Wrong capacitor in 360-12

[rob_u]

I was reading a post a while back about the shunt in the bridge pickup of current 360-12 (and others), so I decided to try putting the cap back into my 360-12 (2003 high gains). It sounds good (just a little volume drop). The thing is, I was just reviewing some posts (and the Ric schematics), and I realized that I put in a .047mf instead of a .0047mf cap. Being no electronics whiz (obviously) I don't know if I'm getting more, or less bass cut with this value. Can anyone clear this up? Maybe my ears are bad, but it sounds good this way (with my MXR Super Comp).

Thanks

[jps]

You're getting more bass and well into the low mids cut with that cap.

[beatlefreak]

Jeff, you'd actually get less bass cut with a larger in-line (series) capacitor - In this case, by a factor of 10 (decimal point moves left one place).

[rob_u]

Thanks for your replies,

there is a noticable "thinning out" taking place, but if I put in the .0047mf it would cut by a factor of 10? It certainly seems logical (the value changes by a factor of ten), but I can't imagine it having ten times less audible bass. I know that when I modded my Boss Blues driver to a higher tone capacitor (more treble cut), I ended up going past Keely's value (I don't remember off hand) to a value ten times the original. But it had far less audible effect than a factor of ten (still less than a Strat tone pot). That was within a pedal circuit. So, does the in-line capacitor on a hot lead have less audible effect than a grounded tone-pot cap? Don't get me wrong, I'm not doubting your advice (in fact, I value it highly), I'm just trying to get the whole picture.

By the way, what type of capacitor does RIC use? I ended up using a poly film because that's all I could find. The RIC's almost look electrolytic, but that can't be right.

Thanks

[jps]

Jeff, you'd actually get less bass cut with a larger in-line (series) capacitor - In this case, by a factor of 10 (decimal point moves left one place).

Oops! That is what I mean't to say.

[beatlefreak]

there is a noticable "thinning out" taking place, but if I put in the .0047mf it would cut by a factor of 10? It certainly seems logical (the value changes by a factor of ten), but I can't imagine it having ten times less audible bass.

Audio doesn't work like that. You're dealing with logarithmic functions to calculate a decibel (dB) level, which then translates into sound pressure level. That's the difference that we can hear. The 10 times factor I was referring to is (of course) the capacitance value, and also the cutoff frequency. Read on...

The in-line (series) capacitor, along with the reisistance from the pickup coil (plus whatever resistance is set by the volume control) forms a first order high pass filter, which blocks low frequencies and passes high frequencies. The resistance and capacitance values set a cutoff frequency (fco). Frequencies below fco are attenuated at a rate of -6 dB/octave. An octave in audio terms is defined as either a doubling or halving of any frequency.

If you have an fco of 2000 Hz (2k), at 1k Hz the sound is 6 dB less, at 500 Hz the sound is 12 dB less, and so on. A point of reference - A difference of 3dB is about the minimal amount of difference the human ear can hear. The actual formula for determining the cutoff frequency is: fco = 1/(2π*R*C). Given your 360 with hi-gains (assuming a 12k Ω coil and volume up all the way, thus no resistance), this yields an fco of about 2.8k Hz for the 0.0047 µf cap, and about 280 Hz with the 0.047 µf cap.

The tone caps (0.047 µf) work differently. As you stated, they are not in-line, but run to ground (through the tone pots). This forms a first order low pass filter, and high frequencies are rolled off above the cutoff frequency. The tone control, in series with the tone cap, allows a shifting of the fco within a range of frequencies.

A polyester, polypropylene, or metal film cap will work fine in the circuit. You could use an electrolytic, provided you could find a value small enough. Electrolytics are usually larger value caps. The cap value in the Rickenbacker circuit is only 0.0047 µf (4700 pf). That's a fairly small capacitance value.

By now, you might be or , but it's really not that hard to understand. Let me know if you have any other questions.

[johnallg]

Jeff, you and Kris both said the same thing, just from different viewpoints - what is taken away as compared to what is passed. You both were right.

Rob, the capacitor forms a filter with the pickup coil and the volume pot. It is a mathematical relationship in that changing the cap value by a factor of 10 changed the rolloff frequency by the same factor, a straight (linear) relationship. But sound is logarithmic so the amount of bass rolloff you hear won't sound like a factor of 10.

To be RIC correct, use the .0047uF cap. RIC uses poly also.

HA! I see Kris types faster and was more prolific in his response!

[beatlefreak]

The bottom line is, the larger the cap value, the lower the cutoff frequency (and therefore, less bass frequencies are attenuated).

[rob_u]

Wow,
Thanks Kris (and John and Jeff),

Although my eyes glaze over when I see equations, I understand the English parts of your explanation. I had an idea it was something like that, but I needed verification. It always helps to know the why, so you can start putting the pieces together.
I picked up a Mylar .0047mfd cap today while I was out, and I'm going to give it a try.

Thanks for all of your help,

Rob