Author Topic: FET's? A Tube Screamer?  (Read 81712 times)

kylecavanaugh

FET's? A Tube Screamer?
« on: February 07, 2007, 04:25:25 PM »
Hi, I'm new here.  Been modding pedals and such for 2 or 3 years.

I was wondering if anyone had tried making the discrete op-amp out of J-FET's or MOSFET's?  If so, how would they compare to the BJT version?

Also, has anyone tried this in their Tube Screamer or similar, I'm curious as to how it affects the tone.

Northwave

Re: FET's? A Tube Screamer?
« Reply #1 on: February 20, 2007, 08:12:27 PM »
Hi! I'm kinda new here too  ;) Not a lot of post at the least.

Seeing as the OPamp is (close too) a perfect gain/block, swapping them should be of no major consequence. Of course there will be some difference, whether its clearly audible or not would depend on the OPs swapped and the application. I`d try to swap the stock ones for other types of monolithic OPs first, before venturing into constructing your own. There`s both JFET and CMOS low noise/audio OPs commercially available. TL7x/6x/8x are popular JFET types, but cant think of any CMOS right now.

johngreene

Re: FET's? A Tube Screamer?
« Reply #2 on: March 16, 2007, 01:22:38 PM »
Hi, I'm new here.  Been modding pedals and such for 2 or 3 years.

I was wondering if anyone had tried making the discrete op-amp out of J-FET's or MOSFET's?  If so, how would they compare to the BJT version?

Also, has anyone tried this in their Tube Screamer or similar, I'm curious as to how it affects the tone.
Look at a Boss Blues Driver schematic.

--john
I started out with nothing... I still have most of it.

blackcorvo

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Re: FET's? A Tube Screamer?
« Reply #3 on: May 10, 2010, 03:13:17 AM »
Hi, I'm new here.  Been modding pedals and such for 2 or 3 years.

I was wondering if anyone had tried making the discrete op-amp out of J-FET's or MOSFET's?  If so, how would they compare to the BJT version?

Also, has anyone tried this in their Tube Screamer or similar, I'm curious as to how it affects the tone.

i have tried a 2N7000 in place of Q3 (the output transistor) for a simple Dist+ like distortion, and it worked just fine.
All you need is to change the 6k8 resistor with a 25k trimpot and adjust it until you get 1/2 of the V+ in Q3's collector.

Transmogrifox

Re: FET's? A Tube Screamer?
« Reply #4 on: October 17, 2010, 05:35:12 PM »
The way to get interesting clipping out of JFETs & MOSFETs is to make the differential amplifier found in the first basic op amp gain block and omit the feedback.  You can drive that with a common source/emitter type transistor amplifier for more gain.  I had a schematic for this on my geocities page, but geocities shut down.  I have not yet posted my stuff back up yet :(

Either way, Northwave is right.  The discrete op amp using FETs does not make  a drastically different sound.  However.... reduce the feedback and/or internal gain of the op amp and its colors will start to show. 

trans·mog·ri·fy
tr.v. trans·mog·ri·fied, trans·mog·ri·fy·ing, trans·mog·ri·fies To change into a different shape or form, especially one that is fantastic or bizarre.

DixonVode

FETs A Tube Screamer
« Reply #5 on: September 08, 2016, 01:24:24 PM »
Ive been looking around at tubes. I was just wondering if anyone has the OBrien Super Screamer tube? If you do what are the pros/cons of this tube?