Capacitor switching in and out

[Ibenhad]

So how do you get the 3 common values of .001, .0022 and .0047 for a capacitor change with a DPDT switch in a circuit? I would have to use .001 as the primary and add the others in parallel and out to that value in order to get the other 2 ( .0022 and .0047 )? I cannot seem to find the other values I would need to make this happen. If I use .001uf/1000pf to start I would need 1100pf and 3700pf to get my needed values? Am I missing something here? Maybe I'm just guitarded...

[R.G.]

Capacitors in parallel add. So to get 0.0022 and 0.0047 by adding a cap to a 0.001, you need a 0.0012uF and a 0.0037uF; you're right. 0.0012 is a standard value, 0.0037 is not; you can get 0.0036 and 0.0039.

However, nothing says that you can't use permanently paralleled caps to make up a 0.0039, or a 0.0012 for that matter. A 0.0012 is a 1000pF and a 200pF in parallel. A 0.0039 is a 0.0033uF and a 400pF in parallel. Ooops - 400pF isn't a standard value either. But 330pF and 33pF are, and so a 3300, a 330, and a 33pF in parallel give you 0.003663uF, which is remarkably close to 3.7nF.

The ugly issue of "how close is close enough" then rears its ugly head. If you had a capacitor with "3900pF" printed on it, what would its capacitance really be? They are not perfect. In fact, you pay extra for anything guaranteed to be closer than 10%. 5% isn't too bad, but 2% and 1% get pricey.

A cap marked "1000pF" may be as bad as 900pF or 1100pF. A "3700pF" if you could get it might be as bad as 3330pF (-10%) or 4070pf (+10%).

The moral to this rambling story is that the practical thing to do is to "switch" in standard value caps and see if you can hear a lot of difference. Only go to high precision trouble if you can really hear it, and not because some schematic on the internet told you to. There's an old saying - "lies, d@mned lies, and the internet."

[frus]

you can do something like this:



you only need to use SPDT on/off/on switch. when it's in center ("off") position, neither of the capacitors above are connected, so you get 1nF. if the switch is "down", you get 1+1 = 2nF (which is close enough to your 2.2nF value), and in "upper" position, you get 1+3.3 = 4.3nF which is close to 4.7

[Enzo]

WHat RG said. These are guitar amps, not NASA space probes, so how close do you need to be? If this is an experiment, note that each step is roughly a factor of 2x - and if the original intent was for .001, .002, .004, then maybe those .0022 and .0047 were simply the closest approximations of those they could find. In other words we may be trying to duplicate what might have been fudged numbers in the first place.

.001 and .001 in parallel make .002. Isn't that really close enough to .0022? And if you have .0012, well then use it to make exactly .0022. Now you have .001 and .0022. And then switching to a .0033 for .0043 ought to be close enough for .0047.

In the old days, when parts were more likely to have 20% (or even higher)tolerances, they established a set of standard values. The numbers were picked so their 20% tolerance bands didn;t overlap. In even older stuff from before those days, "odd" values like 250k resistors were common. And in caps, 25uf caps stuck around longer, but mostly they will be 22uf caps today. Meanwhile, they then started making 10% parts, so they added more standard values midway between the old 20% numbers. And then the same procedure for 5% parts. Nowdays 1% resistors are cheap and available, so you can get just silly values if you want them.

When experimanting with changing parts for tone, such as cathode bypass caps or something, the rule of thumb I learned was start with at least a factor of 2, and get more precise if it actually made a difference. Meaning that the difference between a 22uf and a 25uf cap would be almost nothing, but the difference between a 25uf and a 50uf might be more noticable.

[Chuck H]

Just to attend to the tedius... There is a series option that hits real close to the mark and gets the circuit back down to three capacitors.

[Chuck H]

It looks like Ibenhad dropped this post, wiped, pulled up his pants and left the room. I'd say Webenhad. Because I know no one would take the good and knowledgable information freely offered and not say thank you. You'd need to be a real pud to do something like that.