Capacitor measurement - Jordan Bosstone

[SprinkleSpraycan]

I'm using this schematic to build a one knob bosstone. I want to remove the weight knob and place in the right size cap to replace the setting I like. The pot measurements are: 1 and 2= 69k, 2 and 3 = 35k. I know I could tie some resisters through but it would be cleaner to put one cap in at c6. Problem is I have no idea how to measure it or calculate it.

[Dormammu]

1-2 resistance doesn't matter since it is shortened.
So we have 32k in the series with 2.2mkf.
I would recommend a resistor of 5-10k to prevent oscillations instead of c6,c5 and R weight; and a cap of 1-1.5mkf for soldering at R attak 3-2 pin.  This preserves the ability to regulate bass.
R attak can be lowered to 150-250k.

[PRR]

The 2-cap form gives a different response shape (tone) than a 1-cap approximation.

I know it is another dollar but it may be worth it?

[SprinkleSpraycan]

The 2-cap form gives a different response shape (tone) than a 1-cap approximation.

I know it is another dollar but it may be worth it?

That's what I was wondering. Money is no object at single capacitor price points

[Rob Strand]

That's what I was wondering. Money is no object at single capacitor price points

I think you have the wrong impression about the adjustable cap.   You are going from 10pF (absolutely tiny) to 2.2uF (quite large).   If you went say over a factor of 220nF to 2.2uF then even then it wouldn't really act like a cap between the two limits.   The adjustable cap is a crude approximation at best.

In your case your are really just adding series resistance to a 2.2uF.   That has quite a different effect than a smaller cap.   It's not that it's wrong or sounds bad it's just not the same as a smaller cap - ie. you are interpreting what is happening incorrectly.  Series resistances on the input of transistor amps are very good at cleaning up the tone, especially when the transistor stage has no emitter resistor.

Regardless of size you need a series cap so the guitar doesn't screw-up the biasing of the transistor.

Out of the series resistor and a smaller cap.  You just pick what sounds best.  Often both is better.

[Fancy Lime]

True, two caps and a resistor are different from just a single cap. The former gives a kind of shelf filter response, the latter a simpler "one corner" response. It may still be possible to reasonably approximate your particular favorite sound of the two cap plus resistor arrangement with a single cap. However, the caps are actually part of a more complex interaction with the Attack pot and the guitar pickup resistance and inductance (if you plug straight into the Bosstone, which to me sounds best on this circuit precisely due to the interaction with the guitar).

TL;DR: you can use a single cap but it's fairly impossible to predict, which value sounds best, given the additional unknown factors. You'll have to experiment. It is almost definitely going to be something in between the values of C6 and C5.

HTH,
Andy

[Dormammu]

The 2-cap form gives a different response shape (tone) than a 1-cap approximation.

I know it is another dollar but it may be worth it?

C6 so tiny - that we can ignore him.
Cable capacity may exceed this value by 10-100 times, in the opposite direction and still sound good.

[SprinkleSpraycan]

That's what I was wondering. Money is no object at single capacitor price points

I think you have the wrong impression about the adjustable cap.   You are going from 10pF (absolutely tiny) to 2.2uF (quite large).   If you went say over a factor of 220nF to 2.2uF then even then it wouldn't really act like a cap between the two limits.   The adjustable cap is a crude approximation at best.

In your case your are really just adding series resistance to a 2.2uF.   That has quite a different effect than a smaller cap.   It's not that it's wrong or sounds bad it's just not the same as a smaller cap - ie. you are interpreting what is happening incorrectly.  Series resistances on the input of transistor amps are very good at cleaning up the tone, especially when the transistor stage has no emitter resistor.

Regardless of size you need a series cap so the guitar doesn't screw-up the biasing of the transistor.

Out of the series resistor and a smaller cap.  You just pick what sounds best.  Often both is better.

Great. That helps my understanding a lot. Thanks

[Fancy Lime]

The 2-cap form gives a different response shape (tone) than a 1-cap approximation.

I know it is another dollar but it may be worth it?

C6 so tiny - that we can ignore him.
Cable capacity may exceed this value by 10-100 times, in the opposite direction and still sound good.

Surely that's a typo in the schematic, no? I didn't even realize it was 10pF on the schematic because my eyes autocorrected it to 10nF. Still small-ish but somewhat sensible. Indeed with a 10pF for C6, the Weight control just acts as a series resistor before the input for audio frequencies and makes a second redundant gain control. If that is what you want, a fixed signal attention by adding series resistance, which works great in many fuzz designs, then just take out the 10pF cap completely and replace the Weight pot with a 33k resistor.

Andy