Big Muff excessive feedback; Q2 voltages are low. Help requested.

[aefpv]

I could use some help with my Big Muff project.  I have a lot of excessive feedback and the voltages are off for Q2.  I am utilizing the below layout for the perfboard.  I am making the Triangle version and I have noted the parts that are different from the original layout.  If the part is blacked out, it means that I did not include it in the circuit.  Below is the perfboard layout with changes and the schematic (Tonepad, Triangle version)


My voltages for the Q's are as follows:
Q1
C   4.34
B   0.71
E   0.15

Q2
C   1.1
B   0.69
E   0.09

Q3
C   4.25
B   0.69
E   0.13

Q4
C   3.94
B   1.5
E   1

Below are the pictures of my circuit.  The soldering could be cleaner.  However, I have checked for continuity between elements of the circuit, and it did identify an issue with Q3 that seems to have rectified the voltages for that transistor. 



If you all have any suggestions to help with the excessive feedback, I would appreciate the help.  Thanks!

[antonis]

Sorry but your modified layout makes no sense..
e.g.:
+9V & GND seem to be shorted under the big black circle..
Q4 Collector seems to be "open" - no connectios to OUT..

Also, there are 3 green unidentified wires on your build board..

P.S.
Circuitry around Q2 seems to be OK, from items values & orientation viewpoint..
Reflow 12k resistor legs pads, just in case they are "loose"..

[aefpv]

Thanks for the reply. I should have clarified my layout better. With you example about 9V connected to ground-  That connection was not made and skipped. If I had given the original layout, one would see that I omitted a parallel cap and diode combination for reverse polarity protection. I also did not include an input buffer cap, etc. The Triangle version dropped the input buffer cap and some resistors off of the base of the transistors. When skipping these parts, I made the connections based off of the schematic. I am not confident if the wiring is correct.

The 12K resistor has been reflowed and the problem has persisted. Q4 collector is connected to a 12 K resistor and 100nF cap and then out. The green wires are going to the tone pot. Sorry for the confusion!

Thanks for the help.

[iainpunk]

the bottom left transistor (i guess Q2) originally had a resistor to ground, you left this out, i suggest putting one back, around 100k should be alright.

cheers, Iain

[antonis]

The 12K resistor has been reflowed and the problem has persisted.

If so, make Q2 Emitter resistor 180R and Collector one 10k or Emitter resistor 220R and let Collector resistor value as it is..
(Collector should sit on 4V, give or take..)

[aefpv]

Great ideas!  I will give it a go tonight. Thanks again. AE

[aefpv]

I tried the suggestions above. Adding the 100 k resistor to the base on Q2 got me closest to the desired voltages. However, the feedback (squeal) is still present. The feedback worsens with all pots turned fully on, but much worse with manipulation of the sustain pot. The new Q2 values are as follows:
C.  3.34
B.  0.68
E.  0.05

Can the feedback issue be due to these voltage issues or should I look elsewhere?  If the voltages need more manipulation, any suggestions?  I did try multiple other resistors for the base resister, but not much luck. Lastly, could the transistor be faulty?

Thanks again
AE

[Rob Strand]

One of the collectors looks like it has a 22k collect resistor.

EDIT: After looking up the zillions of Big Muff options, that might be a one of the valid configurations.

[antonis]

Can the feedback issue be due to these voltage issues or should I look elsewhere?

Look at the two 500pF Miller caps..
(start by doubling their value..)

[aefpv]

Sounds good. Will see if I have the caps to make this change.

One other thought or wrinkle in this issue. I put the circuit together with a breadboard and it worked fine. I did not measure the voltages, so I can't compare. However, this feedback issue was not present with the breadboard. With this in mind, my wiring maybe suspect. Would grounding issues create this type of problem? 

Sorry for a rookie question. I have done printed PCB kits with success and this is my second perfboard circuit. The perfboard is much more difficult than the printed PCB!

Take care
AE

[antonis]

Would grounding issues create this type of problem?

A grounding plane could help a lot but, IMHO, it's more of crosstalk mater issue..

[aefpv]

Hopefully, my humble admission will help someone debug their pedal in the future.  I have finally got the pedal to work wonderfully.  When the pedal was providing excessive feedback, I had been testing the pedal outside of an enclosure.  I assume that indicates that my pots, etc were not grounded.  I also discovered, that I had wired the input jack incorrectly.  The signal was wired to the ring and not the tip of the jack.  I do not understand how the pedal was still able to create the sound.  However, as soon as I placed the circuit in the enclosure, the pedal emitted no sound and I found my error quickly.  Once the jack was wired corrected and everything was grounded, it worked really well.  Now, should I try some of the modifications, like switches for different clipping options?....

Thanks to all that provided help.  I really appreciate it.