Big Muff Clone Troubleshooting - Need help!

Discussion in 'The Projects Forum' started by bluesguitar03, Jan 25, 2010.

  1. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
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    0
    I have been working on a big muff clone and have an issue with the output transistor. Keep in mind the circuit works to some extent and I do get signal at the output. With the volume at the max it is almost as loud as when the pedal is bypassed. The problem I'm having is as follows:

    The voltage (relative to ground) is the same (2.03V) at both the collector and the emitter. The base voltage is at 2.74V. My first thought was that either the transistor was defective or I had used incorrect resistors. So I replaced the 4 resistors associated (R22, R23, R24 and R25) as well as Q4 even though they all tested fine (just in case). Note that I tested voltages throughout the rest of the circuit and they all seem normal. I've looked for a short between the collector and emitter traces and can't see one. Even if that were the case, I don't think that would explain the base voltage being at 2.74.

    Any thoughts?[​IMG]
     
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    I'd check for a short around C14, or somewhere on the base connections.

    The base voltage is too high at 2.74V and the transistor is saturating, hence the equal emitter and collector voltages.

    The R24 + R22 bias network should put about 1.6V on the base, giving roughly 1V on the emitter or 0.5mA through the 2K resistor.

    The same 0.5mA flows through the 10K collector resistor, so that should have about 5V across it, therefore about 4V on the collector.
     
  3. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    20
    0
    Well that made sense so I removed C14 from the circuit thinking that if that was indeed the problem, it should go away. When the problem didn't go away I figured it had to be something with the resistors connected to the base. Turns out what happened was when I made the pcb the values for R24 and R25 got switched. So the 10k was going to the base and the 470k was going to the collector. I swapped them out and it works perfectly!

    Thanks for your response... it got me started looking in the right place!
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Great stuff, happy to hear it's working.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    What is a Big Muff? Lots and lots of distortion?
     
  6. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    20
    0
    The originals were fuzz pedals. I made this one with the intentions of being similar but better. It sounds better than the originals did... quieter and i'm pretty sure it has more gain at the output than the original did.
     
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