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making sure I understand OT secondary ground

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  • making sure I understand OT secondary ground

    I'm using the transformers from this radiorca.pdf to build an amp similar to a Silvertone 1482 silvertone1482.pdf
    As you can see the output trans secondary leads on the radio are separated from ground. My plan is to attach either one of the OT sec. leads to ground near the preamp and also make sure the speaker jack has a good ground. Is that correct?
    Thanks
    Vote like your future depends on it.

  • #2
    Correct or not is hard to say. In an amp with no feedback from the speaker output, there is no particular reason to ground the speaker leads for the circuit's operation. You can ground one of them, but if you decide to do it, be sure that you do it properly. Speaker jacks in this kind of amp (i.e. no feedback from the speaker secondary side) can be grounded at will, as long as you don't try to use the chassis for one of the return wires back to the OT. Running speaker currents through the chassis is an open invitation to oscillation. The transformer secondary wires should run to the speaker jack, which can have its bushing grounded. This forces the current that went out of the OT to the speaker to come back into the OT on return without passing through the chassis.
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    • #3
      Thanks R.G. I have a ground bus for the preamp I was thinking I would wire the OT and the speaker jack to that. I also think that I might want to add a neg. feedback loop. Not sure yet.
      Vote like your future depends on it.

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      • #4
        An output transformer can have significant capacitance between one end of the primary and the secondary. If you were to play a loud cord, hold the cord touching the strings on the guitar, and touch the shell of an ungrounded speaker cable, it will shock $#&*% out of you. Ground one side of that speaker.
        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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        • #5
          Also OTs, especially vintages ones that are being tortured in a guitar amp, have the potential to short primary to secondary. A solid chassis ground reference for the OT secondary will protect the user from possibly fatal HT voltage should that happen.
          My band:- http://www.youtube.com/user/RedwingBand

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          • #6
            I decided to sticky this thread for it's safety information.
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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            • #7
              Thanks everyone. The radio that these transformers came from had the OT mounted on the speaker with the B+ coming and going on a three prong plug
              Vote like your future depends on it.

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              • #8
                Originally posted by dmartn149 View Post
                Thanks everyone. The radio that these transformers came from had the OT mounted on the speaker with the B+ coming and going on a three prong plug
                Not that surprised.

                IF it had a field coil, +B goes through it.

                And if permanent mgnet, speakers sometimes had an extra winding called "anti hum", in that case a few turns only.
                Juan Manuel Fahey

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