Substitute JFET for the input and output buffers in Tube Screamer...

[giantsteps]

The board in first modding project bit the bullet (it actually cracked in half...it is so old and I was so bad at soldering back in the day that half the traces are gone...) and I wanted to duplicate it on a new tube screamer board. Here's what it was:

A stripped down tube screamer on a Maxon board (without input and output buffers) with the second half of the op-amp set up like a booster amplifier into a modded big muff tone control.

I want to include input and output buffers on the new project...my question is...can I use J201's instead of the standard 2n3904 type? What would I need to change resistor wise? I am intrigued by the "FET sound" and was hoping these would clip more naturally than the silicon transistors if pushed.

Thanks,

Chris

[Transmogrifox]

Fortunately for the way the input and output buffers are done on a standard TS circuit, you will not likely need any resistor substitutions. 

Unfortunately I don't think mods to the input and output buffers will make an audible difference.  This part of the circuit generally has less than a fraction of a percent of distortion for the signal levels encountered in a TS. 

If you want to add some "FET" sound to your TS, then you will have to work them into the clipping stage.  Doing this essentially changes the pedal to something else entirely (in other words it would be hard to call it a TS any more than BOSS BD-1 could be called a TS).

By the way, I have a circuit on my breadboard which is designed to be a JFET clipper.  If you ever saw the schematic for "differential distortion", then imagine that circuit made with JFETs.  It has a pleasing sound.  Perhaps one of these days I'll post a schematic.

[brett]

Hi
RE: Unfortunately I don't think mods to the input and output buffers will make an audible difference.  This part of the circuit generally has less than a fraction of a percent of distortion for the signal levels encountered in a TS. 

+1 for that.

Mr Transmogrifox: A JFET differential distortion would be interesting.  It would solve the common problem of low input impedance in differential pairs. Any chance of posting it?

[Transmogrifox]

http://www.geocities.com/transmogrifox/DiffDist.pdf

This is a schematic of the thing I have on my breadboard minus the tone stack.  I still haven't decided what to do in the post-equalization stage.

I am using some FNJ598J mic preamp JFETs (really low power, low Vgsoff).  If you use a different kind, you'll need to twiddle resistor values to get the thing to bias correctly.

I call it a "Scorched Meteor".  Perhaps one of these days I'll get a sample recorded.  It's pretty dynamic.

[brett]

Thanks!

[giantsteps]

Thanks for the help!

I'm going to put a switch in for the clipping diodes...

1. green led/1n914
2. JFET/JFET + Germanium diode