A quick pre-summary: basically the 1 thing that I really need to know is what capacitor value I should use.

I have 6505+ and want to cut out some high frequency fizz. I tried an eq pedal in the effects loop for the fun of it and turned down the 6.8k frequency (the highest frequency on my pedal)... that killed the fizz completely but it dulled the tone to much. The fizz is at a much higher frequency then 6.8k, so I should have good results if I can cut out higher frequencies. The fizz seems to be around 10k, so I want to built a simple low pass filter with a relatively steep cutoff at 10k (or around 10k, I'll need to expiriment.) I know how to make a low pass filter such as the ones in guitars, but I need to know a few things:

Can I build a low pass filter using the same circuitry as somebody would with the low pass filters in guitars?

How can I calculate what frequencies will be effected by what value of capitor?

Will the frequencies effected by a specific capacitor value change depending on the impedence of the signal that this low pass filter is effecting?

If this goes well, I'll eventually install this directly inside the amp. I'll have a bit more research to do at that point... but I'll take this one step at a time. Right now, this is just an experiment with a pedal in the effects loop. Also, I'm not sure if I should really be calling this a low pass filter... high frequency cut-off? High shelf? Something like that...

Thank you very much for any of your help!

- if you have an EQ pedal that works, why do something that requires more work?

- Are you looking for a passive circuit (ie, no batteries) that you just put between effect send and receive so that you hit the effect loop and get an instant low pass?

You can make a simple RC filter for your low pass, so it's like your tone knob. That might be easy and what you're looking for. You can relate the cutoff frequency to the time constant tau which is the product of your resistor and capacitor. But you're going to have other resistors and capacitors inside the amp, so it's not going to be that simple. This is something that could get really tricky with the math, so I'll suggest something easier. Just get a few capacitors (like your ripped out of an old radio) and then put a variable resistor (like an old pot from a guitar) in series with it (and the cap going from the resistor "output" to ground). Try a few caps and turn the potentiometer and see if you can get the sound you want that way (it might not be exactly 10K, but as long as it sounds alright). I suspect that really small caps in the pF's will be too small and anything in the 10's of uF will be to big. After you get the resistance dialed in that you want, measure it and then put a fixed resistor in.

Knowing that the cutoff varies with the product of R and C will give you a clue though. If you find that it cuts off too low, increase your cap (or R). I suspect that for a give RC constant, you might find that because of interaction with the other circuits, that it tends to sound better with a bigger cap, or smaller cap (I don't know without experimenting).

You can get much better selectivity with a fancier filter, in which case, you could keep more of your sound and get a faster roll-off at your frequency of interest. But you might get some new phase distortion or other undesirable things happening.

Lastly, you would normally want to control this with the amp's EQ. If you wanted to, you could nudge the frequency that they use (for treble or presence) by tweaking the caps in the tone controls in the amp. I've done that before - it's not for the squeamish though...
As far as the eq pedal that I have goes, there are 2 problems: 1: It's noisy when the amp is quiet and I play quiet pretty often (not a big problem though because when I'm playing quiet, I don't need the perfect tone.) and 2: 6.8k is to low. It takes care of the fizz, but it gets rid of a lot of other high frequencies as a side effect.

"Are you looking for a passive circuit (ie, no batteries) that you just put between effect send and receive so that you hit the effect loop and get an instant low pass" This is exactly my plan.

So it sounds like you are saying that my best bet is to take a potentiometer and a couple of capacitors out of an old radio or something and experiment? That sounds good to me haha. I'll play around with this for a while and maybe I will find something, maybe not.

And I would like to take care of the fizz with the amps eq but even when I turn the treble and presence nobs down all the way, the fizz is still pretty apparent. They just don't effect the really high frequencies. And I don't want to modify with the tone stack or anything because I really like how it sounds except that it just needs a slight low pass filter in the high treble.