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Okay, so I'm getting back into trying to make pedals again. Looking at my old schems and shit, and getting inspired with new designs in my head.

So, to start it all off, I wanted to make the classic Mosfet booster to get me back in the game.

I built it all, but I can't get the damn thing biased for shit.

The voltage at A never drops below 8v. I made a few subs in the circuit, as follows:

R2: 100k trimpot for biasing
R3: 1M since this just sets impedance according to Jack's article
R4: 4.7k since I didn't have 2.7k and figured I could just fix the bias with the trimpot.



Yeah, I can't get point A anywhere close to 5v. Help?

Hmm. Weird. What transistor are you using and are you 100% on the pinout. FETs vary a whole heap

BS170 and yeah I will sheepishly admit when I first put it in I had it backwards, I had looked at a different mosfet pinout before, but it's right now based on the datasheet.

I'm going to try shorting around the 1M and see if that changes anything.

Try removing the protection diode also, sometimes if you use a diode with the wrong threshold it will stop the FET from working.

Mike wrote:Try removing the protection diode also, sometimes if you use a diode with the wrong threshold it will stop the FET from working.

I was just thinking this very thing after staring at the schem for a bit longer. I heard you could sub in an LED there? Yes/No?

Just drop it out, it's only for Static Protection so for prototyping you're OK.

Red LEDs do work fine, I use 1n4148s

FUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU

Okay, both removing the diode nor shorting the 1M did anything to get the A point above 2v. I made a mistake earlier, when I said 8v I had it wrong. It goes between 1.5v and 2v at A, and that's with from 4.5v to 9.1v on the gate pin.

So. Goddamn. Frustrated.

You're trying to get 4.5V at A?

Hmm. Have you a 100K you can parallelise with your R2 100K to make it more friendly?

What's Vr looking like?

According to the article by Jack Orman on this:

"The dc voltage measured from point A on the schematic to ground should be 4.5v to 5.5v with the circuit idling and no input. The value of R2 should be tweaked until the point A voltage is correct. If two or more of these circuits are mounted in the same enclosure, only one R1/R2 network is necessary and the R3 from each stage can be connected to the Vr point."

Vr is going between 4.5v and 9v. A always hovers around 2v in these ranges.

I have some more BS170s coming in the mail. I will try swapping in a new one when I get them. I'm putting this aside for now so I don't make my head asplode from frustration.

Make sure you are looking at the datasheet for the same manufacturer of your BS170's. The pinouts vary on these.

I'll have to check into this. I'm not even sure what manu these are, I ordered them years ago. I have a new batch coming in the mail soon, though. Thanks for the heads up!