Alternative Value Difficulties

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Alternative Value Difficulties

Postby trad3mark » 26 Jul 2009, 13:53

Hey all,
Just a quick question on using alternative values for things. We've all had those builds where we need to use substitute values. I've done ones where the substitutes have been really close. Things like 12k resistors replaced with 15k resistors, or 20nF caps replaced with 22nF caps. As i'm sure a lot of you will agree, a lot of the time, if the values are close enough, it won't really make much of a difference a lot of the time.

My question is, what if almost the entire board were substitutes? Obviously, the sound will be noticibly different. This may be tone, volume, fuzz amount etc etc, the list is endless. But, will it always still work provided the values are pretty close?

I'm just about to debug a Green Russian BMP build, but there's a lot of alternative values. Most are REALLY close, as described above. But there's a few i'm not sure about. The furthest one out in terms of resistors is a 470k replaced with a 600k, or a 2.2m replaced with a 3m. It should work really, right?

I'm so confused.... :scratch:
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Re: Alternative Value Difficulties

Postby willc1968 » 06 Aug 2009, 12:25

when i want or need to have resistor values dead nuts on i just get out the multimeter and try to find 2 resistors or sometimes 3 that will work in series. finding a parallel equivilent is also an option and is sometimes easier and neater because using 2 components where only 1 is allowed for can get ugly at times. hope this helps. will
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Re: Alternative Value Difficulties

Postby earthtonesaudio » 06 Aug 2009, 12:50

There are places where it matters more, like transistor biasing. If you don't have the proper values, it may be beneficial to scale the other values to match what you do have. For example if a transistor's base is biased by 10k and 100k resistors, you could substitute 47k and 470k, and end up reasonably close to the original bias point. Of course then the frequency response is changed, but cap tolerance is generally much worse than resistor tolerance so that part is a bit of a roll of the dice anyway.
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.
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Re: Alternative Value Difficulties

Postby JiM » 06 Aug 2009, 23:05

In DIY fashion you can take some time to test parts for actual value within tolerance.
For example, a 10k 10% resistor will more efficiently replace a 12k if it's close to the max value (i.e. 11k) rather than the min.

[Slightly off topic : i remind an experienced engineer saying that back in the day, a 10% resistor was always more than 5% off the specified value, as these were factory-sorted ... nowdays i'll expect a nice gaussian curve.]

Anyway, for a Big Muff it doesn't matter much, as there were quite a lot of variation in the official schematic along the years ...
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