Adding Artificial Center Tap

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Tony-Cliffton, Dec 7, 2016.

  1. Tony-Cliffton

    Tony-Cliffton Member

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    Why would I want to do this? What would the improvement be? Would it decrease noise? Are there any drawbacks?

    Thanks for your time.
     
  2. bob-i

    bob-i Member

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    We need more info. Artificial CT on what?
     
  3. Tony-Cliffton

    Tony-Cliffton Member

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    Sorry. Power transformer.
     
  4. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Well, we're getting closer... :D

    I'm assuming you are talking about creating an artificial center tap for the power transformer's 6.3V heater winding. Why would you want to do this?
    Either:
    • You had a grounded center tap for the 6.3V winding, but it is now open (not connected anymore), or...
    • You never had a center tap available for the 6.3V winding,
    and you need to establish a DC reference for the heaters to reduce coupled hum.

    Some designs (using either a true center tap or an artificial one) connect the center tap to an elevated DC potential (like the top of a cathode biased power tube's cathode resistor) to further suppress coupled hum.

    In some early Fenders (and other amps) the heaters were wired unbalanced. That is to say, they connected one end of the 6.3V winding and connected it to the chassis, then connected the other end of the 6.3V winding to one end of each of the tubes' heaters. The other ends of those heaters were then connected to the chassis. Cheaper to wire, as you only need to daisy chain one side of the heaters. But it is a bit hummier than if you ran the heater voltage as a balanced signal, with either a real center tap or an artificial one on the heater winding grounded.
     
    Last edited: Dec 8, 2016
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  5. Tony-Cliffton

    Tony-Cliffton Member

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    Yes, its an old silvertone 1482, 1963. No center tap for the 6.3v winding. So you would wire the two 100ohm resistors to the cathode cap on both of the power tubes? And then to ground? Im not sure I follow when you say "the top of the cap".
     
  6. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    If both sides of the heaters are fully wired (balanced) -- that is, if for example you look at the 12AX7 tubes and you see a wire going to pin 9, and a different wire going to pins 4 and 5 -- then you could add an artificial center tap by the following:
    1. Connect one end of one 100 ohm resistor to one side of the heater winding. This can be done at a tube socket, I'll use the example of using one of your 6V6 sockets. In that case the 100 ohm resistor would connect to pin 7 of that tube. The other end of this resistor will need to go either to ground OR to pin 8 of the 6V6 socket.
    2. Connect one end of the other 100 ohm resistor to the other side of the heater winding. In the example above, it would connect to pin 2 of that 6V6 socket. The other end of this resistor will need to go either to ground OR to pin 8 of the 6V6 socket, the same as the first resistor.
    The usual spot for an artificial center tap to be installed is on the power indicator lamp housing, but your amp doesn't have one. Obviously, any work of this nature has to be done competently, without causing any danger of shorting out accidentally against other components, etc.
     
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  7. Tony-Cliffton

    Tony-Cliffton Member

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    Thanks for posting that step by step instructional.:)
     
  8. TimmyP

    TimmyP Member

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    I prefer to use a hum balance pot, just in case a little tweaking is needed. Fender part number 0028421000 (or 028421).
     
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