External bias?

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External bias?

Postby registracii » 27 Jul 2013, 17:15

Hi,
I'm building a fuzz, the schematics calls for a 2k biasing pot, however I don't have one.
My question is can I use a normal external pot instead ?
and closest thing I have is 5k actually c5k which is anti-log?

thanks
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Re: External bias?

Postby Nocentelli » 27 Jul 2013, 17:46

A 5k pot would work fine, might be a bit more fiddly to dial in the bias and some of the pot range may not be useful, that's all.

More involved options include adding a fixed resistor in parallel with the pot lugs to get 2k (might be handy if you plan to mount the bias control as an external knob and want the range to be the as the original circuit), or using the pot to find the right bias value, measuring this resistance, and replacing with a fixed resistor (or combination of resistors) of the same value. This latter option is worth doing if the circuit is such that further adjustment is not neccessary, e.g. for silicon transistors that won't drift with temperature.
mmolteratx wrote:absolutely zero commercial use allowed. If I find anyone selling these, I'll fly to your house and kick you in the nads. And you may or may not find yourself in trouble.
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Re: External bias?

Postby registracii » 27 Jul 2013, 19:01

Thanks Nocentelli,
I think I will mount the bias as external knob.
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Re: External bias?

Postby Nocentelli » 28 Jul 2013, 00:00

What fuzz are you building?
mmolteratx wrote:absolutely zero commercial use allowed. If I find anyone selling these, I'll fly to your house and kick you in the nads. And you may or may not find yourself in trouble.
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Re: External bias?

Postby registracii » 28 Jul 2013, 19:11

a germ fuzz face.
actually it turned out that the trimpot needed is 10k not 2k. not that I have one onhand too..
adding a resistor proved to be beyond my capabilities - still scratching my head over the formulas involved :D
10k I should have somewhere...
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Re: External bias?

Postby induction » 05 Aug 2013, 20:39

No need to do any math if you have a breadboard and a DMM.

1. Put a 20k* pot in place and find the settings that give you the range you want:
- adjust until it doesn't sound good anymore then back off until it does
- measure the resulting resistance at that setting (and the collector voltage, just for your own edification)
- do this for both extremes (max and min) of the bias
2. Find a resistor near in value to the smaller resistance you measured
3. Find a pot near in value to the larger resistance minus the smaller resistance (ok, just a little math)
4. Put them in series.

Then you have a bias pot customized to your own exacting specifications.

This assumes you are changing the bias for sonic variation. If your goal is to adjust for temperature variations, then you can follow a similar recipe but refrigerate the unit for one setting, and heat it with a hair dryer for the other.

Or you can just pop in the 10K and be done with it.

* Any nearby value will do: 25k, 50k, whatever you have laying around. 10k might be enough.

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