Professor Tweed - 3rd jfet problem

[ficelles]

Having a problem with my Professor Tweed build in that I can't bias the 3rd jfet and the pedal sounds horrible!

Have checked, double checked and triple checked the circuit and swapped out the offending jfet but it still won't play ball. The other 2 jfets bias to 4.5v (ish) no problem.

My only departure from the original parts list is a 103J orange drop cap in place of the 1uf electrolytic, can't imagine that would be an issue? I am not an electronics engineer btw, my background is acoustics/music tech. Can usually pick up the soldering iron by the right end though.

Any suggestions welcome... if anyone in the Bristol (UK) area fancies debugging the pedal and telling me how I went wrong I will gladly buy them a beer or two :-)

ficelles

[GibsonGM]

Usually something like this is caused by a solder bridge between 2 places that shouldn't touch!!  It's hopeful that you got the others to bias...I suggest a thorough examination of all the solder contacts in the 3rd FET area.  I know you've probably looked, but that is usually the problem.  Might take 100 searches w/ magnifying glass to find it!!

Can you post the voltages you are getting at that FET?

~Mike

[oldrocker]

After trying to bias my P. TWD I had the same issue with biasing one of the trannys.  It was touchy but I was able to find a spot on the trim that was fairly close. (4.2 volts)  But that doesn't mean you'll be able to.  It's a rather light overdrive that can sound great feeding another distortion pedal or overdriving an amp.  By itself it might not sound like what you'd expect from a single distortion pedal.  It's sounds good though.  Try it feeding a Peppermill.  It gives it a fat distortion tone.  Try it feeding a TS808 it rocks.  Once you get it to bias and you feel it's working correctly of course.

[ficelles]

Usually something like this is caused by a solder bridge between 2 places that shouldn't touch!!  It's hopeful that you got the others to bias...I suggest a thorough examination of all the solder contacts in the 3rd FET area.  I know you've probably looked, but that is usually the problem.  Might take 100 searches w/ magnifying glass to find it!!

Can you post the voltages you are getting at that FET?

~Mike

Guess what, no solder bridge but I did find an error in the build round the feedback loop/lpf area, now sounds much better but that 3rd jfet still won't bias! Best values I can get are D: 2.85v S: 1.22v G: 0v. Not ideal but it least it now sounds like a nice low-ish gain overdrive pedal so I think I will go with what I've got. With the exception that is of replacing the solid braided wire I used for the off-circuit board connections, I have lost count of how many times the ends have broken...

Re the biasing, on all 3 trim pots I am right near the end of the travel, I wonder if there's maybe a better value than 100k?

ficelles

[petemoore]

  The simple way to express this: Read the ROG Fetzer, try a different Jfet transistor [individual or type].
  If they're wierd before and after reading this and no help...disregard, you probably already knew/tried Jfets with various Vg/soff's.
  100k encompasses the bias needs of many Jfets, whether this is the optimum value for fine tuning...
  If the Jfet starts to bias at around say 68k, try maybe a 47k + 50k pot, this should go well over/under the optimal bias point and still give pretty good 'fine tunability' because the pot shaft rotation [and wiper] are covering a smaller R range so the smaller range of available values is 'spread out'.
  Feel free to calculate any small or large R values, pot+stop resistor applications.
  Trimpots can be wierd...I had one woudn't stay 'here' and always seems to bump bias off the mark...a little bit of R added put the potwiper in a different spot on the wafer, enabling 'right here' to be much easier to dail and keep dialed in. 

[GibsonGM]

totally like pete said.  "break the pot" into a couple of resistors + a smaller pot.  When you get the ideal bias, you can put some silicon compound on there to hold the trimmer in place so it won't wander.   Keep trying, it's in there somewhere!!  )

[ficelles]

Yes I'd thought of the resistor plus smaller value trim pot solution, having used orange drop caps however the board is a tad crowded! I guess since the other jfets bias ok with the 100k pots it's just one more little resistor.

Thanks for the suggestions btw, the prompt to go back and recheck the circuit one more time was invaluable...

I should maybe mention this was my first ever project not from a kit - even if it's not quite finished it was rather satisfying when it finally worked earlier today 

ficelles

[petemoore]

  There's bunches of ways...
  With the pot+stop R, it's kind of like roll the dice.
  First to get a Jfet that'll bias [they're not that expensive compared to having only just enough to fill the positions..."Interviewing them' so to speak.
  But that's not always all of it either, finding the one that bias's and sounds good..mostly to do with the gain structure...which of course bias alters...
  There are measurement circuits for the Vgsoff.
  Also it's possible to take a Jfet, an offboard pot [or pot+Stop R], bias the Jfet, measure the resistance between Drain and V+, pick 1 resistor [or parallel 2 R's of about 2x the value for more a exactl single value] to match the bias value, install that resistor [hope nothing changed somehow], done after final bias check.
  As long as it stays biased, dunno why it [bias] seemed to drift after being fixed, probably something else...
  The Jfet, the D/S resistors..power supply, all matters for 'good' bias, which is generally somewhere towards the middle of the loud bias settings.

[ficelles]

Solved the problem with multi-turn trim pots. Easy accurate biasing, and properly biased the pedal sounds great.

I also installed a 3-way toggle switch for a choice of cap values for the feedback loop - quite a versatile little pedal now!

ficelles

[oldrocker]

Glad you've got it working.  I may need to get those trims for mine since it's so touchy.  The PT is my secret magic box.  I had different friends drop by with thier DS or TS and I add the PT in and it amazes them.  When I turn it off they will say "wait turn that on again".  The Professor Tweed is a great circuit.