Vox Tone Bender

This vintage Vox Tone Bender had just a real simple problem. The input jack was bent funny and the ring terminal was shorting to the tip instead of the sleeve. The bend wasn't reliably fixable, so I replaced the input. The symptom was that the output of the pedal would oscillate and you could control the oscillation via the pots. So if you have an oscillating Tone Bender, check your input jack.

Any astute Fuzz student will note that this Tone Bender is remarkably similar to the Fuzz Face and the Sola Sound Tone Benders (Mk I.V, and the second and third transistors of the Mk II). The topology is the same. Component values are quite different, however. The relatively small 4.7nF output coupling capacitor gives this variant a distinctive trebly tone. Any small capacitor in series with your signal will have a high pass filter effect. I noted this on my handwritten schematic. The square wave was observed on the input side of the 4.7nF cap, and the differentiated square wave was observed on the output side.

You can tell by the low voltage at Q1's base (-0.145V) that the devices are germanium. A silicon transistor would measure close to -0.6V there, whereas germanium will measure -0.1V to -0.3V.

These are somewhat tricky circuits due to the direct coupled transistors for beginners. However, the simplicity and low part count is quite alluring, and many beginners start their electronics journey here. For instance, if Q2 is removed or broken, Q1 will not (or should not) turn on. That is a typical point of confusion, especially with DIY builds where it is easy to misplace transistor terminals (sticking them in the wrong way around is very common).

The best way to start troubleshooting such a circuit is voltage measurements. Check your power, then get the voltages of the transistor terminals.

The simple way to record these is by keeping the black probe of the meter to ground, and recording the voltages sticking the red probe at various places. That is how the voltages on my schematic were recorded.

The more advanced troubleshooting way to take voltage readings is to put both probes on the transistor. First one to do is the black probe to the emitter, and the red probe to the collector. This is the "collector-emitter voltage" (V_CE) and it tells you a lot. Second one is the black probe to the emitter, and the red probe to the base. This will tell you why your first reading is what it is. This is the "base-emitter voltage" (V_BE).

In the above, working (not broken) circuit, I can tell from Q1's V_CE that Q1 is saturated, or fully on. This is normal for these fuzz circuits. I can verify that it should be in the ON mode by observing the germanium diode drop, forward bias, of V_BE. It's just over a volt, which is correct for germanium.

Q2 has the more meaningful, and very controversial (amongst fuzz enthusiast) V_CE. I actually did not record the real V_C, but we can just use the -6.23V above it, and the V_E of -0.617V to get close enough. It has roughly a 6V drop across the transistor. If the entire supply voltage of 9V was V_CE, then we say the device is "cutoff" or fully OFF. If there was just a small voltage drop, like we see in Q1, then we say the device is "saturated" or fully ON. Anything between that is just "active." So we've got a saturated Q1 and an active Q2, and this is exactly what you should expect to see in this circuit.

I mentioned the controversy of Q2's V_CE. Maybe controversy isn't quite the right word. I think it is widely accepted that this should be tinkered with "to taste." But I have witnessed builders and repairers ask in forums "what is the RIGHT voltage here?" and seen various answers pounced upon by the "fuzz police" who seem to only allow one answer: "ANY" voltage is OK that your ears see fit. For this reason, many new builds of these fuzz circuits will make either or both transistor collector resistors variable.