beginner question re fuzz face mod

[m7b52000]

A common mod appears to be replacing the fuzz pot with a fixed 1k resistor. Does this mean that the cap sitting on lug 2 of the pot is discarded as well?

Thanks

[willienillie]

No, unless you want "fuzz all the way down" permanently.  For "fuzz all the way up" you put the cap in parallel with the 1k fixed.

[m7b52000]

Thanks for the prompt reply. If I wanted to fix my usual position with fuzz slightly off max I would use a fixed resistor of a bit less than 1K?

Thanks again

[nocentelli]

You need to keep 1k resistance directly between emitter and ground as this resistance sets the bias of the transistor. The amount of fuzz is controlled by the resistance in the path from emitter through the capacitor to ground: The standard fuzzface pot allows the "bias resistance" to be fixed at 1k, whilst varying the resistance through the cap to ground from 1k (minimum fuzz) to zero (max fuzz) and everything in between.

Keep the 1k resistor in place and put the capacitor from the emitter to ground in series with a smaller resistor. So if you like your fuzzface at "90% fuzz", the small resistor should be 100 ohms. If you like it around 80%, use a 220ohm etc. This is probably best achieved by trial and error, or you could try to actually measure the resistance between pin 2/3 of the fuzz pot at your favourite setting (which is practically a bit tricky).

[m7b52000]

Ah, I am beginning to see the light.

Thanks

[antonis]

Leo is "almost" right..
(he should be absolutely right if we dealt with higher value resistors..)

100R bypass cap series resistor could be of size comparable to r_e (0.026/I_CQ) value..
e.g. for 400μA say Collector quiescent current, re is about 65Ω..
At medium to low frequencies,  X_C is also added in series with 100R + r_e, resulting into about 200Ω effective bypass resistance..

The above means that actuall gain is half of the expected one..

P.S.
More precisely, individual stage gain fraction denominator should be in the form of {r_e + [1k//(100R+X_C)]} 

[willienillie]

You could replace the pot with a 1k trimmer, which is a pot anyway, but small, board-mounted, set and forget.  Then you could dial in your sweet spot by ear and always be there.  Easier than figuring out which exact values of fixed resistors you would need.

[Electron Tornado]

You might enjoy this video from that guy at JHS pedals:

[m7b52000]

yes, that's how it all started.

[nocentelli]

You might enjoy this video from that guy at JHS pedals:

Is it just me or are there a few weird things at the start of that video when he goes through the schematic? First off, the fuzz pot appears to be drawn wrongly on that whiteboard (pot drawn as a 1k trimmer to ground and a fixed 10ohm resistor in series with the fuzz cap - I'm pretty sure the Dunlop "Hendrix" version still the standard fuzzace fuzzpot arrangement, albeit with the extra 10r limiting resistor), but he also states that replacing the 470k volume pot with a 500KB will give "much more powerful output": Fair enough, he then says "perceived volume", so I assume the original is a 470kA and this is just a difference in the "feel" of the taper and there is very little difference in output.