Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Axtman on September 19, 2025, 11:07:52 am
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I am currently working on a Fender DRRI with a Hoffman AB763 replacement board. I adjusted the bias pot to eliminate crossover distortion. But when I turn up the vibrato, the sine wave goes into severe crossover distortion. What do you think I should do to prevent this distortion?
BTW - I am looking into modifying the amp to use an LDR for vibrato so as not to affect the bias.
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I am currently working on a Fender DRRI with a Hoffman AB763 replacement board. I adjusted the bias pot to eliminate crossover distortion. But when I turn up the vibrato, the sine wave goes into severe crossover distortion. What do you think I should do to prevent this distortion?
"Crossover Distortion"* doesn't matter when you're using the trem. You're literally seeking to cut off the signal around half the time.
* One could question whether the scope display is being interpreted correctly.
"Crossover" makes the signal absent some period of time.
"Tremolo" makes the signal absent some period of time.
Tremolo is effective when signal is small and trem-signal is large, so "Tremolo" looks like "signal is absent around crossover."
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You can definitely get some grainy tones with a bias tremolo. I find the usual tricks of a diode on the Intensity pot and LED biasing are enough to give me sufficient Depth without having to adjust the series 1M resistor that feeds the oscillating signal into the Intensity pot. It also allows for warmer bias which seems to keep the grainy tones at bay.