Clandestine Secretive Preamp

MichaelW

Well-known member
I gotta say, my first few125B => 1590B builds were easy peasy, so much to the point that I must have gotten complacent and cocky because the last couple have been kicking my butt.

Partly due to my lack of experience in thinking things through and how they will fit together, partly from just being a dumb*ss and not measuring things correctly.

I think going forward, especially for these 1590B conversions I'm going to drill in stages, see how things will fit before committing.

Case in point, I was expecting the Clandestine Preamp to be a super simple build, low parts count, not much too it, well, I screwed up the top jack holes by just a few mm and put the DC jack in the wrong place (right on the same plane as the pcb). I did managed to get it all together and make it work without starting over with a new enclosure, but you can see the battle scars on the jacks from my "persuasion" in getting them in. I had to modify one of the Lumberg jacks by removing a bit of the plastic. Ugh what a PITA this turned out to be! Oh and to top it all off when I finally DID get it all together, and went to snug up the nut for the toggle switch, the barrel came completely off! Arrrrrgggghhhh, I had to take it back apart and replace the toggle.

Anyways, I was inspired by @caiofilipini's build and decided to fit this into a 1590B and also try to match the components that Chase uses (Orange Drop caps, Carbon film resistors). I even put a Silver Mica cap in for the ceramic.

I wasn't really sure what to expect when I fired it up and had to go to the Chase website to see how it all works. It's much more subtle than my Xotic EP Booster but it's also much more transparent. After playing with it for about 30 minutes pushing my Son of Ben pedal into a clean amp I finally started to get it. It's not so much a "boost" as a "tone conditioner". Really starting to dig some of the sounds with the knob up past 3 o'clock that's supposed to simulate a worn, vintage Echoplex.

Not too sure about the chicken head knob, I might replace it with something else. (You may have guessed I'm pretty wishy washy when it comes to knobs hahah)

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One question for you guys that have built this pedal. What did you wind up biasing the transistor at?
I biased the JFET by ear but my voltages are way off when I measured them. I'm getting 23.6v Vref at R101 but I can't get anywhere near 12-13v at the drain or the bias pad. The lowest I can get it is around 17.8v with the trimmer completely counter-clockwise. At that point it's clipping. Where I have it sounding good measures around 20v. Plenty of clean headroom and volume and the sound if nice and full and fat.

Not sure what I'm doing wrong?
 
TIS58 has a different pinout (SGD) vs DSG of the 5457.

I suppose you could get it in there with some pin yoga if you insist. 🤣
Thanks for replying. You just saved me from potentially frying it.

I thought it was DGS from the dimensions pic on the data sheet but I guess that’s just general T092 info. There is clearly a pin out icon right next to it so silly me.

So it’ worse then I thought. Great !! Haha.

Thanks again much appreciated 😀
 
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