Biasing a fuzzbox

Biasing is to set the DC potential of some critical points in the circuit so that the AC signal which will be superimposed can be held wholy and without deformations. This issue is evident in the case of HiFi or mixing equipment. It is the same in an Overdrive or a Fuzz : even if their output signal is saturated, it has a specific shape and must not be damaged.

This article should help fuzz users to bias or rebias their equipments more accurately than common use. It is specially devoted to owners of my LikeYourFace and Hi-GeFuzz

What for ?

The bias setting determines the amplitude and the balance of saturation.
Saturation is pleasant to the ear if it is symmetrical or almost. Heavy asymmetries, truncated signals cause an unpleasant perception.

At first, biasing is a purely technical adjustment. 
The aim is to get a complete signal that stays symmetrical when saturation increases.

It is also a sonic adjustment that adapts the fuzzy texture to the guitarist’s playing. 
A guitar signal goes through successive stages : attack, sustain, release.
The aim is to get the most homogeneous sonic texture at the best moment of this sequence, depending of your way of playing : soft, heavy, plain, contrasted…

Biasing a LikeYourFace fuzz : The access hole to the trimpot…

How to ?

The first fuzzboxes had no internal adjustment. It was like a lottery, because the germanium transistors are not constant. Nowadays, manufacturers fit an adjustable resistor in the place of a fixed resistor, and so each fuzzbox can be individualy adjusted.

First : technical adjustment

The Stompbox is on, no input signal (guitar volume to zero).
0. Adjust to get 4.5 to 4.8V between the collector of the second transistor and the + battery.
Biased that way to near the half supply voltage, the collector of the second transistor will provide a balanced signal at full level.
In my LikeYourFace fuzz, a test pad eases the measuring.

Then : sonic adjustment

1. play an open string note, not too loud : just let it go…
Edit the bias for a better sustain, more body, without frayed “bzzz”.

2. Play usual guitar licks, loosed or nags, chords and single notes.
Edit bias to get the best magical moment when the signal is squared.

3. After 5 to 10 minutes, repeat the test, and if needed make final touch-ups.
In principle, you won’t have to edit anymore, unless you change the transistors.

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Example : biasing a LikeYourFace
fuzzbox by Guitar Poppa

Techno - biasing - mode et oscillo

Left :

Biasing a LikeYourFace : transistors, trimpot and test pad…
The trimpot is also accessible from the outside …

Right :

Signals on the collectors of T1 and T2 in the case of a well biased fuzz.
– Above : nipple shaped signal, typical of a good quality T1 transistor…
– Low : balanced output signal on T2…
Waveforms obtained with a 50mV input level, Fuzz potentiometer halfway.

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Guitar Poppa’s tips and tricks

My Fuzz have been individually biased at workshop after 10 minutes switched on.
This setting can be re-adjusted if necessary : extreme temperature, maintenance …
•  An internal BIAS trimpot is placed near the center of the PCB.
•  A hole on top of the box allows adjustment using a thin screwdriver from the outside.
•  Engage the screwdriver, turn it gently until you feel it clicks, and adjust.
•  At one end of track, the sound is clear and disappears; to the other, it becomes chipped.

It is useless to do adjust bias immediately after putting the fuzzbox on.
You must wait 10 minutes as currents and temperature are diffused in germanium crystals.

During technically setting, it is better to adjust a little up : between 4.8V and 5V.
This compensates an asymmetry in the circuit, little known to people who advise 4.5V.
(voltage on T2’s emittor resistor, for those who know…)

During musical setting, the best criterion is to get a homogeneous body and sustain.
The decision should come from the plaisure of playing the fuzz, in all ways and styles.

When a good setting has been found, one should better stick to it.
Often, guitarists have the temptation to touch the bias at the slightest discomfort feeling.
And as they do it hasty manner, nothing works no more properly.

A well designed Fuzz recovers its character each time when reconnected.
The transistors assembling and specific devices help get thermal stability, even with germanium.
Just avoid sudden temperature changes, and let time to get a cruising temperature.

Extreme conditions such as full sun or cold wind may require a temporary rebiasing.
This can be done with a screwdriver as described above.
LikeYourFace BIAS version features a potentiometer specially devoted to biasing.

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