The Germanium Hybrid Fuzz pedal… technical stuff

The GHF is based on the well published “fuzz face” circuit.  To follow along with the description, download the PDF of the schematic HERE.

Nearly all of the magic in a fuzz face happens in the transistors. In the GHF Q1 is a carefully selected vintage Russian military surplus transistor. These are purchased by the thousand and every one has the gain and leakage measured.  Measuring the characteristics for a Germanium transistor is quite difficult. Modern transistor testers give false readings for two reasons:
1) The leakage in a vintage Germanium transistor is high enough that it skews the reading.
2) The transistor tester does not use a high enough current (Germanium gain, or “Hfe”, varies with collector current).

Because measurement is so difficult we use a purpose built test-box that operates the transistor at close to the actual operating point in the pedal. We take two readings (leakage with no base current, and pass current) then combine the readings in a purpose built spreadsheet. This guarantees that the gain and leakage values are correctly measured, at the actual operating point, for every transistor.

Based on the gain and leakage, over 90% of the transistors are rejected. We use transistors that sit in the generally acknowledged gain sweet-spot of 70-to-85. The transistors that pass are then checked for noise in a second purpose built test rig. The ones that pass this test are used in the GHF pedals.

Q2 is a very special Silicon transistor.  We spent months auditioning hundreds of transistor types before we found “the one” that matched perfectly to the vintage Germanium Q1. Q2 has a gain of around 140 (with some slightly higher) giving a warm, “active” fuzz.  It has high junction capacitance which works well in the hybrid circuit.

Some other points to note in the GHF circuit:
1) An input “trim” control is added primarily to manage the “woofiness” of humbuckers. Turn this all the way CCW and it is removed from the circuit.

2) There is a 100pF high-quality mono-block capacitor from the base of Q1 to ground. This helps control RF interference.

3) The Q2 collector resistors (the 10k Bias, 3k3 resistor, and 1k resistor above Q2 in the schematic) are slightly different to the “standard” resistors. The 3k3 resistor allows broader control by the Bias control. The 1k resistor gives a slightly fatter tone. This allows for possible mods to the pedal… solder a 5k pot across the 1k resistor to give control over the mids, and/or move the 10k Bias control to a real pot that is adjustable from outside the pedal (all mods are done at your own risk though, and they will void the warranty).

4) The “drive” control is a reverse-log pot. This gives much finer control over the amount of fuzz than a linear pot.

5) There is no external power. The pedal only draws a couple of milliamps so a battery lasts hundreds of hours. The last thing you need is a blown pedal because of a bad power-supply hookup (those vintage Q1 transistors are limited supply and every one has a slightly unique personality). There is a “power” switch on the pedal (not shown on the schematic) that allows your to turn off the pedal without unplugging the input lead.  A discussion paper PDF about battery power can be found HERE.

So there you have it. A fuzz face with really nice transistors and some useful upgrades.

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Dingotone field-testing philosophy… DingoTone’s field-test philosophy is to ship between 6 and 10 field-test pedals at the same time to different testers around the world (DingoTone testers live in the USA, Canada, Australia, Ireland, the UK, and the Netherlands). Sending pedals to everyone at once results in almost-real-time interaction between testers. This approach is very expensive (build and ship 6 to 10 pedals for each design iteration, instead of sending a single pedal from one tester to the next), but it results in a higher quality design.