PHOBOS GERMANIUM FUZZ
5
BUILD NOTES
Transistor notes
For this circuit, as with other vintage fuzzes, it’s not so much the part number of the germanium
transistor as it is the properties—specifically, gain (hFE) and leakage.
The Tone Bender Mk III is less finicky about transistors than the Mk II, but still moreso than other
germanium circuits like the Fuzz Face or Rangemaster. You can save a lot of time by just buying a
matched set from Small Bear Electronics
or somewhere else. However, if you don’t have access to pre-
matched transistors or you just want to sort your own, here’s what to look for.
Characteristics
This is just a general guideline. There are some transistors that meet these characteristics that won’t
sound right, and others that are outside this nominal range that will work just fine.
•
Q1:
hFE 50-100, low leakage (50-150μA)
•
Q2:
hFE 50-100, low leakage (50-150μA)
•
Q3:
hFE 90-120, medium leakage (150-300μA)
The characteristics of Q1 and Q2 are less important than Q3 due to their Darlington configuration. In
fact, some people have even experimented with low-gain silicons for Q1/2 with no reported change in
tone since this stage just boosts the signal a bit. This can also significantly reduce the noise since silicon
is much quieter than germanium.
Biasing
The Phobos Fuzz project is set up to allow for easy biasing of the Q1/Q2 and Q3 transistors via trim
pots without having to swap out resistors. As a starting point, set the two bias trimmers to around 90%
rotation (approximately 10k total resistance when combined with R3 and R7). Then, with a multimeter,
touch the black and red leads to the pads marked “Q1/2 TEST”. This is equivalent to the collector of
Q1/2. Rotate the trimmer until the multimeter reads
-3.5V
(either positive or negative depending on
which lead is touching which pad).
Next, touch the multimeter leads to the pads marked “Q3 TEST”. This is equivalent to the collector of
Q3. Rotate the trimmer until you read
-2V
on the Q3 collector (again, either positive or negative).
Then, with the black lead touching ground, touch each leg of each of the transistors and see how they
line up with these target voltages.
•
Q1:
Collector -3.5v, Base -1.5v, Emitter -1.4v
•
Q2:
Collector -3.5v, Base -1.5v, Emitter -1.4v
•
Q3:
Collector -2v, Base -0.05v, Emitter 0v
The -3.5v target voltage of Q1/Q2 is just a baseline. Some have reported great sounds with collector
voltages as high as -6.5v. Use your ears and don’t be afraid to stray from what’s listed.
