blackfire fizzle

[rtill]

Hey guys, built the blackfire and another boogey pedal.  They sound GREAT!!! except they both have a weird fizzle when playing.  Any ideas of what it could be......the biggest difference I have in the blackfire from the original schematic is, because I ran out of parts, ....I used a 3.9k resistor in place of the first 2.2k.  Could this be the cause??? or is it a weird ground issue?

[jonathan perez]

what kind of fizzle?

[petemoore]

  Blackfire is known for developing sizzle in the high end.
  Mods for sizzle include anything that reduces it.
  >From signal path:
  Small Cap to ground
  Smaller caps to ground from multiple signal tap points
  Cap to ground through a pot [.1uf / 10k ?...output?]
  A popular BF add on is a Tonestack, the circuit can push the passives and still have enough left over for some boost.
  Any combination of these is applicable.
  In signal path-:
  Big resistor [~330k>?] will attenuate, and attenuate slightly more treble than bass.
  Bypass a BF stage, that'll make it an inverting circuit, reducing gain...whether that's enough gain reduction to eliminate oscillation caused by inverted signal routing or whether in phase matters...?
  Bypass two BF Gain stages, IIRC there are four.

[rtill]

it sounds like the .....for example....... on the fetzer valve ..... if the trimpot isn't set correctly, you get the farty fizzle noise.

[petemoore]

  I'm sorry, that sounds like gating?
  Know by taking voltage measurements on the transistors...audio probing, if there's a misbiased stage, good chance the drive of a previous stage would 'force' it to pass signal, if mangled somewhat.
  Four stages, if one is 'off', you'll see incosistant bias voltage on that stage compared to the others.

[rtill]

would the mismatched 3.9k to ground in place of the 2.2kI used in the first stage be the culprit?

[petemoore]

I used a 3.9k resistor in place of the first 2.2k.  Could this be the cause??? or is it a weird ground issue?
  Emitter bypass resistor, sets gain on the transistor probably, don't know what were talking about without a schematic, [e bypass cap?] but yupp, almost doubling that one's value will change the gain/hence clipping sound.
  Take a 3k9 and do math or find a parallel resistance value that sums to about 2k2 [using DMM and a buncha resistors...something about 6k8...very rough estimate. Add the ~6k8 resistor [whatever you find that makes 3k8 =~2k2] to the 3k9 you have there.
  Alternately you can find the resistance with a pot what you want to make 2k2, adjust say a 20k pot as VR [one outside lug not used] across the 3k9, read the pot only, find and use a 'like' fixed value parallel to the 3k9...something to bring that closer to 2k2..