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Fender Tone Stack With JFETs?

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Precisionb87

Dec 15, 2010
212
85
Philadelphia, PA
#1
Hey Everyone.
How would I achieve the same amount of gain from a discretely built JFET circuit, as I would from a single stage from a 12AX7 (wired in a way you would see in a fender amp)

For many years I have been using an Alembic preamp clone that I built. It sounds fantastic, but I was thinking of making a JFET version.

A simple schematic would be very helpful. Thanks!

P.S. - What type of JFET would be the best choice?
 
#2
Precisionb87 said:
Hey Everyone.
How would I achieve the same amount of gain from a discretely built JFET circuit, as I would from a single stage from a 12AX7 (wired in a way you would see in a fender amp)
Click to expand...

Generally you're going to wire a JFET as a common source amplifier.

[Invalid or Expired Link Removed]

is a good bit on analysing a JFET gain stage.

[Invalid or Expired Link Removed]

is a good bit on analysing common cathode triode gain stages.

Use the second to determine gain in the circuit of interest, use the first to construct a JFET circuit with similar gain.
 
#4
@Lineman: I have actually applied this exact circuit to what I am trying to do, but it didn't seem like enough gain. Plus I would like to run the circuit on something more than 9V for more head room. I'm not that savvy and I'm not sure how to reorient to do that.
Needless to say, I am having trouble digesting the first post...:help:
Has anyone ever done what I am trying to accomplish here? I'm not trying to re-invent the wheel here, I would be fine with just copying somebody else...
 
#5
Precisionb87 said:
@Lineman: I have actually applied this exact circuit to what I am trying to do, but it didn't seem like enough gain. Plus I would like to run the circuit on something more than 9V for more head room. I'm not that savvy and I'm not sure how to reorient to do that.
Needless to say, I am having trouble digesting the first post...:help:
Has anyone ever done what I am trying to accomplish here? I'm not trying to re-invent the wheel here, I would be fine with just copying somebody else...
Click to expand...

Try adding a capacitor for more gain. I think it's explained in the link.

IME, testing that circuit with a function generator and scope, it did seem to have quite a bit of headroom before a sine wave distorted.
 
#6
line6man said:
Try adding a capacitor for more gain. I think it's explained in the link.

IME, testing that circuit with a function generator and scope, it did seem to have quite a bit of headroom before a sine wave distorted.
Click to expand...

Yes, but I would not be using this circuit for just a "boost". It would have to be able to drive a tone stack, and then another would have to be able to amplify it further.

What is a average ballpark peak to peak voltage of the output of a fender preamp? (after tone stack and second stage of gain)
 
#9
Precisionb87 said:
Has anyone ever done what I am trying to accomplish here? I'm not trying to re-invent the wheel here, I would be fine with just copying somebody else...
Click to expand...

I've done it several times, but using BiFET opamps rather than discrete FETS. Here's the last DIY build I detailed on TB several years ago. It uses impedance scaling of the tone stack to suit the opamps, but it definitely sounds Fender/Alembic-ish. ;)
 

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