wich J-Fet equivalent for overdrive?

[mohsinddd]

Hello everybody,
i am going to build a simple overdrive box, its the Trotsky Drive from Beavis Audio, but i was wondering. from the KT312 and other transistors he mentoined, wich Jfet could be used in his circuit?
i am interested because i would like to experiment with the sound of a jfet in overdrive. Could it be the J201 or MPF102? the other transistors he mentoined as subtitutes for the soviet chip KT312 are the 2n2222 and the 2n3904 both sillicon, non jfet...

http://www.beavisaudio.com/projects/TrotskyDrive/

[lopsided]

hello,

you can not easily change bi-polar transistors for a jfets, because they are biased differently.
I would recommend building the Trotsky and try out the bi-polars and then choosing another circuit designed for jfets and experiment with them.

[mohsinddd]

i understand, the problem however is that i could not find any simple jfet overdrive circuits.
can you recommend me any? do you have any schematics with jfet overdrive?

i have seen the Fetzer Valve, but it seems to be a booster, would it be good to use as a overdrive/distortion? or could it be modified a bit to do overdrive?

[CynicalMan]

You can make an hybrid Fetzer/Trotsky:

For a gain control, replace the 1M resistor on the input with a 1M log volume pot. You'll probably want a J201 for this, as a MPF102 won't have much gain.

I have a FET OD on my site:
http://sites.google.com/site/distorque/home/projects/atomic-overdrive-ii

Runoffgroove has some other FET ODs, although they're more complex:
http://runoffgroove.com/professor.html
http://runoffgroove.com/eighteen.html
http://runoffgroove.com/articles.html

There's also the AMZ Mini-Booster which is a dirty booster:
http://www.muzique.com/amz/mini.htm

[mohsinddd]

the hybrid looks great, thanks

[petemoore]

  In order of expected output amounts:
  MPF102 [soft' output],   
  2n5457's,     
  J201,     
  NTE458 [hi-loud, but hard-glass ''sounding''].
   Build the Fetzer testjig, take notes on the measurements to decide which individual candidates can be more easily 'shoehorned' into 9v bias arrangements, sticky-notes or tape makes it easy to pin the transistors to the numbers it produces.
  Starting with a small handful [>10?]of say 2n5457 measured/ the notes will produce a wide range of Bias needs, some Jfets will be easy to bias @9v, some won't...choose candidates from 'the batch' of Jfets for best results.
  Setup: Bias each stage individually, some variance of bias circuit resistances makes setting up bias on the transistor pins easier, but trimpots can be finnickey. Replacing a 'testpot' [used as a temporary/variable bias resistor] to determine the corrected R value for proper bias] with a fixed resistor [meausure the corrected pot value, swap in a fixed resistor of the same ohmage] may make it easier to keep a number of stages biased. I mounted trimpots for a while but found that using a trimmed 200k regular pot [with wires to testclips] allowed quick bias measurements, then soldering in the correct value for the stage [use 2 fixed resistors to build a more exact value, or very carefully 'ballpark' the value to stay within the bias-pocket.
  MuAmp notes: low output current? [what input impedance will it feed?], it is a self-biasing, a non-linear, 2 transistor stage.
   Input distortion and output distortion of Jfet can be varied, are controllable, and 'sound' different [technically, electronic analogs don't qualify as 'sound']. Stronger input peak voltages [boost 'em] will produce greater stage non-linearities [harmonic content = 'distortion].
   Note soundclips/voicing techniques on various 'string-of-Jfet-stages' type circuits, bias, enjoy, perhaps re-voice, over-complicate...or perhaps even simplify the circuit down to to just what is needed, no more. 
   





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