Output volume control pot value on fuzz face

[joelap]

Hey all, I just finished up taking a dunlop fuzz face reissue and gutting it out, build a vintage correct circuit on perf and am using the dunlop fuzzface case for the housing.  I followed the circuit on fuzz central (fuzzcentral.ssguitar.com).  Anyways, the only difference between mine is that I dont have a 500kLog pot that fits the fuzz face enclosure, so I am using the stock 100k one that comes with the reissue.  The effect sounds muffled, and not as fuzz as it sounded when I used the same parts on my breadboard.  I checked for continuity, everything is fine.  Used the same transistors too.  I know this circuit worked, and so far my only guess would be the 100k vol pot as opposed to a 500k.

Could the problem be in the 100k pot?  Does it need a 500k?  If so, for future reference, how does one know which value pot is needed for a volume or gain control?

[John Lyons]

Joe
The Fuzz face is sensitive to this volume pot value. I think there is something about it on fuzz central...Maybe I saw it posted here.
Search for it and you should get an answer.
I'm not 100% on the "why" of it but 500K = better high end response.

John

[joelap]

Thanks John, I figured that'd be it.  I'll have to just get a 500k pot that fits the fuzz face enclosure.

[John Lyons]

If you have a 1M pot that fits you can put a 1M resistor across the outer lugs and that will make your pot 500K.
2 resistances in parallel divide by 2.

John

[joelap]

Nope, all I've got is Alpha 16mm's here.  Thats ok though, I dont mind waiting for one to come in 

[Barcode80]

why wouldn't a 16mm fit? the shaft size is the same, is it not?

[petemoore]

1 Pot is a resistor to ground, 100k shunts more to ground, 500k less.
2 when the pot is at less than full on, a series resistance [depending on setting] is introduced to signal path, in series.
 So if you have a 500k turned low, you may notice the series resistance rolls off a bit more highs as it generally attenuates, a 'bright' cap can be put across the signal path lugs of the pot to bypass highs.
 If you have a 100k turned up all the way, and a 500k turned up all the way [which is louder, I never even turn the 100k past 3 oclock] the 500k may sound louder/brighter because less signal is shunted to ground across the potwafer.
 Then there's the 1k, and in the Axis face 1k/.047uf [iirc...is 470 in the original FF, goes between collector resistors], making this 1k from 470 increases volume, hence you'll have your output volume knob set differently for same output when diddling with this resistor's value.
 I go with an Axis Face type setup, 100k, [though 50k'd probably do well], and...not sure but I think I use that little cap, and it has a 1k 'top' resistor.

[joelap]

why wouldn't a 16mm fit? the shaft size is the same, is it not?

Nah, the FF reissue has a wider drilled hole.  Its actually kind of odd.  And I dont believe the shaft is long enough.

[Roobin]

I have one theory since building a FF.

I notice that when the vol pot (100k) is at low volume ie low resistance to ground, it is very trebbly. Conversely, I got a sweet tone from lowering amp volume and turing up the FF volume. But obviously I don't want a really quiet bypassed volume. So I plotted some Excel graphs, and found some interesting things. (I won't clog up the page with a graph, but i'll post it if anyone is interested)

Using a 500k pot, from about 150/200k to 500k, the frequency response is roughly linear. (Note i'm using a log scale for freq). AS Resistance increases, frequency knee of the high pass filter formed decreases. Before that though, there is a steep rise as resistance approaches 0. (Which is what you'd expect, it's like a y = 1/x graph).

To explain the effects then: as volume increase, resistance to ground increases. It's a high pass filterTherefore frequency 'knee' decreases. At lower volume (i.e. lower resistance to ground) the -3dB knee is quite high, nearing 1kHz! So, if you were using a 100k pot, you'd be looking at some major highs, fewer lows. A 500k pot means that for the same volume (same ratios), the frequency response will be lower, since although the ratios are the same (and thus the volumes), the freq, knee is lower because of higher resistance. Hence the excruciating trebly from a 100k, and a nicer tone when volume turned up.

Is this right? I'd hate to give this knowledge out if it were wrong. Conversely, I'd like to congratualte myself if it is right!

[John Lyons]

Roobin

Go ahead and post the graph please. I'm curious to see what this looks like on (virual) paper.

Thanks

John

[joegagan]

FWIW< every single  90s Dunlop FF reissue i have seen ( at least a dozen) , the vol pot pot measures roughly 350K

I know it's sacriledge , but a gainstage after the FF avoids all of those issues, plus gives you some make up gain for a big muff style tone control

[Roobin]

Basic audio - its only a Freq Vs resistance graph! Not sure if it's right, but still. It's in my album - http://aronnelson.com/gallery/album67/image002

[John Lyons]

Hmm.. I was under the impression that a larger value pot had a better high end resopnce and that a 100K was muddier.
This graph is saying the opposite. I must be missing something.

John

[joelap]

From personal experience, the stock FF reissue output pot (100k according to Fuzz Central) was very muffled and indiscernable when used with the vintage fuzz face circuitry.  Not sure why it would have been otherwise, but putting a 500k pot in its place solved the problem.

[joegagan]

joelap, did you see my post?
did you measure the pot that came in your dunlop?

[Roobin]

From GEO Tech of the FF:

"More importantly, it cuts more lows out by it's interaction with the output coupling cap, and is less subject to stray capacitive loading cutting highs than the 500K; a 100K should sound somewhat brighter."