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October 31, 2014, 06:48:47 AM
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DIYstompboxes.com  |  DIY Stompboxes  |  Building your own stompbox  |  External Bias Adjustment Pot -- Fuzz Face 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: External Bias Adjustment Pot -- Fuzz Face  (Read 1318 times)
phillip
Posts: 1132



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External Bias Adjustment Pot -- Fuzz Face
« on: April 01, 2004, 01:18:30 PM »

I want to add an external bias adjustment pot to the collector of Q2 in my next Fuzz Face project (like Analog Mike's Sun Face).  I was thinking about using a 10KB pot for the bias adjustment.  My concern is that if the pot gets turned all the way down (0 ohms), there could possibly be damage to the transistor.

Should I add a fixed resistor in series with the pot, like in the Fulltone '69 with a 1K before the internal bias adjusting trimpot?

TIA!
Phillip
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B Tremblay
Posts: 1545

runoffgroove.com


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External Bias Adjustment Pot -- Fuzz Face
« Reply #1 on: April 01, 2004, 01:53:14 PM »

I suggest using a smaller pot, either 1k or 2k, as well as the standard bias trimpot.  Set the external pot to the midpoint of its rotation, then use the trimmer to set the voltage of Q2's collector.

By using this approach, you can deviate a bit above or below the pre-set collector voltage.

I seem to recall a Joe Gagan design with this feature.
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B Tremblay
runoffgroove.com
phillip
Posts: 1132



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External Bias Adjustment Pot -- Fuzz Face
« Reply #2 on: April 01, 2004, 02:06:30 PM »

Thanks B.  I went back through all my schematic files on my hard drive.  Joe G's Nitroburner is the one with a similar setup.  he uses a 10K trimpot and a 5KB pot to control the bias.  I'll use this setup Smiley

Phillip
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