Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Axtman on September 11, 2025, 11:06:17 am
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I am working on a friend's Fender DRRI with a Hoffman AB763 turret board My friend bought the amp with the board already installed and I am correcting a few wiring mistakes and component values. I was puzzled why the vibrator was on the Normal Channel. I then realized it was wired like a Princeton, and the vibrato varies the bias on the output tubes. My friend only wants vibrato on the Vibrato Channel and not on the Normal Channel.
Does anyone have a wiring diagram for me to remove the vibrator from the bias and add an optoisolator, like a typical Deluxe Reverb? Thanks.
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Use a Deluxe Reverb layout diagram
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Those are two rather different circuits, so it may not be a simple switch of components. Could you post a photo of the circuit board? You may well find it easier to get a new circuit board to get to the classic AB763 Deluxe Reverb circuit rather than re-wiring what's in there now.
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How simple do you want this mod to be? This is the simplest arrangement that springs to mind, based on Hoffmans schematic (https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf).
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Thank you!