BSIAB II noiseless biasing issues?

[theman]

I have an issue with the BSIAB II when using noiseless biasing -- or I think so. I'm using a 10K/10K resistor divider and taking 470K resistors to the gates of Q2 and Q4. For some reason, Q5 is very difficult to bias. I'm using a 100K trim, and it works great over a wide range when I play hard power chords, but soft passages has the sound crackle and then act like a noise gate and shut-off. The sound is amazing with a 2.2uF bypass cap on the source of Q5 also, but this is even harder to bias on soft passages. I can only get it to sound good over the whole input range when the trim pot is something like 10K (using a J201), and it's really sensitive (and the volume is a lot lower also). I've swapped out a few JFETs (J201s and 2N5457s) with the same issues. Any suggestions on trouble shooting?

Thanks.

[R.G.]

Maybe. Got a pointer to the schematic?

[theman]

The schematic is the same as the BSIAB II (found at http://www.generalguitargadgets.com/diagrams/bsiab_2_sc.gif)  except that instead of Q2 and Q4 being biased at the gate by the 1M resistor dividers, they are being biased instead by a 10K/10K resistor divider (midpoint voltage stabilized by a 47uF cap) which then goes to a 470K resistor to the gate of Q2, and another 470K resistor to the gate of Q4. I'm currently using all J201s for Q1-Q5. I'm thinking the biasing problem on Q5 is not due to the noiseless biasing scheme, but I'm not sure. I've read that J201s are higher gain, and maybe harder to bias. Also the 5.6K source resistor in Q5 could cause some biasing issues. I'll have to go back and try using 2N5457s and see if that fixes things ...

Any other suggestions?

[petemoore]

  Q5 is biased separately ffrom the two Mu Amps that precede it.
  I've heard that swapping Jfets may make it easier to bias a Jfet 'position', may or may not help, have you rechecked the Bias Resistors with the DMM on Q5?

[theman]

Thanks for the suggestion. The mu-amp stages definitely affect how the last stage biases up a bit. I changed Q1, Q2, and Q5 to 2N5457s, and there's less gain, is less noisy, and a bit easier to bias up. The last trim pot resistance is about 15K. The tone has more clarity, more open, less compressed, and is more suitable for rhythms. But I'd still like more gain and compression for leads.