Sunn T preamp

[notworking]

Hi there,

I am having issues with biasing the transistors.
The trimmers reduce the voltage only from 9.3 to 9.2. It is the same for all 3 j201 that need bias.
I have checked the trimmers, they are 20k and work fine.

It is driving me crazy, any idea what might be causing this type of issue?

Thanks!

[notworking]

I managed to take pictures of the board

[aelling]

Those pictures are too bad quality to really tell anything from them, take the pictures up close and straight on rather than from an angle so we can see the components clearer, and make them higher quality.

[notworking]

I will tonight

[notworking]

Hopefully these images look better, I am not the best photographer

[rocket88]

I could be wrong, but it looks to me that your links from the trimmers don’t go to the drain of the JFETs, and instead go between lug 2 and lug 3 of the trimmer, which makes them do nothing. But, doing that the JFETs wouldn’t get any power. What I would do is go through the board again carefully and check that the links from the power and trimmers to the drain, as well as the source to ground at there. You definitely have a build error somewhere at those two points if the JFETs aren’t biasing at all.

[velouriafx]

You could increase the trims for 50k. I have a similar problem with this and now i always use 50k trims. I built it a lot of times and works perfect.
A video for reference:
https://www.youtube.com/watch?v=FSK7DbI6mrs

[aelling]

I see that the bright/normal input wires are hanging loose, try connecting them to the input switch.

What JFETs are you using? I saw some people on the main page having issues with TO-92 J201s, if you have any 2N5457, try those.

And the pictures still aren't that great, if you use a phone to take them try a different Flash settings to see if you can get them less grainy. Last, are you resizing the pictures before uploading? They seem really small.

[notworking]

thanks i will give it a try

[notworking]

the links are properly connected.
the minuscule decrease in voltage makes me think that the +V reaches D in parallel but I can't see how. If replacing the trimmers with 50k trims fixes it I guess the issues is with the trims.

The input wires are usually connected to my testing board, not floating around.
I am using J201, they are not the issue 'cause I have tested them in other builds. I will try the 5457 before swapping out the trims. But still, the real issue is that I can't decrease the bias voltage. I have built dozens of similar projects and this is the first time I encounter such issue.

Don't know what to do about the pictures, may be without flash. The website asks me to resize them, doesn't let me upload them as they are.

Thx everybody for the comments

[rocket88]

If the JFETs are good and you’re saying that the links are there, you’ve got a major build issue. I would site down with the layout and the board and go column by column to make sure you’re build is correct; nothing out of place and  all correct component value. Trust me, besides being the one that created the pedal and the layout, I’ve built a few of them and a few of the other preamps I posted, which are all similar layouts with component value differences, and each one fired up and biased no problem.

[notworking]

I have checked many times, I can't spot any issue. I will try 50k trimmers and 2n5457 and will declare defeat and rebuild on a new board.
Still annoyed that I can't find the issue

Thanks