English channel with BF245's instead J201's

[sheena]

Hello good people!

  Well would like to wish Marry Christmas to all DIYers on this forum and ask for some help.

  Maybe someone with more experience than I have could help me. I've built EC pedal from www.runoffgroove.com but have used BF245's because in my place it is really impossible to find J201's. Of course the thing doesn't work. I've managed to get all drain voltages (excluding q3) to around 4.5v but all source voltages seem too high - (not only q4):

q1 source: 3v
q2 source: 3.6v
q3 source: 7.93v
q4 source: 4.5v (while drain is 4.6v)
q5 source: 0.58v

So what do you think? Could this be caused by the fact I'm not using J201's or is could be some mistake in build? I've checked every resistor and capacitor and all seems correct.
Every stage by itself has some gain but coupled together they don't do anything.

Well I would be grateful to get some good ideas what to do before I engage with buying J201 fets somewhere abroad which is more expensive than it should be.
Thank you for every answer.

Greetings,
  Marin.

P.S. I've tried to replace every particular transistor with two or more but voltages didn't change dramatically.

[nooneknows]

On geofex.com there's a Jfet matcher that is very helpful.
However, try using BF245A not C if you can't find anything else.
(For the real thing try at diystompboxes.com store or Smallbear, I've bought from both and I'm happy)

[PerroGrande]

Hi Marin,

Well, in theory, you could get the BF245A to work, but you're going to have to be prepared to work with each stage one at a time to get it to bias properly.  Even if it biases properly, the gains may be problematic, as in FET world, the transconductance other key characteristics of the device gets into the act.  Even if you get it to bias properly, the gain of a given stage may be different.  I don't know if it will be different enough or not to destroy the sound characteristics of the pedal.

Stage Q3 is just a follower and should work without issue, although I usually employ a lower value resistor than the 10K one shown (2K7, for example).

Let's take a look at some of the key characteristics of the two devices:

J201:

IDss:  0.2-1.0mA
Vp : (-0.3) - (-1.5)V
Gfs : .5 mS (or better)

BF245A:

IDss: 2-6.5mA
Vp: (-0.5) - (-8V)
Gfs: 3 - 6.5 mS

The BF245 has both higher Idss current *and* higher transconductance (gain), at the sacrifice of a wider range of Vp. 

The FET matcher might help here -- especially if you have more than a few BF245's to choose from.  Start by trying to pick BF245s with Vp (Vgsoff) in the range that would be closer to the J201's  (.3-1.5v negative)

In general -- you're probably going to find that you need lower values of *both* source and drain resistors to get this thing to bias with BF245's.  If I have time later, I'll throw this thing on my sim and see what it does with typical J201 and BF245 FETs.

[sheena]

  Well thank you guys, you've helped me a lot. I'm not so in jFET main characteristics and have overlooked
the fact I need A variant of BF245 instead of C which I've used. I'll get a bunch of BF245A's in local store and
I believe it will work - will post report after reinstallation. Thanks about pointing out jfet matcher on geofex
and everything else!

Best regards, Marin.

[mac]

I always use BF245A. I never had bias problems, they simply work. Orange Squeezer, ROG fet simulations, Phase 90, BSIAB, etc, etc.
On the other side, I had to tweak bias when using other FETS.
But BF245A have a Vgsoff a little higher than the J201, up to 2V IIRC, average 0.8V - 1.2V. I use Philips as they have lower Vgsoff.
If you want some very high gain, low Vgsoff fets go for a 2SK117 Toshiba. The ones I got have an average Vgsoff  about 0.5v - 0.6V.

mac