SOLVED Mofeta preamp - Issue with 2N5088?

[drowrgh]

Hello all,
I just assembled a Mofeta Preamp and something weird happens:
The pedal has 6 transistors, 4 2N5457 JFET, and 2 2N5088 BJT.
When I first tested it, the output was ridiculously low even with everything at max, just a mosquito distortion with the tone pots barely affecting the signal, but surely doing something. I tested the resistor values just in case I misplaced something, but everything was ok.
Then I took out the two 2N5088, and, boom, a thunderous sound. Mostly clean with a slight breakup, tube-like, which is nice since I feed it with an octave fuzz, but the eq section was definitely achieving some interesting sounds, and the output was so damn loud past noon. Which is what I was expecting, honestly.
But yeah, now there's nothing where those two transistors are supposed to be, so my questions are:
- What am I missing without those two transistors? Maybe their task is to overdrive both -normal and brite- channels?
- Why is it working? I'm just a hobbyist, I haven't traced the circuit and if I did I won't quite know what's happening there, but leaving "open points" just seem to render some part of the circuit useless. Why does it sound good then?
- Should I try something else? How can I know my 2N5088 transistors are working? I just bought 4, and all of them "misbehave". Would it be dangerous if I flip them over, just in case the pinout for some reason is reversed? Will I blow up my circuit?
Thank you for your feedback.

 

[BuddytheReow]

The build doc doesn’t have the schematic so my help will be limited. Flipping the transistors around you should be fine to experiment.

 

[PJS]

All the 2n5088 that I saw all had the same pinout. As that is the specified transistor in the build docs I doubt that flipping them will help - unless there is an error in the board silkscreen. I did notice that there are 3 bias trimmers. The build docs don't specify how they are supposed to be set (usually what I have seen is a specification for bias to be set at a particular voltage). If those biases are not correct then the transistors could be not working at all, or not well.

 

[drowrgh]

All the 2n5088 that I saw all had the same pinout. As that is the specified transistor in the build docs I doubt that flipping them will help - unless there is an error in the board silkscreen. I did notice that there are 3 bias trimmers. The build docs don't specify how they are supposed to be set (usually what I have seen is a specification for bias to be set at a particular voltage). If those biases are not correct then the transistors could be not working at all, or not well.

Those three trimmers seem to adjust the threshold of the eq section, like how many lows, mids and highs get to be amplified by the JFETs, which work fine. They allow you to kinda set the range of the three eq knobs. If you set them too "low", ugly grains sound in place of those frequencies, and if you leave them to "open", you increase their volume, but even with everything maxed out, the output was at the same volume than with the pedal disengaged.
I've seen other diy schematics of more or less the same pedal, and they use 4 JFETs too, but no 2n5088 BJT.
P.E. if you check out this one, Coda Effects Black Hole, It's more or less the same circuit, but they just have one boost and no op-amp. The 2n5088 of the Coda Effects seems to be at the end of the circuit with the collector going into the 100K trimmer which is also affected by the master volume pot, and then it goes out.
So I think the EAE Model FET approaches this with one boost per channel, which goes into the op amp with its own volume control, that's why I think the 2n5088's task is to overdrive each channel and that's why I think my pedal sounds too clean (which is OK though).
Of course these are all suppositions, why I don't get is why it's sounding when nothing is connected there.

 

[drowrgh]

Great, I think I found the error.
I have some resistors but I had to buy a few uncommon values for this project. I bought them from Banzai, usually they have great customer service but it's not the first time they get mistaken with something. Instead of 562K resistors, they sent me 562ohm resistors, and those are connected directly to the transistors -my mind seemed to be somewhere else while soldering those- so definitely there's a huge biasing error there, maybe a lot of volume goes to ground or something.
I'll get the new ones and post the results here. Sorry if I wasted some of your time.

 

[BuddytheReow]

Our time is not wasted if it helps you. In future builds you should test your components with a DMM before installing. You don't need to test every single one before installing (though no one will try to stop you. I tend to do that sometimes), but you should at least test one on each strip of resistors.

 

[music6000]

Great, I think I found the error.
I have some resistors but I had to buy a few uncommon values for this project. I bought them from Banzai, usually they have great customer service but it's not the first time they get mistaken with something. Instead of 562K resistors, they sent me 562ohm resistors, and those are connected directly to the transistors -my mind seemed to be somewhere else while soldering those- so definitely there's a huge biasing error there, maybe a lot of volume goes to ground or something.
I'll get the new ones and post the results here. Sorry if I wasted some of your time.

For Future Troubleshooting : https://forum.pedalpcb.com/threads/...s-when-an-issue-arises-with-a-pcb-build.7081/

This is why I posted this as one of the Members would have picked up on your Resistor values being wrong in the area that your 2N5088 are connected.

Cheers music6000

 

[drowrgh]

Yep, I fixed those values with a 470k ohm and a 100k ohm resistor in series and DUUUUUDE this thing truly roars!
Lesson learned, always test components.
Thank you all, good people.

 

[JamieJ]

Good to hear you have sort it @drowrgh
I’ve got into the habit of using my TC1 component tester to check every resistor before inserting onto a board. It’s a bit more of a hassle but safeguards against people sending you the wrong part.

 

[fig]

safeguards against people sending you the wrong part.

Actually, it doesn't.

 

[JamieJ]

Actually, it doesn't.

Haha oops. You know what I meant!