tonebender 3 knob q3 voltage divider

[mac]

in a tonebender 3 knob q3 collector resistor (18k) is tied to the junction of two 10k resistors from vcc to gnd.
since there is a little current going to the transistor it looks like q3 "vcc" is a little less than 4.5v.
at first sight i thought that it has something to do with thermal runaway, but finger touching q3 sends collector to gnd quickly without altering the voltage divider voltage, ie about 4.3v.
i can think of 2 reasons why the voltage divider is there, to make it clip referenced to about half vcc, or some interaccion with the tone control.
opinions?

pd, lovely pedal btw. i made one with 2n388 Texas Inst very low leakage Ge, q1-2 collector at 3.8v, q3 at 2v, new 1n60 diode, but i had to place a 1m5 resistor from base to collector because leakage was very small to self bias the thing, 25uAmp.
Ge are isolated with remains of a plastic carpet i use in a house i've just finished, cave effect

mac

[Derringer]

I just turned my attention to the 3-Knob Tonebender design last night.
It really is an elegant design.

I looked at the original schematic design(s) here ...
http://fuzzcentral.ssguitar.com/3knob.php

...and tried to breadboard a silicon NPN version using some low gain 2n3903's. And I totally had an 'Aha' moment when I realized why I wasn't getting signal past Q3. I've never played with germanium transistors before, but now I understand how their leakage could be figured into a design. So to get Q3 to bias, I first played with a 1M pot between q3's collector and base and then seized upon your design here mac
http://www.diystompboxes.com/smfforum/index.php?topic=84315.0
and borrowed your fuzz-pot solution in conjunction. Very clever btw to use the wiper as the input thus creating a fixed 250K resistor from Q3's base to ground. With the 1M pot maxed out I get 3.2V on the collector.
I don't really know how a 3-knob is supposed to sound, but what I have breadboarded sounds cool and different from my other fuzzes. I dig.

But in reading through Fuzz Central's literature as well as posts here, I see that low volume is an issue. In my breadboarded version, the wiper of the tone pot goes directly to the volume pot, and I have the volume pot maxed out to get a signal that's "just about enough" unity.

The bias arrangement on Q3 is really cool. 10K - 10K voltage divider and 18K collector resistor tapped to the divider junction. But that 18K collector resistor connected to that one 10K to ground creates another voltage divider on the "Bass" output of Q3. It reduces the output by about 70%.
So it does interact with the tone control because it attenuates the bass side.

I'm going to give your simplified version a try too and then make a decision on which to build.
Thank you for sharing it!

[mac]

and borrowed your fuzz-pot solution in conjunction. Very clever btw to use the wiper as the input thus creating a fixed 250K resistor from Q3's base to ground. With the 1M pot maxed out I get 3.2V on the collector.
I don't really know how a 3-knob is supposed to sound, but what I have breadboarded sounds cool and different from my other fuzzes. I dig.

Take a look at the Burns Buzzaround and the Big Muff Pi first stages. The 250k to gnd helps a lot to make Q3 stable by draining excess leakage; if not present all leakage would go directly to the base thus changing collector current and bias.

mac

[PRR]

> touching q3 sends collector to gnd quickly without altering the voltage divider voltage, ie about 4.3v.

If Q3 is off, the 10K+10K point should be 4.5V.

Q3 normal bias may give near 4V at the 10K+10K point.

If Q3 is full-ON, collector to ground, the 10K+10K point should be 3.6V.

Any other answer suggests your 18K isn't 18K.

> why the voltage divider is there

Perhaps to reduce clip-point.

Perhaps this was random-design (modify until happy).

But note that the 10K+10K is not bypassed. Good thing, because the tone control takes full-band from the 10K point. Versis highs-only from the 18K point. If you lose the lows, the sound is weaker. So the high-pass taps a higher gain from the 18K, the full-pass taps a lower gain point up at the 10Ks.

[mac]

If Q3 is off, the 10K+10K point should be 4.5V.

Q3 normal bias may give near 4V at the 10K+10K point.

If Q3 is full-ON, collector to ground, the 10K+10K point should be 3.6V.

Any other answer suggests your 18K isn't 18K

PPR,
It was a 18k.
I just finger touched without going to the extremes you noted, on and off.
I should have written the range I observed to be more clear, 4.4v-3.9v iirc (I do not have my notes, on vacation)
Thanks for the info.

About the tone control, highs are taken from the collector by using a 222 cap (I prefer a 472), and almost full range "18k away" from the collector via a 104 cap. The signal is stronger at the collector than at the junction of the 18k-10k-10k. The 10k to gnd after the 222 cap makes the high and full range path of about equal volume, and at the same time form a high pass filter.
I experimented with the value of this resistor and it has a great fx on the tone and volume.
The pot just makes a path more or less resistive.
Well, this is my view of the tone control.

mac