Fuzz debugging: why sound is gated

[pingponggwen]

Hello,

I built my first fuzz pedal based on the Tone Reaper but I can't handle with gated sound (or no sound).

On the scheme here is what I did with no results:

1. Replaced both Q8 and Q9
2. Replaced C23 and C24
3. Checked every component between Q8 and Q9
4. Disconnected D7
5. Shuntd fuzz pot

With an audio probe the signal is earable on C23 but nothing from the fuzz pot.

Since I read about bias I measured voltages on transistors:

for Q8: C at 4.74v, B at 0.72v and E at 0.13V (seems ok to me)

for Q9: C at 8.3v, B at 0.03v and E at 0v

So Q9 seems to be the problem but since I double checked components and replace both capacitors between transistors I don't know where to look at (maybe missed something)




Thanks for your help

[ElectricDruid]

Can you post some pictures of your build, please? Both sides of the board would be helpful.

The voltages certainly suggest that Q9 is not happy. I might be looking for a short across the diode D7. Is it definitely in the right way around? Something must be dragging that base connection down to ground.

[Rob Strand]

What you have is a Tonebender Mk III type circuit with a silicon transistor in the last stage (Q9).   It never works like a germanium.  A silicon Q9 gets cut-off too much without some form of biasing on the base.

What you need to do is add a large resistor from the base of Q9 to either the +V rail or the collector of Q9.   Connecting to the collector will be a bit more stable.  You can get an idea of the value by picking the resistor to get about 7V to 8 V on the collector of Q9.  However, at the end of the day you need to play around until it sounds good to you, and it still might not sound right.

For some rough estimates:  7V to 8V on the collector of Q9 requires a collector current of 100uA to 200uA.  Let's say the transistor gain is 70 that means we need a base current of 1.4uA to 2.9uA.  The resistor value (when connecting between b and c) will be about (8-1)/1.4uA = 5MEG to (7-1)/2.9uA = 2.1MEG.   So somewhere in the 2.2M to 4.7M region would be a good start.    Don't imply any precision in the calculations or that it implies a good sound.  They are just starting values you need to play with in the hope it will sound better.

[pinkjimiphoton]

What you have is a Tonebender Mk III type circuit with a silicon transistor in the last stage (Q9).   It never works like a germanium.  A silicon Q9 gets cut-off too much without some form of biasing on the base.

What you need to do is add a large resistor from the base of Q9 to either the +V rail or the collector of Q9.   Connecting to the collector will be a bit more stable.  You can get an idea of the value by picking the resistor to get about 7V to 8 V on the collector of Q9.  However, at the end of the day you need to play around until it sounds good to you, and it still might not sound right.

For some rough estimates:  7V to 8V on the collector of Q9 requires a collector current of 100uA to 200uA.  Let's say the transistor gain is 70 that means we need a base current of 1.4uA to 2.9uA.  The resistor value (when connecting between b and c) will be about (8-1)/1.4uA = 5MEG to (7-1)/2.9uA = 2.1MEG.   So somewhere in the 2.2M to 4.7M region would be a good start.    Don't imply any precision in the calculations or that it implies a good sound.  They are just starting values you need to play with in the hope it will sound better.

temporarily graft some trimmers in  can make it easier to dial in a useable bias.
like rob said, mess with the resistors til it sounds good to you. there's usually multiple sweet spots that will bias up to a useable point, but your ears will guide ya. all bets are off with fuzzes. work with what ya got... be fearless

[antonis]

Just about I was getting worried, Jimi..!!
(Fuzz topic without your presence..) 

[amptramp]

Q9 only has leakage current for biasing, so it doesn't turn on until there is enough signal being rectified by D7 to pull the base voltage of Q9 up to a level where it conducts.  You need additional bias current to the base via a resistor to either +9V or Q9's collector.  The signal is gated off until there is enough bias on Q9's base.