5F2a questions - impedance selector

Discussion in 'Shock Brother's DIY Amps' started by JSS14, Apr 27, 2010.

  1. JSS14

    JSS14 NEW MEMBER!

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    Hey everyone,

    I'm a newcomer here but I've gone through many of the forums and I can't find an answer to some questions I have. I just built a 5F2a clone that follows the schematic pretty much exactly except for the addition of an impedance selector (4, 8, and 16 ohms). I'm using a hammond 1760C output transformer and a hoffman impedance switch. I'm wondering how to connect the transformer to the switch and then to the output jack. More specifically, where does the negative feedback loop tie into the switch at?

    My second pertinent question is can I increase the fuse rating to 1A? I'm using a 5F2 Pts power transformer if that helps. Any information would be much appreciated. Thanks!!!
     
  2. celeste

    celeste Friend of Leo's

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    If you are using the stock 22K feedback resistor, take the feedback from the 8 ohm tap. It is conventional to take the feedback from the highest impedance tap, so if you do that, take feedback from the 16 ohm tap, double the feedback resistor.

    The Hoffman switch should be a single pole, 3 throw switch, so it should have 4 terminals on it. At any given time, only 2 of those terminals are connected together, and one of them is always connected to one of the other three. The terminal that is always connected goes to the jack contact that contacts the plug tip, it is the hot. The 4 8 and 16 ohm wires get connected to the other three switch terminals such that in the fully anticlockwise position the 4 ohm wire is connected to the jack, in the middle position, the 8 ohm is and in the fully clockwise, the 16 is. The OT common/0 ohm wire goes to the jack sleave.
     
  3. M Fowler

    M Fowler TDPRI Member

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    Look at the ceriatone schematic it shows how simple the Hoffman switching is.
    Dynamo label on the back of chassis for 4 8 16 ohm indicators.
    Install the switch in the chassis and attach a knob. Turn the knob to find all the setting now attach the OT wires to the corresponding terminals on the switch. The fourth terminal is the one you wire to the jacks.

    Marking the outside of the chassis first makes it easy to know how you wired the switch, just my 2 cents worth.[​IMG]
     
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