Distortion 250 Gain Issue

[pivr]

Hello, experts!
I recently built up a "Distortion 250" PCB/pedal and followed the build guide with the exception of the clipping diodes. There weren't any 1N270 diodes available so I used D9E diodes instead. After I finished the pedal and tested it out I noticed that all of the distortion is at the very end of the gain potentiometer. Is this a result of the diode change? If so, any suggestions on what I can change to have the gain potentiometer work as intended? Or if this is a known issue with the overall design, how are you all fixing it?
Thanks in advance for any help or suggestions!

 

[Aleph Null]

It could be a result of the diode change. I'm having a hard time finding specs for the diodes you mention, so I can't characterize the difference. If you are confident that all the other values are stock, you could reduce, or even jump, R5 for more clipping. I'd probably start with 1k instead of 10k, but that's a wild guess.

 

[thomasbe86]

Maybe the taper of the gain pot?

 

[BuddytheReow]

A C tapered pot will give you more control of the gain in terms of the sweep

 

[pivr]

Thanks, all, for the suggestions! When I was trying to find a replacement for the 1N270 I found the specs for the D9E and I ran it through Gemini for a "second opinion (see below).
The Vf was very similar, but I know that isn't everything to consider (leakage, current, etc.). Obviously it isn't too close since it's kind of wonky. But I thought it was interesting so I mentioned it here. Great idea on trying a different taper pot. I'll give that whirl to see what effect that has on it. I really think Aleph Null is on to something with trying to change out the 10K with a lower value resistor. I think putting a lower value resistor in parallel will be a good test and requires no rework (just hold it on and see if it changes).
Anyway, I'll have a chance to try it tomorrow and will report back. Thanks so much for the info and the great suggestions. I truly appreciate it!

 

[thomasbe86]

Thanks, all, for the suggestions! When I was trying to find a replacement for the 1N270 I found the specs for the D9E and I ran it through Gemini for a "second opinion

Try and see here to compare diodes Vf.

But D9E are totally fine, just like any schottky would by the way, do not pay attention to "harder, bright, smooth, warmer", or any other hype words Gemini is repeating from legends and myths spread over the internet about diodes, especially germanium.
The "I noticed that all of the distortion is at the very end of the gain potentiometer" has nothing to do with your diodes choice.

 

[Erik S]

Is it possible your diodes are backwards? I think some of those Russian diodes have the stripe at the wrong end.

Someone here pointed out to me that with glass germ diodes you can usually peek inside at the germanium nugget and make sure that’s toward the square pad.

 

[Coda]

Are you running into a clean amp? The Distortion+ style drives are designed to push an amp over the edge, not so much to create variable distortion out of nothing…

 

[pivr]

Is it possible your diodes are backwards? I think some of those Russian diodes have the stripe at the wrong end.

Someone here pointed out to me that with glass germ diodes you can usually peek inside at the germanium nugget and make sure that’s toward the square pad.

I believe so, but great to know there is another thing to inspect for manufacturing issues!
I took a picture of them (see below), do these seem to be installed correctly? I assumed the blue band was the cathode indicator.

Thanks for taking the time to help!

 

[pivr]

I had a few minutes to spare and ran out to the garage to test out a few items that you all suggested. I thought I'd start by hooking it up to my new (first) scope to see what's happening with the signal. It's my understanding (probably incorrect) that I should see the the top/bottom of the signal sine wave start flattening out as gain is added. And it becomes more of a square wave as more and more gain is added creating more distortion. I ran a 1kHz wave into the pedal and took a picture of the screen as I added more gain. For the most part, this first image is what I see as I add gain from lowest to almost the highest setting of the gain pot (although the sine wave does increase without clipping). The second image show the clipping kick in at around 95% of the gain pot's travel.


Clipping mode:


I have no idea if these are good/bad looking signals. But I was surprised at the first image, pre-clipping. Looks like a signal within a signal, perhaps there isn't enough filtering/decoupling?

Anyway, I thought these were interesting and wanted to share what I found so far. I'll get back out there and try some of your other suggestions this afternoon!

Thanks again, all.

 

[jwin615]

They are backwards but it wouldn't matter. They're symmetric anyway. Functionally equivalent, just reversed in position.
Those solder joints look cold. Your iron may not be hot enough. Solder should flow freely through the pad and not blob up.
Check the datasheet for your solder for recommended temps

 

[Erik S]

do these seem to be installed correctly?

Looking at the schematic it makes sense to me that it wouldn’t matter that they’re backwards like jwin615 says above. I’ve never been very good at keeping cathodes and anodes straight in my head, but the post-it on my parts bin says “nugget end = stripe end” so I always look for the little germanium ball and face it toward the square pad if I’m not sure.

 

[pivr]

They are backwards but it wouldn't matter. They're symmetric anyway. Functionally equivalent, just reversed in position.
Those solder joints look cold. Your iron may not be hot enough. Solder should flow freely through the pad and not blob up.
Check the datasheet for your solder for recommended temps

Thanks for verifying! And yes, it shouldn't matter since they're symmetric. I'm not sure why that didn't occur to me! C'mon coffee, you had one job to do!

For what it's worth I have my iron set for 370C for my 60/40 spool. I think it looks worse since I soldered them in on the other side and you're seeing what ran through. But I will certainly double check. Thanks for the note!!

 

[pivr]

Looking at the schematic it makes sense to me that it wouldn’t matter that they’re backwards like jwin615 says above. I’ve never been very good at keeping cathodes and anodes straight in my head, but the post-it on my parts bin says “nugget end = stripe end” so I always look for the little germanium ball and face it toward the square pad if I’m not sure.

Hahha, nugget end! Love it.