif
since the original transistors were GE, my understanding is the the ge diode in this design has some temperature stabilizing properties, but not if the transistors are Si...ge and si are not the same
obviously, since the diode goes to ground it would clip any signal bigger than one diode drop. and with the transistor emitter going to ground too, I guess there isnt much room for the base to swing..?
..clearly PRR has a MUCH better understanding of these things than I do, so I hope he steps in again and explain now that the correct schematic is linked to...

..I built mine with regular Si transistors in Q1 and Q2 (MPSA92 i think it was) and had a AC128 as Q3. this design seamed to me much more stable and forgiving than the Fuzzface and not picky at all about transistor choises,  allthou lacking some of the "fire" the ff has...but that is just an opinion and up to taste...
J

[PRR]

Still making clipping more symmetric, not throwing the bias off.

The transistor and diode "should" be similar: 0.2V Ge, 0.6V Si. (However you may find musical use for a 0.2V:0.6V asymmetry.)

Temperature is not in the picture. In the Ge version, Q3 is biased only by self-leakage. Yes if the diode leakage were EXACTLY matched to transistor C-B leakage, we could force transistor "off" and hold it there with temperature... but we don't want transistor "off" and it would be far too expensive to do such matching in production.

Look at DynaComp and friends: http://www.geofex.com/FX_images/dynacomp.gif

In this drawing, Q3 Q4 are level detectors and rectifiers. For simplicity they are cap-coupled to their driver. Without the diodes: On the first positive transient, the cap current kicks the Base, but also charges-down the cap, and subsequent transients have little effect. With the diodes: on the first positive transient, the cap current kicks the Base, but also charges-down the cap, but then the subsequent negative transient is clamped by the diode and charges-up the cap, no net charge.