Wednesday, 6 February 2013

JFET Vulcan Overdrive

Great circuit by Joe Davisson, but should probably be called a distortion rather than an overdrive because it is apparently quite high gain.  The trimmers should make biasing the JFETs easy so don't let inconsistent JFETs put you off.

In terms of mods, Joe said of the circuit:  For higher gain, change the 2.2M/100pF to 470k/470pF
and conversely if you increase the resistor you should get less gain if you so desire.


  1. Happy to say this is the first jfet circuit that fired up first time. Trimmers rule!! I'm going to adjust the R/C network as mentioned above to give me a bit more gain. Anyway tag this, an ok sounding overdrive so far. I'll update you as soon as I've had my tea. Yum, yum!

    1. Excellent, thanks for verifying Mark

    2. Just popped in 470k/ 470p combo and to be honest it's very disappointing. Instead of a nice meaty distortion just a weak flabby sad sound. (Probably my guitar playing!) I'stick with this but I doubt I will box it- no tone stack either. oh well, I had a lovely curry so not all is bad!

    3. My comments above are a little unfair. Dropped the 470p cap to a 330p (with 470k resistor) and it seems to lose some of the flab. I think my disappointment in this one is that I was expecting something more high gain which it isn't. Not everything has to be screaming, pinched harmonics (does it!?) With a little tweeking this could be very musical. Definitely needs some sort of tone shaping, though.

      Out of curiosity, if the source resistors were changed, either up or down, what effect would that have on the gain?

      Anyway, yes I will have that last naan!


  2. 5 years later...
    Lowering source resistors will increase drain current which increases gain. You will have to dial down the trimmers to get the correct drain voltage, but the net effect is more gain. You could also add more stages for more gain. A good place for a tone network would be between the 2nd & 3rd stages. NB: tone networks reduce the gain, sometimes substantially.

    Garlic Naan!