Special Overdrive transistor question/mistake

[soothsayer86]

I built a couple Special Overdrives and both are having the same issue. The overdrive itself is extremely sputtery and fizzy sounding and not smooth at all. I have realized I got BC548B versions of the transistors and the circuit calls for BC548C transistors... ugh. I am wondering if this mistake could be causing the issues with the pedals. I have everything socketed and have tried swapping transistors and IC's and it stays the same.

Also, if this is indeed the issue, if someone could point me to the correct transistors at mouser that would be awesome because there are a few different varieties and I am still learning which parts can be swapped and which can't. Thanks in advance.

 

[Chas Grant]

Here are the rest of the pictures:

Had an emergency at work I have to babysit for, so I looked over everything on my phone. I don’t see anything obvious. All your components look correct. Have you taken any bias voltage readings? If they are good you may need to check for audio through the circuit with a probe.

 

[Chuck D. Bones]

Measure the voltage at IC1 pins 1 & 7, should be close to +4.5V. Check voltages on Q1 & Q2 emitters, should be around +3.9V. You used a blue LED for D1. The BOM does not specify LED color; the layout on Guitar FX Layouts shows red. This will only affect the tone when you turn up ACCENT.

 

[Chas Grant]

Looked aat the pics on a real monitor, everything does look correct. I think Chuck nailed it with voltage checks at those points above! Take those and we can move forward from there.

 

[soothsayer86]

OK first thing is to remove the jumper for the LED. Not sure if this will help or not, but the jumper will alter the current flow through the circuit. If not using the LED just leave it and the CLR out of the circuit.

Without a jumper the circuit doesn't function. The LED footswitch with the 3pdt breakout board keeps the CLR in the circuit so those pads need to be jumpered.

 

[soothsayer86]

Measure the voltage at IC1 pins 1 & 7, should be close to +4.5V. Check voltages on Q1 & Q2 emitters, should be around +3.9V. You used a blue LED for D1. The BOM does not specify LED color; the layout on Guitar FX Layouts shows red. This will only affect the tone when you turn up ACCENT.

Thanks Chuck, I'll check this when I get home from work. As to the blue LED, I tried various colors and the blue one just happened to be in there when I took the photos.

 

[soothsayer86]

Had an emergency at work I have to babysit for, so I looked over everything on my phone. I don’t see anything obvious. All your components look correct. Have you taken any bias voltage readings? If they are good you may need to check for audio through the circuit with a probe.

I get audio through the pedal totally fine, the issue is that it just sounds terrible ?

 

[soothsayer86]

So, reading through the comments on Guitar FX Layouts (https://tagboardeffects.blogspot.com/2014/03/shins-music-dumbloid-standard.html#comment-form), it seems like the issue I am describing is just standard for this design...sigh. So I am gonna try out some different op amps and see if I can find one that clips better.

 

[Chas Grant]

Without a jumper the circuit doesn't function. The LED footswitch with the 3pdt breakout board keeps the CLR in the circuit so those pads need to be jumpered.

My Bad! I didn't look at the 3PDT board. I use a different breakout board that has the CLR mounted on the 3PDT board and, so I don't install the LED or the CLR on the circuit PCB, just on the breakout board. I just assumed yours did also, but it doesn't. So you are correct with needing them.

 

[soothsayer86]

My Bad! I didn't look at the 3PDT board. I use a different breakout board that has the CLR mounted on the 3PDT board and, so I don't install the LED or the CLR on the circuit PCB, just on the breakout board. I just assumed yours did also, but it doesn't. So you are correct with needing them.

No worries, just wanted to make sure someone who comes along and reads this later has accurate info!

 

[FloydianPulse]

About to build this circuit and realized that I bought BC548 instead of B or C, looking at data sheet seems the unlettered one just has a wider gain potential. Since not being used as gain am I ok to proceed with the unlettered BC548?

 

[Chuck D. Bones]

Should be fine, that circuit is not sensitive to the transistors' hFE. For that matter, they don't need to be BC548. Any of the usual suspects will work.
If you're curious about what you got, you could always try measuring them.

BTW, Soothsayer, did you ever measure the voltages I asked you to measure?

 

[odourboy]

So, reading through the comments on Guitar FX Layouts (https://tagboardeffects.blogspot.com/2014/03/shins-music-dumbloid-standard.html#comment-form), it seems like the issue I am describing is just standard for this design...sigh. So I am gonna try out some different op amps and see if I can find one that clips better.

Did you ever resolve this? I built one recently and it exhibits the same characteristic. I hadn't noticed it but after reading this thread, it jumped out at me. Sadly, it's something you can't unhear once you've heard it. LOL! I love this pedal except for that weirdness.