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Post by night0wl on Feb 13, 2006 13:39:54 GMT -5
John, the trimpot is still scratchy after I set it and leave it. It starts off making a small amount of noise then gets worse. Maybe try another trimpot?
Shane
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Post by JohnH on Feb 13, 2006 15:23:13 GMT -5
Yes, it sounds dodgy. You can also use a pot to test the right bias setting, measure its resistance and put in the nearest fixed resistor. Thats what I do, although best the value varies for each individual JFET, since they are not very consistent devices.
John
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Post by UnklMickey on Feb 13, 2006 16:33:32 GMT -5
John, that sounds like a good plan.
some of those trimpots can't take much heat when they are initially soldered in, and become very sensitive to vibration. especially the ones that look like little plastic cubes.
i'll wonder, in hindsight, it would make sense to wire in a full sized pot with a knob for set-up? then replace it with a fixed resistor.
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Post by night0wl on Feb 13, 2006 18:34:32 GMT -5
Cool Thanks guys.
I replaced the trimpot with a 10k linear pot and got the boost just right (sounds great to!). I measured the resistance (hopefully I did it right) and it measured 2.2k. Unfortunately I don't have a 2.2k here and the nearest electronics store is about 100km away so I won't be getting it any time soon. Which brings me to this question.... Can someone please make a list of popular resistor and capacitor values that are used commonly in distortion/overdrive effects and guitar mods? If I can get a list of them I can stock up next time I get the chance. Also, Is there a better quality trimpot I can buy or is it better to do John's suggestion with the fixed resistor?
Thanks, Shane
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Post by JohnH on Feb 13, 2006 19:00:40 GMT -5
I keep a stock of everything from 100 ohms up to 10M, and D!ck Smith sells bulk packs with 10 of each value from say 1k to 8.2k, or 10k to 82k, 100k to 820k etc, for $2 per pack
You could also make that Fetzer work by increasing the source resistor, and increasing the drain resistor in proportion, it is the ratio between them that is most important, so if they currently are 1.5k and 2.2k, then probably 4.7k and 6.8k, or 10k and 15k would also work fine. As these values increase, current consumption goes down. Output impedenace goes up, which ultimatly can be detrimental, but within this range, it is probably not going to make any difference.
John
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Post by night0wl on Feb 13, 2006 19:05:45 GMT -5
Thanks John I'll try that.
Cheers, Shane
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Post by UnklMickey on Feb 13, 2006 19:53:24 GMT -5
Shane,
i'd like to offer another little bit of advice.
consider mounting a few full sized pots on an angled bracket and attach knobs to each one. maybe 1k, 10k, 100k, 1meg.
mechanically attach the bracket to a block of wood, or to your breadboard. use light-gauge solid wires (small enough to plug into your breadboard) soldered to each of the pots.
when you are prototyping a circuit that requires trimming, plug in the appropriate value pot. when you get the right value for that circuit and that particular device. use the needed value fixed resistor in the circuit, when you transfer it to a permanent home.
if you do much experimenting, this will be much easier to adjust than a trim-pot. and worth the initial effort of drilling the bracket and the cost of the pots.
if you don't do so much, plugging a trimpot into the breadboard will make more sense.
as i mentioned before, using a resistor in series with any pot in any experimental set-up, to limit it's minimum resistance, is a prudent safeguard.
and as always, i defer to John's judgement in these matters. he has been involved in more prototyping these days than, i have been.
unk
P.S. i see this thread has become increasingly long (5 pages already). don't be surprised if some moderator comes along and moves it. and perhaps renames it "Shane's Corner" .:lol:.
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Post by night0wl on Feb 14, 2006 0:12:51 GMT -5
Thanks unk, great idea! I will definately do that.
I found a used 2k2 resistor I salvaged from an old powered mixer a few weeks ago and it worked a treat! The % thing is finally going! lol
Thanks for all the time and help unk and John, I really appreciate it.
Can I now officially close "Shane's Corner" ?? lol
Cheers, Shane
P.S. I just realised I can't say dãmn in here!
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Post by JohnH on Feb 14, 2006 1:28:56 GMT -5
Great news - well done! John
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