Bias issue on an Octave fuzz circuit

[HH1978]

Hello,

I have traced my favorite octave fuzz, mostly out of curiosity. That pedal is still in production, so I won't reveal what it is.

After tracing it, I built it on a breadboard to see if it works. I could only get gated sounds (with a distinct octave effect though), so I thought there must be a bias problem somewhere. Voltages on Q1 and Q2 are very close to what I measured in the original unit, but they are too high on Q3 (c: 9,20Vdc, b:3,73Vdc, e:2,88Vdc). Trimmer provides very little adjustment.

Is there any obvious nonsense in that part of the schematic, that would mean I made an error when tracing?

If not, how can I tweak the resistors around Q3 to get closer to schematic voltages?

Many thanks!

[HH1978]

Sorry,

Realized it is my first post and i didn't introduce myself.
I'm Hugues, from Belgium. I've been on the forum as a reader for a long time.

[antonis]

Hi Hugues and Welcome..

What is the value of R14 resistor..??
( 'cause if it is 2M2, as I suspect, you've find the issue..)

P.S.
You actually need an Emitter resistor of value equal to Collector one, for phase splitting..
So, place a 10K resistor from Emitter to GND and see what happens then..
(you can see that, as it is, Collector voltage drop is 230 times lower than Emitter voltage - about 9mV..)

[HH1978]

Thanks for the reply!

R14 is 680K, not 2M2.

I'll try to add the 10K from emitter to ground.

[HH1978]

Voltages are indeed better with the 10K resistor from emitter to ground.

c: 6,60V
b: can now be adjusted efficiently with the trimmer
e: 2,34V

However, it still sounds totally gated.

[duck_arse]

also welcome. your 500k pot at the output is shown as a variable resistor instead of voltage divider. this will load and kill your output.

[anotherjim]

Additionally to ^...
The rectifier diodes are basically acting as charge pumps into the 68n output capacitor. Once pumped up, signal will struggle to get through. There needs to be resistance to 0v on the diode cathodes so the charge can bleed off between pumps. Not sure about the resistor networks around the phase splitter output caps. You really need the negative plates of those caps isolated from the phase splitter DC and allowed to go to 0v dia the diodes and the missing bleed resistor. Usually, the output pot can act as the bleed and you don't need an output cap at all.

[HH1978]

also welcome. your 500k pot at the output is shown as a variable resistor instead of voltage divider. this will load and kill your output.

Actually, on the original pedal, the output pot act as an attenuator (and is labelled so), so it's probably what it's intended to do... killing the output.

That's the only part of the original circuit I don't like, to be honest, so I always have that control on 0, using another pedal later in the chain to tame the volume if needed.

The rectifier diodes are basically acting as charge pumps into the 68n output capacitor. Once pumped up, signal will struggle to get through. There needs to be resistance to 0v on the diode cathodes so the charge can bleed off between pumps. Not sure about the resistor networks around the phase splitter output caps. You really need the negative plates of those caps isolated from the phase splitter DC and allowed to go to 0v dia the diodes and the missing bleed resistor. Usually, the output pot can act as the bleed and you don't need an output cap at all.

I wonder, I had a resistor missing on the emitter, and now one missing on the diodes cathodes. I'm sure the total number of resistors on the schematic is correct. I might have confused one 10k for a 100k (the emitter one), and mistraced both. I'll try with a 10k on the emitter and a 100k on the cathodes, omitting the 100k on the anodes, and see what happens. I don't have the original at hand right now as it is at the rehearsal room.

[HH1978]

It works!

Thanks a lot for your help!

Here's the updated schematic :