Help with FF Gain Pot

[Ben Lyman]

Again with my Ge PNP FF 
In an attempt to make it brighter when turning down the gain here's what I came up with after first trying to do a treble bleed from a guitar schematic. Can anyone tell me what is happening here and/or how I can improve it? It sounds good but I lost some of the lower gain in trade for a brighter tone. Gain pot wide open still sounds unchanged, which is good, lots and lots of fuzz.

[Groovenut]

You might try reducing the cap value from .15uF to .033uF

At .15uF your HPF F3 knee is ~220Hz at min gain, at 50% gain it 425Hz. So you are getting almost full guitar bandwidth even at 50% (the reverse pot should show 50% gain about 15-20% from min rotation).

If you change to .033uF the HPF F3 will be 965Hz at min gain, and ~ 1.9KHz at 50% gain. This should just liven up the high freqs when the harmonics are missing at lower gains.

This may take a bit of fiddling with the cap values to get the balance you seek, but should be a good starting place.

You can also lose the 100k reistor in parallel with the bright cap, unless you like how it's changes the gain pot sweep (it's in parallel with the input of the pot to wiper).

My two cents..

[Ben Lyman]

Thanks Lawrence, it sounds WAY better! I didn't have any .033uF caps so I used a .022uF.
I nixed the 100k and that helped too. I didn't like it without the 1k in series, it got kind of brittle sounding but I reduced it to 470ohms and it smoothed it out just enough.
I don't know what any of this is doing, I really wish I had a better grasp on
the technicalities but all I can do is experiment and listen.
I've always wanted a FF with a treble bleed on the gain,
is the name "Bleeding Face" or "Bloody Face" already taken?
I think this one is almost ready to box up, here's what I changed

[Groovenut]

Ben,
Great to hear you have it sounding where you want it.

You can view the gain pot treble bleed like this,

Since the treble bleed cap is across the input to wiper of the pot, it forms a high pass filter (HPF) just like any RC combo. The biggest difference is that it's variable. So you can find the frequency using F=1/2piRC R in Ohms C in Farads, but you'll want to find the frequency along the sweep. So I usually figure min, 25%, 50%, 75% and max resistance of the pot.

The series resistance to the bleed cap is slowing current of the signal through the cap. This has the affect of making the F3 knee sound more rounded and gradual, plus it tends to extend the knee a bit as well.

Hope this helps!

[Ben Lyman]

Thanks again, that really helps. My math is very rusty, so is F=1/2piRC, something like this:
F=1.57x(RxC) 

I also googled "Bleeding Face" and "Bloody Face" fuzz pedal. It seems those names are both available. I think "Bloody Face" has a nice ring to it but "Bleeding Face" sounds more vintage 1960's British 

[Kipper4]

F=1/2piRC
this confused me at first too seeing as I'm old school and not used to computer speak in terms of maths,
F = 1 divided by 2xpi  x (RxC)
F - 1 divided by 2x 3.142 x (RxC)
ergo
resistor = 10k
cap = 1uf
F =         1                        1
    -----------------   = ---------
    6.284 x 10,000          62480

unless i made a mistake. I'm sure someone will put it right if i have.

[Groovenut]

Thanks again, that really helps. My math is very rusty, so is F=1/2piRC, something like this:
F=1.57x(RxC) 

Where R=5K and C=.022uF, I do 1 / 2(3.14)5000*.000000022, 1 / 690.8=1447.6Hz
Do everything to the right of the division sign first, then divide 1 by that product

Edit: Kipper beat me to it

[Ben Lyman]

Super helpful! thanks, are you guys teachers or something? 
Now I need to better understand what a "Hertz" is and how it relates to musical notes 

[PRR]

> understand what a "Hertz" is

It is a Cycle Per Second.

It was all CPS when I grew up. But there's a long-term plan to honor the people who pioneered the field. Hertz is a guy all musicians should know; his 1880s book is on my shelf and *still* spot-on.

> and how it relates to musical notes

Set your stop-watch for one Second. Count how many vibrations your guitar string makes in one second. Or if you have a A=440 tuning fork, you should count 440 vibrations in a second.

Yeah, that takes very fast eyes.

While strings have a "base pitch" (Hertz), they also spray a lot of overtones.

When working with audio filters (tone controls), you actually want the chart which plots 20Hz to 20,000Hz in terms of "boom", "mud", "body", "screech", etc. The "flavor" of the different frequency zone.

[JustinFun]

At the risk of sounding too simplistic, I find that the best sounding and easiest solution is to hard wire the fuzz pot full on and put a 500k pot in series with the input - goes from all out fuzz to crunch to clean and never gets muddy.

[Ben Lyman]

Paul,
Thanks, I think I understand a little of what you are saying.
Is it possible to set a filter that only cuts some of the low frequencies from the open Low E string up to the 5th fret (A440) but every note above A440 remains unaffected?
And/or: cut some high freq's when playing above the 15th fret on the high e string?
I imagine, even if this is possible, all the notes in between would be affected in some way.

Justin,
Great idea, and thanks to my handy new "Creation Station" it only took about 30 seconds to try this out. Sounds great, like using the guitar volume to control gain.
Is this a common practice and are there any drawbacks that I should be aware of? I noticed a little more noise but I figure that was just the extra wires or something.
Also, why 500k? Will anything else work? How about a 100k reverse audio?

[JustinFun]

Hi Ben,

Any pot will work but to a different extent and with different response. I used to use 100k, but I prefer 500 as it allows you to get that clean treble-boost-y sound you get from turning the guitar down on a stock fuzz face. YMMV, of course. I don't know if it makes it play more badly with other effects in front of it (not that FF is great with that anyway) as I always put it first. Haven't noticed extra noise, but i've not A-B'd it.

I've built a bunch of them for friends, and whilst i used to have the input resistance as a third knob i now just remove the gain control and use input resistance and volume.

[antonis]

Is it possible to set a filter that only cuts some of the low frequencies from the open Low E string up to the 5th fret (A440) but every note above A440 remains unaffected?
And/or: cut some high freq's when playing above the 15th fret on the high e string?
I imagine, even if this is possible, all the notes in between would be affected in some way.

Actually there isn't something like "sharp" cutting or so...

Simple filters (active or passive) use "negative gain" (attenuation) on some specific frequencies..

It should be a square wave but in reality is more something like a curve with a slope that is determined of the filter class (1st, 2nd, 3rd e.t.c.)

Meaning that, more or less, there is a group of neighbor freqs that are also affected (the closer the more..)

[Ben Lyman]

Okay, thanks Antonis and Justinfun. I just had another thought, ive been putting a 1M pull down resistor at the input.
If I put a 500k pot at the input it will be in parallel with the 1M, will it not?
Should I do away with my 1M pull down resistor?
How about using a 1M pot at the input instead?
Wouldn't that give me the 1M pull down plus the gain control?

[Groovenut]

The pot should be in series with the input cap. You would probably still need the input shunt 1M resistor to prevent switch pop.

[Ben Lyman]

Oh okay, thanks again Lawrence! I guess that about wraps it up for now on the saga of my first Ge FF. I'm going to the soldering station next, hopefully I won't have any problems with it!

[antonis]

The pot should be in series with the input cap.

Not necessarilly..

He could use it as a shunt pot (reverse wired) but it should be taken in mind for total input impedance calculation..
(in parallel with whatever follows next...)

[Ben Lyman]

Dang it you guys! Now I'm thoroughly confused again 
Sorry about the iPhone finger paints, but the first way is how I assumed it would go.
The second way is the only other way I can think of, which is it? Or is there yet some other way?

[JustinFun]

Both ways will act as alternative gain controls, but the second one is what i do (the first is used in some fuzz pedals though, such as the Harmonic Percolator).

If you were doing the first one on your breadboard, i understand better why you were getting more noise. Give the second one a try and see how you like it.

[Ben Lyman]

OK, thanks. I've actually already lain it out on my perf and begun soldering but after that is done I can experiment with the pots using alligators.
I thought maybe the constant 500k was going to reduce my guitar signal a bit and take away some of the gain potential... no?