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  • Centre Tapped Full Wave Rectifier

    I have an early solid state stand alone reverb unit by Fender.

    It has a centre tapped full wave rectifier.

    Normally the centre tap is used as the 0V/GND. However, here it uses the centre tap as the 'positive' and the upper and lower part of the secondary winding as the return.

    I'm wondering if there is a reason why? Any reason to change it to the more common/modern standard?

    I'm having trouble finding more info on this uncommon setup.
    Attached Files

  • #2
    If take the circuit separately from grounding its function is absolute identical with the classic one. The difference in circuit should be subtile- probably they don.t want to put directly the transformer to the ground, from noise consideration...I think.
    just my 2cents

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    • #3
      How about mentioning the amp model? With only that schematic snippet to go on, we can only really guess.

      But as for guessing, maybe they use stud diodes with anode to case, so a "backwards" rectifier was the easier for heatsinking to chassis vs having to isolate/float the rectifiers?
      Doesn't matter either way, center tap as ground is just convention, no reason NOT to use it as positive instead.
      Last edited by Greg Robinson; 04-14-2022, 05:31 AM. Reason: Corrected errors - sorry!

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      • #4
        He said Fender Reverb - solid state version. The standalone reverb unit. I don't have it in my files.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Is a Fender FR1000 unit.

          Click image for larger version

Name:	Schematic FR-100.png
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          • #6
            https://music-electronics-forum.com/...-repair/18085-

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            • #7
              https://warosu.org/diy/thread/894949

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              • #8
                Hard to tell, I can't get the schematic to enlarge, but those diodes look backwards, the Cathode (end with the stripe) is on the transformer side of the circuit. The diodes go to ground, so the center tap is the plus output from the transformer/rectifier circuit.
                WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                REMEMBER: Everybody knows that smokin' ain't allowed in school !

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                • #9
                  I'm with Greg - it's a legacy from vintage diode heatsinking, so no issue with a modern power diode like a 1N4002. At least that schematic has the positive rail at the top - the older schematics based on germanium transistors usually had a negative top rail, so some mental gymnastics needed.

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                  • #10
                    Even so I cannot estimate from circuit how much current it draw to justify to use the chassis as diodes heat sink (supposing it conduct from anodes). Maybe someone may do a current estimation, then using a power diode which asked for a good thermal disipation have a sense, and the extra costs of mounting isolators justify the reversed configuration from economy reason. Otherwise nothing wrong with it even don't need it.
                    Last edited by catalin gramada; 04-14-2022, 02:00 PM.

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                    • #11
                      Originally posted by catalin gramada View Post
                      Even so I cannot estimate from circuit how much current it draw to justify to use the chassis as heath sink. Maybe someone may do a current estimation, then using a power diode which asked for a good thermal disipation have a sense, and the extra costs of mounting isolators justify the reversed configuration. Otherwise nothing wrong with it even don't need it.
                      Didn't necessarily need to be this particular design that needed heatsunk diodes, but when designing a series of products for a line-up, it's common to use the same conventions across all related designs. If another, higher powered amp like the SR2100 solid state Twin Reverb needed them (don't know, can't recall if I've ever been inside one of that era Fenders, if so I don't remember specifically), and they chose stud diodes with anode to case, then they likely would've done the same with all other amps in the series. Remember, the only reason we normally see the diodes on the "high" side is convention. There could very well be an alternate universe out there where all full-wave rectifiers have the diodes on the "ground" side.
                      Last edited by Greg Robinson; 04-14-2022, 04:29 PM. Reason: Fixed typo

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                      • #12
                        Yes it is a FR-1000.

                        Originally posted by catalin gramada View Post
                        Is a Fender FR1000 unit.
                        Not sure where this schematic comes from but looks like they have misidentified the 'Death Cap'. Hopefully no one is blindly following it, replacing the cap across the switch and leaving the actual death cap in place..

                        Originally posted by Greg Robinson
                        Doesn't matter either way, center tap as ground is just convention, no reason NOT to use it as positive instead.
                        True, I had not seen this before, so it threw me off. I think it's more intuitive to think about current flow with the standard CT grounded wiring.

                        The diodes are small old PCB mounted diodes, no heat sinking.

                        I guess there isn't a particular reason they wired it this way, also no reason not to.

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                        • #13
                          I don't know where the death come from but those cap is a random snubber cap over the switch present in almost any electric equipment. The potential danger may come if any have any ideea to service the device without to unplug it from the mains but just switched off. The schematic looks allright, seems someone added some remarks on it.

                          Attached Files

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                          • #14
                            Originally posted by catalin gramada View Post
                            I don't know where the death come from but those cap is a random snubber cap over the switch present in almost any electric equipment.
                            Yeah exactly. C25 is between chassis and line. I hope having this schematic floating around doesn't lead to folks removing the snubber cap while leaving C25 in.

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