Jfet and resistor question - Eden bass preamp repair

[MAProTulz]

So, I've got an Eden Navigator preamp that got a shot of some AC through the outputs somehow. Blew out most of the JFETs, scorched a couple of traces off the board, nothing that can't be repaired. I'm going to jump over the damaged traces with some wire - point to point.

Am I correct in assuming that if the JFETs that are in the unit are marked J112A, that ANY J112 from any manufacturer will do what I need it to do? Or do I need to carefully match what's in there, for fear of destroying the tone of the unit?

Also, there's a footswitch control that kicks in a high gain circuit. 12dB of boost is far too much, though - I've found the JFET that I think they use to switch a resistor into the feedback loop of a TLO72 - which I'm assuming reduces the negative feedback which allows more gain from the opamp. I'd love to cut the boost in half, so that it only gives 6dB of extra gain. Would I just cut that resistor value in half (say if it's a 15K resistor, substitute a 7.5K or so) for half the gain?

I'm kind of a novice at this, but the Eden's a very well made piece of gear and I can find everything pretty well. I can solder pretty well and have good mechanical skills - just trying to wrap my head around the electronics part of things - thanks!

Dan Roth
Tripp Street Studios
Framingham, MA

[JonCraig]

i know different manufacturer's versions of opamps vary in tone, so i assume the same would apple to fets. while you're soldering away, it may be worth your while to just solder in a socket of some sort that would let you try different compatible fets, and see which one you like the best.

as for the feedback loop, what about (temporarily) putting a pot in there, and see how it reacts?

the usual disclaimer applies, though... if you blow it up, it's not my fault ;-)

--jon

[MAProTulz]

Hm, a pot, now that's a DAMN smart idea! I could even run it out to the back panel so he could pre-set it and have the ability to crank it up should he want that 12dB of gain at some point...hmmmmmm....

I think we're just going to install some J112s and see what we get - it's broken now, so anything's an improvement over that!

[tINY]

I wouldn't run a pot out to the front pannel that is in a feedback loop. Way to much possibility for noise/oscilations.

I'd stick with the same company's "A" bin parts. JFETs are notoriously inconsistent.



-tINY

[MAProTulz]

So, how would you do a remotely adjustable setup to replace the fixed
resistor that's in there now? I'll leave the JFet in that circuit and just
use (what) to adjust the resistance that the JFet is switching in and out- -

Preamp needs to be up and running for this weekend, so in this pinch, we're using Radio Shack FETs, leaving the pot mod till later and probably replacing the FETs again later too :P