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Thread: Fender AB763 Vibrato Channel Square Wave Frequency Testing

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    Fender AB763 Vibrato Channel Square Wave Frequency Testing

    A friend of mine with a BF Super Reverb was asking me about the Vibrato Channel of his amp because he doesn't like it as much as the Normal Channel. As far as I can tell, there's nothing wrong with it.

    Since then, I've tested a few AB763 Vibrato Channels with a square wave, and though I can tweak the Normal Channel tone controls for relatively flat response, I can't dial in flat response on the Vibrato Channel. There always appears to be some treble boost on the Vibrato Channel, a spike at the front of the square wave. It's not ringing, but it's not flat.

    By reading the circuit, you can see why it's not flat. I was just hoping someone else has seen the same thing to confirm my result -- that the AB763 Vibrato Channel is a lot more "colored" than the Normal Channel.

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    Old Timer Leo_Gnardo's Avatar
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    Yes, the reverb/dry mix circuit where dry signal is extracted from the second preamp stage through 3M3 paralleled with 10 pF, you'd have to expect a little hi frequency peaking there. You could remove the 10 pF cap, or reduce it for a flatter response. Or increase it for more zing if that's your thing.

    Try as you might, there's always going to be a difference because of that mix stage plus the makeup gain stage - one of V4's triodes - that follows it. C'est la vie. Somehow Fender has millions of fans in spite of these quirks.

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    Lifetime Member Enzo's Avatar
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    Yep, nothing wrong with it. At a local taern, there are two nice waitresses. Nothing wrong with either one, yet I prefer one of them over the other. What do do, what to do...?

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    another thing that is peculiar to that channel is the vibrato pot always attached to the third stage,if you disconnect that probably you will get a fuller sound.
    But Fender ab763 are not flat by default,for a reason,and you don't play square waves,or at least you play distortion pedals with big low pass filters and no bright cap engaged.

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    All the above plus don't forget about the huge effect the speaker has on the waveform. I'll bet the output of that doesn't look very square.

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    If that 10pF cap was on a switch and its effect could easily be ABd, I suspect that most would switch it off.

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    I think that 10pF cap is there to compensate for the treble loss somewhere above 6kHz caused by the Miller capacitance of V4b (roughly 100pF). Its value might be a little high, but I like the effect.

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    Last edited by Helmholtz; 03-12-2020 at 05:51 PM.
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    Quote Originally Posted by alexradium View Post
    you don't play square waves
    Quite true, but square waves are a good diagnostic tool for determining whether or not an audio circuit has flat frequency response or not or if it's exhibiting any other problems like ringing, etc... In this case, it's a good way to observe objectively that, yes, one circuit is changing the shape of the input waveform more than the other in a way that's visible on a scope.

    I just want to be able to tell this guy with confidence, "Yes, they all sound like that. You have indeed noticed a difference between the Normal and Vibrato Channels, but there's nothing wrong with your amp."

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