fetzer or till preamp with 2n5458?

[jogina111]

I got a 2n5458 FET from an old box and I've decided to use it for a preamp.. My questions: 1. what would be a better preamp to build?don tillman's discrete preamp or the fetzer valve?
2. What tweaks should I make on either circuits should I do to make them compatible for a 2n5458?
3. I cant find a 100k trimpot for the fetzer(Im gathering materials for both circuits) and 50k is the highest I found. Maybe a fixed value resistor? But what value?

Please help me.

[Davelectro]

I wouldn't use a trimpot -it's better to actually measure the fet- but 50K it's ok for a 2n5458. At least in a fetzer valve.

[Earthscum]

http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm

read through that concerning Tillman amps using a different fet. The process he uses can be applied to any fet. I much prefer the sound of MPF102's and 2N545(x) to J201's in either Tillman or Fetzer.

[jogina111]

how do I know if the trimpot is in the correct setting? Should it sound clean or overdriven?

[Jdansti]

The Tillman preamp bias voltage at the drain should be about 6V and it should sound clean unless you're hitting it with some really hot humbuckers. Tillman used a J201 and 2N5457.  His drain resistor is 6.8k.  I'm not sure how the 2N5458 would react, but if it were me and that's all I had, I'd start off around 6V at the drain and tweak it until I got a nice clean boost.

Here's Don Tillman's explanation of his preamp:
http://www.till.com/articles/GuitarPreamp/


Copyright 1992, 2001, J. Donald Tillman. 
All rights reserved.  email: don@till.com
web page:  http://www.till.com


BTW- I put some J201s in the shipment I'm sending you.