Hi, I want to use an external bias knob for the Mangler. What kind of pot and rating should I use? Linear, Audio, Reverse? Thanks!
If you intend to replace the on-board trimmer pot with an external pot, just use the same value (or as close as you can get). The schematic uses a 20K linear, which is a difficult value to find, so I'd just use a 25K linear. The BYOC ESV Fuzz uses a 25K linear trimmer, so that value should be just fine.
If you want to make a really precise bias adjustment, you could put a 10K trimmer in series with a 10K linear pot. In a controlled environment (i.e. medium room temperature for the germanium transistors), set the external pot to 12:00, and then bias with the trimmer. That way you can pretty much know that your external bias knob's 12:00 setting is ideal, and that you'll probably only have to do minor tweaks to the left or the right.
Hey, thanks for the really fast response. This is a great forum!
Let me take this a little further into a generic application. In general, when using a linear pot
to adjust bias what is the difference between using a 10k, 25k or 50k? Do higher values provide
a greater range or expand the accuracy of the biasing? Thanks!
Higher values make it harder to dial in.
Take the fuzz face as an example. If you put a bias trimmer in the power supply to Q1, it's likely going to be that you bias that transistor somewhere between 5K and 10K. Using a 10K trimmer, that means you have 50% of the trimmers rotation to use. If instead you use a 20k trimmer, you are now down to 25%, and with a 50K it's 10%. Do you see how it might be difficult to dial in exactly 8.35k on a 25k+ trimmer, while being easier with the 10K?
Jacob
Thanks Jkokura. That's exactly the info I need. :D
late hijack. what about the 20k W taper tube screamer tone pot? I think its weird path might be nice here. or are we better off with a 10k for the external bias pot. or 25k?