The Tone Bender circuit

A few weeks ago, I ordered a few parts from Tayda Electronics for an unrelated project and stumbled on their Tone Bender clone PCB. From what I could see, it's most likely a Mark II or a modified Mark III with no tone circuit.

Sadly, that shipment got lost in transit and they had run out of stock of that particular PCB, so I couldn't get a replacement. Their instructions set provides a diagram of the circuit, and it's a relatively simple one. I decided to try my luck, breadboard one and then solder it on a protoboard I have.

First attempt

I pulled out my breadboards, my spare parts, printed the diagram, and started my adventure.

One thing to note on this circuit is that like most early effect circuits, it is built around a positive ground rail. It means you cannot power these pedals with a daisy-chain power supply otherwise you'll introduce noise on your signal chain (and we don't want that). One way to work around that is to build a charge pump to invert and clean up the voltage going to the circuit, but this makes for a bulkier circuit. Today's circuits are usually built with a negative ground rail which makes things a lot easier.

The other way around the "positive ground rail" problem is to make some slight changes to the circuit. This also requires you to change the type of transistors the circuit calls for. The original design of the Tone Bender calls for three germanium PNP transistors like the AC128 or the AC116. To make a negative ground version of the Tone Bender, you need to do 3 things:
  1. Feed +9V where the circuit expects -9V
  2. Use equivalent NPN transistors (like the AC187 for germanium or 2N3904 for silicon)
  3. Flip the electrolytic capacitors polarity around
Here is the circuit diagram I borrowed from Tayda Electronics:

The circuit is relatively simple: two transistors (Q1 and Q2) in a Darlington configuration to amplify the input signal twice, which is then fed into Q3 for extra saturation. Q1 is amplifying with a fixed voltage, while Q2 and Q3 have their own bias trim potentiometers. The output of Q3 is then sent to the output through a volume potentiometer. The fuzz potentiometer acts on the base of Q2 and the emitter of Q3. The 9V rail has a filter cap, and the input rail bleeds and filters some of the signal to ground. All in all, fairly standard.

I managed to breadboard the circuit, and get the dry signal to the amp. Note that at that point, there is no footswitch involved. The fact I have signal going through is great, however: there is no fuzz or saturation going on... I played with the trim pots and made sure the voltages on Q2 and Q3 were correct and that all parts were connected in the right order, but no amount of changes caused saturation to happen. Bummer.

Takes 2, 3, 4, and 5

Not satisfied with this lack of fuzz, I carefully removed all jumper wires and components from the breadboard, made a cup of tea, and tried again. This time, I minimized the amount of jumper wires and made a much more compact version of the circuit. I also pulled out my function generator, which is a much better way to trace a signal instead of plugging in a guitar and strum the strings once in a while.

I start the function generator, and I hear the 300Hz sine wave in my amp! It seems to work! Great! Then I noticed the output cable was not connected to the circuit. That's the moment I discovered my function generator leaks signal like a ham radio antena. Using it is of no use since I can pick the signal with my oscilloscope probe on every single lead I touch. Back to strumming a guitar to test, I guess...

With the "maybe it's working now" excitement gone, a dreadful realization sets in: this time I have absolutely no output. Not even the dry signal. I can trace the signal to the base of Q1, and then nothing. I double-checked the connections and voltages and everything looks fine. Still no output, though.

Tone Bender Mk II-ish, as-the-diagram-is-drawn breadboard built. No, it doesn't work.

I dismantled the whole thing and tried again from scratch 3 more times with zero success. I get slightly farther each time, but there's something that's not quite right and I can't put my finger on it. I ended up testing the breadboard to make sure there are no issues with it and it's fine. I changed the transistors in case one burned out. I flipped the transistors around in case the pinout was incorrect. Removed the fuzz and volume pots. Used a fixed value resistor in place of the trim pots. Still no output. I give up after 2 days and let it sit for a while.

Take 6: The AionFX circuit

While researching some more details about the Tone Bender circuits, I stumbled on a Coda Effects article on an AionFX Tone Bender build (that you can read here). He does a much better job than I do to explain the circuit of the AionFX Phobos fuzz. I borrowed its circuit diagram because it's great:

You can see the overall layout is similar to Tayda's PCB. Beside the component values and a extra tone control circuit, both are closely related. The main difference is how Q1 and Q2 are configured, and where the fuzz potentiometer is connected to. The input signal is also slightly different, as it only bleeds to ground from one point and goes through one non-polarized capacitor instead of being grounded through two resistors and two capacitors. It's not much more complicated than Tayda's circuit, so I decided to take a crack at it and got the breadboard out again.

Tone Bender Mk III on the breadboard after a few tries. This one didn't work either.

I made the circuit, except the tone control bits since I don't particularly need it. I'm still facing the same problem with that circuit: no output at all. This time, I can probe the input signal all the way to the output of Q3, but it's somehow disappearing before it reaches the volume. I'm unsure as to what could cause this since there's only R7 in the way. I'm sure it's something stupid and it's most likely due to the breadboard, but it's just not working out.

Take 7?

I pooled my spare change and browsed eBay for some germanium transistors. I found a set of 5 Telefunken AC116 PNP transistors for about 13$, straight from Germany. With that out of the way, I then proceeded to purchase an AionFX Phobos kit. The kit should have all the parts required (except the transistors, but the ones I found should do just fine). I'm not sure when I'll receive the parts, but take 7 is definitely going to be the working first time. The nice thing is the AionFX PCB has an inverter circuit built-in so you can use standard center-negative power supplies with it. I will definitely make an article on this build once I have all the part (and hopefully a working unit).


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