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Thread: Modded Fender Deluxe reverb - Bias adj pot wiring design question

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    Modded Fender Deluxe reverb - Bias adj pot wiring design question

    I have a modded 1979 Deluxe reverb and i am looking at the 6V6 output section. All is working well however. Just figuring out exactly what mod is doing.

    In a normal situation, the center tap on the bias adjusting pot (approx -35 v) goes directly to the 220 K resistors that bias the 6V6 control grids.

    I my case the center tap of the bias adjust pot goes to one side of the intensity pot, and then the intensity pot center tap goes to the 220 K resistors that feed to 6V6 grids.

    Any idea what the deign was intended.

    I have not traced the other side of the intensity pot but I am sure it goes into the vibrato circuit somehow.

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    Incidently the mod added a master volume, fx loop, and a 6 position fat switch.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by LoveChet View Post
    the center tap of the bias adjust pot goes to one side of the intensity pot, and then the intensity pot center tap goes to the 220 K resistors that feed to 6V6 grids.

    Any idea what the deign was intended.
    It's possible that the bias pot is wired as a variable resistor and is the last load on the bias supply voltage division and that whoever wired the circuit used the center lug as ground. Is the bias pot center lug going to the grounded lug on the intensity pot? If so then this would be my guess. Otherwise...

    Maybe whoever did the mods also changed the vibrato to a bias wiggle type?

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    Lifetime Member Enzo's Avatar
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    Agree, the trem was changed to the bias wigle type.

    First example I could think of was old Fender Princetons, like AA964 or 6G2.

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    From what you are saying then, I now have a bias wiggle type tremolo.

    The bias pot center tap goes to one side of the intensity pot and nothing else goes there. Then the center tap of the intensity pot goes to the 6V6 bias grids via 220 k res.

    The other side of the intensity pot goes thru a large orange cap and a 4 meg resistor and then to the plate of one of the tremolo tubes.

    Paul Rivera must have done it for a reason, possibly because of the other mods. I am assuming it didn't affect overall tone in a negative way.

    I can see that I need to draw myself a diagram of my entire deluxe as I've since embraced doing all my own repairs. Quite a challenge but its fun. Doing the FX loop diagram was a real hoot.

    Thanks for the info

    Gary - cold in Colrain

    ps: getting ready to swap out 6v6 sockets. Also changing out the two preamp cathode caps on channel 2 since thats a common recommendation. I see that the bias supply circuit already has a Mallory 100 v capacity filter cap so Im leaving that alone as it appears that was probably changed out in the past, since they used a 50uf 50v as stock I thought.
    If it aint broke, dont fix it.

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    Lifetime Member Enzo's Avatar
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    Seriously, look up those old Princeton schematics, that will show you how Fender drew the circuit. Yours will be very similar.

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    I checked out the Princton diagram. Sure enough its intensity pot is tied directly to the 6v6 grid bias resistors and there is no bias adjust pot.

    So with the Princton since there is no bias adj pot you have to change components if you want to adjust the idle current for any set of 6V6 tubes.

    Seems I have the best of both worlds. wiggle tremolo with bias adj pot. According to what Ive heard, the wiggle tremolo is a very nice tremolo. Probably why Rivera did the mod as part of my master volume and FX loop mod.

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