Would you please take a look? (Power transformer related)

Discussion in 'Amp Tech Center' started by The Guy, Sep 17, 2016.

  1. The Guy

    The Guy Tele-Holic

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    Last edited: Sep 17, 2016
  2. jazzguitar

    jazzguitar Tele-Afflicted

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    It's not so clear what exactly your question is.

    Typical power transformers for vintage guitar amps have one 5V filament supply for the rectifier, one 6.3V filament supply for the other tubes, and one high voltage supply.

    With tube rectifiers, the high voltage supply usually is center-tapped and named eg. 660V CT which means that the center tap is grounded and the other two terminals alternatingly deliver the 330V AC to be rectified, thus supplying the actual circuit. Thus you could also name this a 330V-0-330V secondary.

    Likewise, to reduce hum, the 6.3V secondary is center-tapped in the better transformers. Thus here you could also designate it 3.15V-0-3.15V supply (later Fender tried to save money, dropped the center tap and installed two 100 ohm resistors instead. Some Silverface amps have a control pot here labeled hum balance, which is the best version).

    Maybe the Heyboer transformer is available in a center-tapped version too, and they took the labeling for this one without it.

    Does that help?
     
  3. The Guy

    The Guy Tele-Holic

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    It does!! thank you.

    I would like to understand how that center tap actually works though... why does it take away hum? Does it affect tone in some way? But maybe that's too long to explain.

    Thanks again in any case!


    The Guy.
     
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The 6.3v for the heaters is AC or alternating current. If you have balanced voltage, +3.15v to -3.15v AC RMS running through parallel or twisted lines the AC radio magnetic interference (RMI) noise (heard as 60Hz hum) cancels out. A real grounded center tap or an artificial center tap made up of two 100 ohm resistors connecting each wire to ground ensures a balanced voltage referenced to the amp's zero volt reference--the chassis. Without a center tap you can end up with +5v to -1.3v AC RMS with less than perfect hum cancellation. Because of how close the heater filaments are to the cathode it's easy for the RMI transmitted by the filaments to generate a 60Hz current in the cathode.
     
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  5. The Guy

    The Guy Tele-Holic

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    Thank you for the explanation! :)

    Does a real grounded center tap have any advantage over that artificial center tap? Or they end up having exactly the same effect?

    Some of the old tweeds didn't have the center tap in the heater coil, why is that? Cost or lack of knowledge?
     
  6. clintj

    clintj Friend of Leo's

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    Some old tweeds use the chassis as the return path. One 6.3V lead goes to one side of the socket, and the other is grounded to chassis. One pro is less wire used, one con is greater chance of heater hum.

    A true center tap all but guarantees the two halves are equal and opposite. The dual 100R artificial center tap can vary based on resistor values and the physical properties of the transformer, and may not be perfectly balanced. One way around that is use a low value pot as an adjustable center to achieve minimal hum.
     
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  7. The Guy

    The Guy Tele-Holic

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  8. BobbyZ

    BobbyZ Doctor of Teleocity

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    Read somewhere that the two 100 resisters have the advantage of acting like a fuse if a tube shorts to the filiment.
    Never felt the need to lift the center tap and put in the two resisters in amps without them though.
    Probably more of an "on paper" advantage than anything.

    An extreme example of what a center tap does happen to me one -40 night when the ground for the electrical transformer out on the pole snapped.
    Everything in the house went hay wire because the voltage started going up and down. A pretty extreme swing in fact.
    A few things got fried but the furnace kept going till morning thankfully. Just sped up and slowed down all night.
     
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  9. jazzguitar

    jazzguitar Tele-Afflicted

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    Here's an explanation about filament hum and remedy:
    http://www.valvewizard.co.uk/heater.html
    Older amps did not have as much gain so the filament hum was not considered as serious.

    I have no problem with a good center tap (you can cancel out the hum even better with an "hum balance adjust") because if a tube develops a short from heater to filament, it is bad anyway and the filament voltage will melt away the cathode connection. Rarely happens though.
     
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