Author Topic: External Bias Pot for Mangler  (Read 1683 times)

Bluestrat

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External Bias Pot for Mangler
« on: September 26, 2011, 06:59:48 AM »
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!

mgwhit

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Re: External Bias Pot for Mangler
« Reply #1 on: September 26, 2011, 07:31:11 AM »
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.

Bluestrat

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Re: External Bias Pot for Mangler
« Reply #2 on: September 26, 2011, 08:27:37 AM »
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!

jkokura

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Re: External Bias Pot for Mangler
« Reply #3 on: September 26, 2011, 09:42:56 AM »
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?

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Bluestrat

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Re: External Bias Pot for Mangler
« Reply #4 on: September 26, 2011, 09:48:12 AM »
Thanks Jkokura. That's exactly the info I need.  :D

calciferspit

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Re: External Bias Pot for Mangler
« Reply #5 on: June 03, 2012, 10:56:47 PM »
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?