Trimmer and Jfet (J201 in this case)

[arma61]

Hi guys

need help with a ROG Condor CabSim http://www.runoffgroove.com/condor.html

looks like the trimmer is affecting the voltage at J201's D only for less than 1mm, I mean I don't need to make pressure on turning the screwdriver, just touch it and the voltage change drammatically, from 2 to 7 volts, that's for all the 100 trimmer I have, blu, black, vertical, orizonthal, new and old ones.  What can be wrong? I cannot get the right sound from it..... the "G" point  is hard to find!!

...I remember on other 2 Condor CabSim they worked fine, I can just turn left and right for about half of the travel's shaft to get the right voltage.

Thanks

[deadastronaut]

hi armando:  have you tried a regular 100k pot?...change the fet?.....

[petemoore]

  Perhaps there's a miswire...but Jfets 'jump' into and out of bias pretty easy, sometimes the trimpot misses the sweet spot...
  Perhaps build the R up near where it would bias [say 25k or 50k ?...whatever puts the drain over/under/near bias point...then use a small trimpot to fine tune the ''final summation'' ie adding the fixed resistor to get very near the R value, then adding a small value pot to that so the trim/knob spans only a small R range which covers the R value around the correct bias point for 'this' Jfet...this will make a much more 'finetune' bias adjuster.
  The next Jfet will require the same 'special' attention because they vary so much from one to the next. For this reason a different Fet device [the next J201 or whatever], may make it easier for bias/performance @9v supply.

[diydave]

Or take a batch off j201's (e.g. 10 fets) and try each one until you get the right fet for the job.

It's possible that the 1k source-pot is the culprit. In that it's to big for a smooth baising of your fet.

[frank_p]

...but Jfets 'jump' into and out of bias pretty easy, sometimes the trimpot misses the sweet spot...

Yeah, it happened to me too.  Even with multi-turn trimpot.
You could try to measure at what Ohms it jumps,  then take a multiturn trimpot of lower value then the 100k and a bit higher than your measured resistance (if you can find one).  Lower value for the pot plus multiturn gives you more sensibility per turns.

[arma61]

thx guys for help 

got 10 mins yesterday night to try out the few J201s I have, they all act the same! tried also some 2N5457, little better but still hard to "take" the sweet spot. Will try with a standard pot, a brand new one, may be all my trimmers are "old"

I also compared it to the other CabSim I have, same layout, same components, same jfet lugs orientation! that's weird, I don't want, for the moment, to take out the fet from the working one, I use it on my peavey rage 158? so I can connect my hearphone, so I want it to stay as it is!!

Will do other test tonigt, also comparing with the working one the gain, it seems to me the gain of this new one is far low than the working one.

Thx again
Armando