Big Muff Pi: Sustain missing on low notes

[doii]

Hi

I build the Big Muff Pi from General Guitar Gadgets on perfboard.
http://www.generalguitargadgets.com/index.php?option=content&task=view&id=27&Itemid=52

It's my third build and everthing went well. It sounds OK except for the low notes. They have almost no distortion and no sustain. For the higher notes there's enough sustain.
I took the voltages on the transistor pins (in brackets the voltages from General Guitar Gadgets). The values are right except for Q4:

9V power battery: 8.6 (8.

Q1
C=3.8 [3.9]
B=0.6 [0.6]
E=0.03 [0.03]

Q2
C=3.8 [3.8]
B=0.6 [0.6]
E=0.03 [0.03]

Q3
C=3.8 [3.8]
B=0.6 [0.6]
E=0.03 [0.03]

Q4
C=3.7 [4.0]
B=1.6 [1.8]
E=1.0 [2.5]

Transistors are 2N5088. C15 (100pF) is not on the board because I don't have it yet. C10 (0.0039uF) consists of 2 caps in parallel (3300pF and 680pF).
What could be wrong?

Thanks in advance!

[GibsonGM]

Hi Doii,

Given that your voltages are in range except for Q4, I'd expect to have a healthy distortion going with all freq's....check the resistors around Q4 maybe, see if any have shorts and are the right values.  This is the boost/recovery transistor, and coming out of the tone stack, it has a large effect on output.  While you're there, check the tone stack for shorts and/or weird part orientations.

C10 missing MAY affect to rolloff at the input but I wouldn't expect it to have a great effect.  However, it helps if all the parts you need are in place, even if it's 400pf, or whatever, for now.

Try audio probing along the signal path..this should help you find the short or whatever that is causing these symptoms.  It'll at least narrow it down to 1 area.  I suspect something may be wacky in one of your feedback loops (C2/R3, etc. etc.).  This ALWAYS works for me, especially when building from a known working schematic      I perfed mine too, and it sounds Great, but took me some debugging to get it right! Good luck.

[doii]

Thanks for your quick answer and your help. I'll check my board using your tips.

[GibsonGM]

Good luck, doii...if you keep at it, you'll get it ;o)  The fact there's any sound at all coming thru tells me it shouldn't be anything major, just an error or component issue somewhere.   I make everything on perf (even an Easyvibe!) because I don't have a drill press, nor time/inclination to etch boards.  Quick and easy, but there's a ton of potential for stupid mistakes!!   

[Cliff Schecht]

Crank up the gain man!