JFET emulation (J201 or 2N5457) source resistor value

[richon]

Hi,

I'm playing with JFET emulation (Dr. Boogie and "his brothers") and after reading http://runoffgroove.com/fetzervalve.html ...

...I've found that for J201 the IDEAL source resistor was 1,8K and for 2N5457 was 360 ohms.

BUT, if a J201 stage is used (with 1,8K source resistor) with a 2N5457 instead...  I get confused... 

should the source resistor change to 360 ohms to work good?
would the 2N5457 work/sound bad in place of a J201 (obviously changing bias to make it sound good)
besides changing bias point (at drain) what does the source resistor stand for in dBs and frecuencys???

[blackcorvo]

it will work if you properly adjust the trimpot to polarizate it right... this can be made by checking the voltage on drain OR by ear, plugging the guitar to it and adjusting the trimpot till it sounds good.

[richon]

looking again at the FETZER page at ROGs...   they say that the 2N5457 needs a 360 ohm Source resistor and a 820 ohm Drain resistor...

but...  would it be the same if I use a multiplier of those values??

5 x source resistor  = 1,8K
5 x drain resistor    = 4.1K (or something near)

would the FET still work good with the new values (by keeping the balance between the source and drain resistor)

[fuzzo]

I'm working on a JFET overdrive and after I read your question , I tested 330ohm at source with a pot to the drain....Nothing , any sound . 1K seems to be the good values (2N5457 / J201 ) 

[richon]

with a 360ohms resistor in source there should be a 1K trimer (or pot) to set the bias....

maybe the 5X factor (or any factor at all, would do the trick)

[richon]

what about starved bias?

does this explain why it still sounds good?

[electrosonic]

This looks like good information, but I'm no expert.
http://www.rason.org/Projects/jfetamp/jfetamp.htm

Andrew.

[Wild E]

I am also playing with the Fetzer valve using 2N5457's. After measuring the Vp and Idss and plugging the numbers into their calculator I got the following value for the one I'm using: Rs=641, Rd=4745. I got the trimpots close then started adjusting the values. I didn't have a 220pF cap so I used two 100pF, but by mistake I put them in series instead of parallel. The sounds ranged from pretty clean to ugly mud. Rs=800, Rd=2.37K gave a nice sound but not very tubey. This is a great learning circuit for someone new like me. If you set your Rs then start adjusting Rd you may find it hard to put down! I guess it's time to say thanks ROG!

[petemoore]

  Great starting point for a distortion too.
  Sounds great driving distorters.
  Is a great first, and with some fiddling with Jfet's, second stage of a distorter.
  Any of these two stages can be a keeper or part of something else.
  As a way of saying it, try to find what amount of input content [varies in frequency, with dynamics, pickups etc.] starts to distort a Jfet gain stage, go for less, or more than that..as part of the overall considerations of ''gain structuring'' if you will, how much is fed into each Jfet input [in terms of distorted waveforms, frequencies that add to 'average gain' numbers].
  I think the trick is to first figure out how much gain it is to target, then how many transistors would sound cool getting there, subtract for voicing and add frequency control [''sprinkled is cool, many schematics tend toward rolled off bass early and treble adjusted later, YMMV].
  It's not for lack of options, that's for sure, for any certain amount of boost, there are a thousand ways to get there.