Substituting values

[bass_econo]

What is the rule of thumb for substituting values for resistors and capacitors?   I can never seem to find the exact value when I need and I was wondering if I should go up or down in values.

Also how far should I go either way?

[Mark Hammer]

First, that's an excellent question.

Second, it depends.  Great answer, huh?

Probably your best tool in determining the acceptability of a substitute is going to be some of the common formulae like the one for calculating corner frequency in a simple RC filter ( F = 1/[2*pi*R*C] ), or the ones for calculating the gain of an inverting or noninverting op-amp.  These won't take you ALL the way, but they will get you to the city limits.

In many instances, components are "over-rated" in certain ways.  So, for instance, a particular series cap (i.e., in the signal path) is intended to block DC voltages and roll off bottom.  It may happen that the cap rolls off low end at 4hz.  In that case, since you wouldn't really notice any loss of bass end from a guitar until you started to get up past 100hz, you can probably afford to sub a value as small as 1/4 of the one indicated.  Of course, you may want to be a little conservative since such slight rolloffs way down deep can add up once a dozen or more are factored in over the course of several pedals.

(Of course, it needs to be mentioned that you can often simulate a value you don't have by placing other values in parallel or series)

In some instances, it may even be detrimental to go with the value shown, although I recognize that it may take you a while until you can easily discern where it does and doesn't matter.  But here is an instance.  You may occasionally see a circuit where the last stage is an op-amp.  Let's say it has a 10k resistor in the feedback loop, in parallel with a 47pf cap.  You could drive yourself bananas looking for 100pf caps, or you could do the math and realize that the cap provides for high-end rolloff at 33.8khz to provide immunity against oscillations, and that, quite frankly, for guitar distortion you don't really need any more bandwidth than maybe 8-10khz at best so a MUCH larger cap (maybe even as high as 220pf) is likely to be even *better* sonically.

I would not want to offer any sort of +/- tolerance  since sometimes the unit needs certain values to achieve a specific effect, and sometimes values well beyond such a tolerance are perfectly acceptable.

Having said all of that, however, in most instances, caps used to smooth power supply lines can afford to be larger in value, and only rarely smaller than shown.  Caps used to block DC can often afford to be larger, and depending on how the math turns out, smaller.

Resistors, on the other hand, because they are so much cheaper than caps (hence more likely to be plentiful in your parts bin), and because they come in so many more values, can often be combined to mimic a missing value.  Very often, what limits the acceptability of a substitute can be physical space on the board rather than actual value.

When it comes to voltage ratings on caps, you can always use a higher value, but not necessarily use a lower one.  Of course, just because company X bought a truckload of 10uf/25v caps to use on all their products, whether +9v-powered (overkill) or +15v powered (sensible), doesn't mean that you HAVE to use a 25v rated cap.  Caps come in a limited number of voltage ratings.  General rule of thumb is to use at least the next one up that is at least 1/3 higher.  So, a 10v-rated cap for a 9v circuit is playing with fire, but a 16v rated cap for the same circuit is fine.

IN many instances, effects pedals can use 1/8W resistors in a lot of places, except for maybe current-limiting resistors between the battery and the circuit.

That's not everything, but it's a start.

[bass_econo]

Thanks Mark, I can always count on your great responses.    Although they tend to raise other questions, making realize how little I think I know.  This is a good thing though cause this is learning for me because I don't have an electronics background.

I'll experiment with the values trying for near target value.

Thanks

[Mark Hammer]

Thanks, and you're welcome.  It is way too easy for beginners here to believe that they have to have absolutely every possible value to even get started, or every component exactly as listed before they can complete a circuit/project, or that the project will sound awful UNLESS the precise values are used.  Worse, given the many different dumb ways that a circuit can be prevented from working, it is too easy for beginners to mistakenly think the component values used are the source of the problem.  Finally, the vast majority of commercial pedals are a manufacturers guess at what will work acceptably for 90% of the public, and the vast majority of DIY projects are what happened to sound good on the designer's rig, and conform to the designer's tastes, both of which may be very different from yours.

Component values, much like biblical laws, are intended to be treated as conceptual ideals.  Sometimes they work exactly as written, and sometimes they need to be interpreted in context and tweaked so that the intent of the design is achieved.  Fortunately, in most instances, the design will stipulate when certain values can not be diverged from.  Like I say, though, you can find no better companion on this journey than the math.

[Pedal love]

I was wondering if I should go up or down in values.

Also how far should I go either way?

The rule of thumb I follow is normally + or - 20 %. in non critical circuits. In some what critical circuits I go + or - 10%. In pretty critical or very critical circuits, it should be pretty much as dead on as possible.pl

[MartyMart]

Thanks, and you're welcome.  It is way too easy for beginners here to believe that they have to have absolutely every possible value to even get started, or every component exactly as listed before they can complete a circuit/project, or that the project will sound awful UNLESS the precise values are used.  Worse, given the many different dumb ways that a circuit can be prevented from working, it is too easy for beginners to mistakenly think the component values used are the source of the problem.  Finally, the vast majority of commercial pedals are a manufacturers guess at what will work acceptably for 90% of the public, and the vast majority of DIY projects are what happened to sound good on the designer's rig, and conform to the designer's tastes, both of which may be very different from yours.

Component values, much like biblical laws, are intended to be treated as conceptual ideals.  Sometimes they work exactly as written, and sometimes they need to be interpreted in context and tweaked so that the intent of the design is achieved.  Fortunately, in most instances, the design will stipulate when certain values can not be diverged from.  Like I say, though, you can find no better companion on this journey than the math.

Mark, you should write a book on "Stompbox/electronics" !!
I always find your posts a joy to read, even when fairly technical, they are easy to follow and full of great information and insight

My rule of thumb is "Use whatever I have that is close enough"  and I have not had any major problems because of that.
All my "errors" have been just that  ...... "MY ERRORS"  !!
Plus an occasional "off the net schematic error" - Dont trust 'em !
If you need 2.2uf and have 1.5uf use it, if you need 10k and have 12k, use it !

Marty.