need help biasing English Channel

[jamiefbolton]

So, the trim pots i bought to bias the J201s were not correct. i figured i'd just replace the trim pot with a fixed resistor. the trim pot is only acting as a variable resistor anyway right? but every resistor i try does nothing to the Drain voltage of the J201. it should be 4.5v. but i only get 9.6. so correct me if i'm wrong, the normal 100k trim pot allows 9.6 v through when its wide open and zero when its closed. the 4.5 will come somewhere in between. whatever that value is, say 47k....i can replace the trim pot with a fixed resistor to bias the J201.

any ideas? i've seen this before in a Peppermill i've built. i didn't have a trim pot so i wanted to use a fixed resistor, but to no avail, it never worked either.

please help.....

[phector2004]

Sounds like the trimpot ain't trimming!

What value trimpot are you using? Does it not vary at all when you crank it? If you put a 50k, it might get close to the 4.5V, 10k probably not, but 9.6V sounds like its not resisting one bit. If you've got it on perfboard and have a lot of resistors, you can probably socket the two connections and stick in different resistor values till it gets close


EDIT: post # 666

[jamiefbolton]

i should clarify. i've removed the trim pots already because they were not working well. they were crappy trim pots. instead, i've put resistors from the 9+ to the drain of the J201. so there is 9+ power getting to the drain through a resistor. this is built on a pcb i bought from OLC, so there should be no layout error.

[phector2004]

You can definitely replace the trimpot with a fixed resistor. Companies do this all the time as they get huge batches of FETs at a single time, with exhibit roughly the same characteristics. In your case, it might be a nuisance finding the resistor that biases the J201 just right. I guess 47k would be a good starting pot to see if it needs more/less.

[CynicalMan]

Use a multimeter to test the resistors you put in to make sure there aren't any invisible shorts. Make sure you have the FETs the right way around. And, of course, do the usual umpteenth check to make sure everything is connected correctly.

The trimpot will make the drain voltage 9V (or more) at one end but at the other the drain voltage will be somewhere between 0 and 4.5V, it will not be 0. But the characteristics of FETs are highly variable, so, a 47k resistor often won't give a drain voltage a 4.5V. That is why there are trimpots. If you don't like the trimpots you have, I'd suggest getting better ones instead of having to solder and resolder in new resistors until you get the bias right.

[jamiefbolton]

yeah i'm going to get new trim pots. no resistors i try are working at all and its too tedious to go about it this way. new trim pots will fix everything.

[PRR]

> i only get 9.6.

EXACTly equal to battery voltage?

Then the JFET is NOT conducting current.

MAYBE real-crappy trimmers, but unlikely to be THAT bad.

Use a fixed 47K Drain resistor (instead of the trim-pot). Tack-solder (temporary) various source-bias resistors, 100, 220, 470, 1K, 2K. One of those should set the Drain between 3V and 6V, and it should work. If not, then something else IS wrong.

Wrong JFET pin-out.

Source resistor not connected or WAY wrong value. (Try 470 ohms, Yel-Vio-Brn.)

Measure voltage on ALL three pins.... could be a clue there.

[drummerman15]

So, what was the issue?

I have a similar problem with my build.  The trim pot feeding the drain of jfet 2 is not dropping the voltage.  I double checked everything in the area and all seems to be OK - correct resistor values and no shorts.  The 4 trim pots are 100k, but only this particular trim pot is not dropping the voltage.  I removed the pot and tested on dmm and it was OK.  I replaced with a different pot and same problem - I get the exact battery voltage whether in circuit or out.  I tried a 47k resistor and the same thing! - battery voltage.  What am I doing wrong here?

[jamiefbolton]

i replaced the trim pots and everything is fine. i'd like to have the first two FETs as MPF102s, but the second one won't bias properly. impossible to get 4.5v. so, yeah, trim pots were the problem.

[drummerman15]

I would think that I could see the voltage drop just with the trimpot connected to the +9v feed. I connected both a 100k trimpot (several actually) and a 47k resistor to +9v and i get hardly any voltage drop - from 9.14v to 9.10v.  I've also just connected both trimpot and resistor directly to the battery - not in circuit- and get the same result.  Is the power feed to the D (of jfet 2 and the G of jfet 3) affected by the interaction of the remainder of the circuit. I just find it weird that the 3 other trimpots are dropping the voltage to their respective jfets just fine. The only difference I can see is that jfets 2 and 3 are connected to this pot vs. the other pots that are just feeding one jfet.

[petemoore]

  trimpot is 1 of many things that help create operational bias.
   It can seem like somethings wrong with it when there's nothing actually wrong with it, measuring the R values may help in determining its goodness.
  Otherwise a 100k or what regular sized pot, and careful 'tack on soldering' can usually spot tack on contact points to connect little wires to relevant nodes, ..and drain bais resistor.
  Once operational bias is confirmed and 'it works optimally' is reasonably satisfied, the then untouched pot value can be measured and a fixed resistor[s value can replace the 'find the value pot'.
  However there are other things that can ''push'' or ''pull'' voltage, mistrace or open, misvalued R.
  When the trimpot has difficulty, suspicion by association of the that stage node/components may have to come into the search-for-problem work area to determine if problem's there.

[drummerman15]

I changed the trimpot again with another 100k and was getting 8.9 v (full counter clockwise) to .4 and .3 volts (full clockwise)- very little in between.  No matter how finely I tried to adjust the pot, I could not get anywhere near 4.5v - just one extreme or the other.  I then tried a 47k resistor and got .3 v.  I changed out Q2 and Q3 with a number of different jfets and no difference.  Why is this one part of the circuit behaving like this?