FET Abuse!

mictester · Post by **mictester** » 24 Oct 2009, 21:39

A quick, simple, cheap, smooth-sounding Valve (Tube - U.S.) emulator. There are various tweaks you can do - change the 150pF to vary the amount of treble fizziness, mess with the bias values to change the shape of the resultant waveforms, make to 5k preset a variable so you can radically alter the sound, add a simple tone control if you want... The possibilities are endless! The distortion really cleans up well with the guitar volume control.

Enjoy!

mictester · Post by **mictester** » 26 Oct 2009, 06:22

biffa wrote:That's the 2n3819 tube OD isn't it?
What's it sound like?
I've got shed loads of 2n3819's!

More or less - I found it as a pencil drawing in one of my notebooks from 1971. The values have been tweaked a bit. It sounds very good for such a simple circuit. It's also very open to lots of modifications - try changing the 150p cap for as low as 47p or as high as 680p - the tonal changes are remarkable. Also experiment with the biasing of the input FET. It sounds pretty damn smooth as drawn.

RnFR · Post by **RnFR** » 26 Oct 2009, 18:36

looks similar to the stages in the new catalinbread pedals.

mictester · Post by **mictester** » 26 Oct 2009, 19:27

RnFR wrote:looks similar to the stages in the new catalinbread pedals.

That just proves the old saying that "there's nothing new under the sun"! As far as I remember, the circuit came out of a discussion with my University Tutor about the similarities in the transfer functions of FETs and Valves ("Tubes" - US). Some years later, I found something similar in an "Elektor" article. I also have a "cascode" FET circuit that's not too far removed from the FET u-amp circuits on the AMZ website - again it's from my early '70s notebooks. That circuit was for a low consumption instrumentation amplifier!

RnFR · Post by **RnFR** » 26 Oct 2009, 19:53

cool! well maybe you could answer this question then-

does the source follower at the end have any other function than to deal with impedance issues? could it be used to add distortion and compression to the circuit without adding volume?

here's a cascode circuit that i made up that i think might be interesting in.

viewtopic.php?f=13&t=7182&hilit=assload

mictester · Post by **mictester** » 27 Oct 2009, 05:35

RnFR wrote:cool! well maybe you could answer this question then-

does the source follower at the end have any other function than to deal with impedance issues? could it be used to add distortion and compression to the circuit without adding volume?

here's a cascode circuit that i made up that i think might be interesting in.

viewtopic.php?f=13&t=7182&hilit=assload

The source follower was there just to drive longer cables by lowering the output impedance. I found that there was less treble loss down the guitar lead (the original was built into the body of a tatty telecaster). The original also used "phantom" power down the guitar lead, so no battery was needed in the guitar. I have two ways of doing the phantom power - the obvious one is to use a stereo guitar lead, with one core for power and one for audio, but it's also possible to do it down a conventional cable - if anyone's interested, I'll post here.