Why does the Tone Bender circuit even work at all? - diyAudio
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Old 7th July 2012, 10:03 PM   #1
miallen is offline miallen  United States
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Default Why does the Tone Bender circuit even work at all?

Regarding the classic "Tone Bender" fuzz circuit:

Fuzz Central -- Colorsound Tonebender MKII

I'm just curious but how can this circuit even work at all if there is no where for the base current to go?

Specifically, consider Q1. Being PNP with positive ground, is there not a current going from the emitter and out the base that needs a path to the negative supply?

Or am I out of my brain on the train?

Mike
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Old 7th July 2012, 10:26 PM   #2
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If you consider Q1's base - emitter junction is acting like a rectifier, letting only half of a sine wave through to its collector. This is capacitively coupled to the next stage, rich in harmonics, for further processing (tuning) to get the rquired sound.
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Old 7th July 2012, 11:41 PM   #3
miallen is offline miallen  United States
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Ah, interesting. So there is actually something behind this "fuzz". And I suppose it's rather important that the transistor be Germanium with it's 0.2 V drop since the range of the guitar signal is probably only ~2 Vpp (and that's when you're really banging on it). And since the input impedance of this circuit is probably low with no Re, I wonder if the coupling cap is charging up at all to bias Q1 as a function of signal power.
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Old 8th July 2012, 12:42 AM   #4
kevinkr is offline kevinkr  United States
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This clearly musical instrument related and in the wrong place. Moving it to Instruments And Amplifiers. Please see headers atop both forums for guidance.

Edit: Moved
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Old 8th July 2012, 02:55 AM   #5
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Many germanium transistors would pass a distorted signal without any bias at all. Even the ones with bias and DC feedback like the Vox Tonebender responded differently to different transistors. I still have one of the two transistor versions that I made in the late 60's. It used transistors borrowed from an old Sony radio. I put sockets in the board and swapped until I got the sound I wanted.

Also remember that most of the germanium transistors of the day had leakage currents around 1 mA, which also varied considerably with temperature. That's why the old fuzz boxes sound different when cold.
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