How can I build a low pass filter to go in the effects loop?

[manvspig]

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. 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!

[WRGKMC]

Those amps should have plenty of range for adjustment using the built in EQ. If you arent getting what you want then I'd have to believe you have something else wrong. Not every amp sounds good with every guitar or foot pedal. Making wise choices buying is essential for good tone. If you've narrowed down it being your amp then the first thing I'd do is choose different speakers, not mess with the amp. If the speakers are too bright get warmer speakers that have a lower frequency roll off.

This forum thread pretty much confirms the speaker is the cause of the fizz as I suspected.

http://www.sevenstring.org/forum/gear-equipment/175394-peavey-sheffield-speakers-diffrence.html

I'd get a good celestion or eminance to stick in there and sell the Sheffield on ebay and get some or all of my money back. The head itself is likely voiced properly but unless you are an electronic tech, I wouldnt go messing with its electronics. Swapping a speaker for your tone is not only a better way of changing the amps tone, its the preferd method. Using a cap to kill the highs will also kill the harmonics, and essential part of the musical sound. Choosing a speaketr that rolls off at a lower frequency will attenuate the highs gradually for a natural sound vs rolling it all off and having it sound luke a blanket has been placed over the amp.

Even after the speaker replacement, the amp may just not be the best choice for your guitar, stomp boxes or playing style. The high gain Peaveys are pretty much a one trick pony for getting a heavy metal tone so their gain staging and voicing is designed to cut through a loud mix. Solo they can sound harsh as hell.

If you do want that metal sound, One trick they use with these amps is to use a BBE sonic maximizer in the effects loop. You can pick one up for around $69. (I just bought one on ebay for $39) What a BBE does is act somewhat like a smart EQ. Plus it fixes issues with frequency phasing caused by speakers. Guitarists use them all the time with metal amps to get that bass thump happening .

Not to get too technical,  but frequencies projected from a normal amp and speaker when overdriven suffer from a phase problem. The listener hears/feels the bass before the trebble and in a room. This is because high frequencies are reflective and bass frequencies are direct. (Horns are used to direct high frequencies in a compressed beam so they dont radiate out and get absorbed by the air as much.

 What the BBE does is adds a slight delay to the bass so the trebble and bass reach the ears at the same time. This makes the impact of the sound more solid. The device also has high and low frequency processing knobs so you can adjust the amount of high and low end. The result is both the high and low harmonics are put back in phase and what you hear is more like what you'd hear on a recording. (basically you're using a is a device used in recording to get those good metal sounds in a live rig so you get those same sounds live.

So those would be my two suggestions. Try a different warmer speaker and/ir use a BBE if you want those common metal tones you hear on recordings. Both are relatively inexpensive and non invasive. You can easily undo them whenever you want.

[manvspig]

Thank you for your thought out response. I'm not using Sheffield's though. I have 4 Eminence speakers: 2 Governors and 2 Swamp thangs in an x pattern. The Governors do seem to have a particular fizz. The Swamp Thangs certainly have less fizz than the Governors. So perhaps the Governors are causing the problem to a point... I could switch them out for V30's, but the last time I tried a V30 loaded cab, I didn't like it. No definition. The high frequencies just sounded like fizz, although a different fizz then what I'm dealing with now.

The thing is, I love my tone except for that the very high frequencies are to apparent and fizzy. If I swapped speakers, I'd have changes I wasn't looking for. And I have read many times that the 6505 is very fizzy. So It doesn't appear that the speakers are the problem.

Sometimes I use the rhythm channel for my metal tone. It lacks some intensity in the low end but has no fizz. After I tried an eq pedal in the effects loop, I realized that the fizz on the lead channel is curable.

I don't intend on modifying the amp unless I can make this low pass filter work very well in the effects loop. So for now, this is just an experiment with a pedal in the effects loop.

[Barry Wuthrich]

I'm not an amp tech, but I have done enough reading to probably electrocute myself if I were to try a build.

Here is what I had my tech do for my champ. If your amp isn't cathode biased, it won't work. I wanted to be able to affect the tone without corrupting the guitar signal and losing harmonics. No signal goes through the cathode. So I had the tech add a 3 position toggle wired into the cathode bias on the preamp tube. One position selects the stock resistor to ground bias. Another selects the resistor along with a cap in series for a bright switch. I forget the value, but the switch says 25 on that position. The other cap in series (position) 3 was a .47 for a fat boost. Some might recommend .63. Season to taste. This works well for me. I only have a volume knob on the amp and like the circuit as  simple as possible.

If this isn't feasable for your amp, I'd just agree that speaker selection is huge. My amp through a celestion greenback is way brighter than it is through others. The higher wattage speakers are darker.

The other thing with fizzy highs is this. The bigger tube amps tend to get set with pre-amp distortion instead of output tube distortion and then adjusted with the master volume. Pre-amp tube distortion is just fizzy by comparison, and any distortion in the first pre-amp tube gets amplified in every other stage. I use a analogman KOT4 to compensate for that, but that's a pricey fix. That's why I have mostly 6-15 watt tube amps now. I can drive my output tubes and I just have my tech add a line out in case  I need more power.