input capacitor substitution?

[spi]

I am building an effect (bsiab2) that has a 250pf on the input.  I'm assuming this is to filter out noise coming into the circuit.  I don't have any 250pf right now, but I have a 220pf.   Is there any gotcha I should be concerned about by using this substitution, or is it good enough for rock?

[antonis]

More than good enough..

Go for it..!! 

[garcho]

I don't have any 250pf right now, but I have a 220pf

There's an engineer's rule of thumb, "the 10% rule". If a component is within 10% of the value called for, it should work. In fact, much commercial and professionally built circuitry is designed with that rule in mind. There are quite a few caveats, but those are mostly for equipment that is beyond the scope of guitar pedal world. In your case, it's really close to 10%.

I'm assuming this is to filter out noise coming into the circuit.

Right you are. Is there a series input resistor as well, and is the cap going to ground? That's often used as a low-pass filter to get rid of radio frequency interference. Usually, in those instances, the cap going to ground is in the picofarad range, whereas the input cap going to the op amp or transistor is in the nanofarad range.

[spi]

Thanks!

[antonis]

Is there a series input resistor as well, and is the cap going to ground?

There isn't any in particular pedal but there is always a source output impedance pluged in..

http://generalguitargadgets.com/pdf/ggg_bsiab2_sc.pdf

[anotherjim]

Funnily enough, the BSIAB Antonis linked has a series resistor - but on the wrong side of the 250p cap. I see no reason the cap couldn't have been on the JFET gate. Then, assuming the input has zero output impedance, you would have a RF filter that cuts over 1.5Mhz! Changing the cap to 220p would have negligable effect.

[antonis]

Quite right, Jim.. 

390R (R2) could also be placed between C1 & R1 or even before C1 with no different effect on Q1 Gate bias..

But, it's placed there so let it be there..