Fender - Vibrolux Reverb Boost (Late 70's)

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fuzzfiend
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Post by fuzzfiend »

In the late 70's Fender added a push/pull boost switch to the volume pot of several of their amps. I was having a hard time finding a schematic that included this circuitry. I'd seen several folks on the interwebs comment about the reverb being employed, but nothing specific. So, I opened up a 1979 Vibrolux Reverb and traced it out. There was also an unlabeled 1/4" jack on the back of the amp that looked like it might have been a mod until I looked inside. It was related to the boost circuitry (see the lower right hand corner), but what could it have been used for?
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79VibroluxReverbVolBoostCkt.JPG
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teemuk
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Post by teemuk »

Oh, the infamous "pull master"...

It's an overdrive feature implemented to some of their amp models (ca. early 1970's), which employed driver stage of the spring reverb as additional signal booster (a traditional Fender preamp didn't have enough gain for overdrive).

Here's a similar "factory" schematic:
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http://www.thevintagesound.com/ffg/sche ... _schem.jpg

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lead2203
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Post by lead2203 »

There are a few different versions of this circuit. Look at the Super Champ.....IMO its the best sounding one.

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Post by enorbet2 »

OP asked what could the JACK be used for. I think he's aware how the overdrive functions. Incidentally 2 12AT7s in parallel are not as much gain as a single 12AX7 so thats not why this circuit was chosen. It was a result of a mod called the Ice Cube mod in which a shorting set of plugs were cast in epoxy in an ice cube tray that connected the reverb input to the output at the phono jacks on the back panel. It simply evolved to provide a dedicated overdrive circuit.

OP - because the output tap is taken off the transformer which makes the circuit like a mini output stage to drive the very low impedance (~2 ohms) spring transducer this jack can be used t drive literally anything from a recording console to an extension amp, whatever. It is just a nicely convenient preamp out jack in this schematic. odd maybe but at least usable.

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