Changing 4 Ohms Champ to 8 Ohms

Discussion in 'Shock Brother's DIY Amps' started by robtking, Oct 18, 2020 at 3:55 PM.

  1. robtking

    robtking TDPRI Member

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    Hi All,

    I built a 5F1/2 a few months back and I'm really enjoying it, however, I now want to build a new cab to accommodate a 12" speaker as there's a lot more choice at 8 ohms.

    My output transformer has 4, 8 & 16 ohms secondary taps and my question is - should I install another 1/4 output jack and connect the 8 ohms secondary and jump the common ground tab back to the first 4 ohm jack? Do I need to be aware of anything else?

    Many thanks,
    Robert.
     
  2. glenlivet

    glenlivet Tele-Holic

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    yep, if your OT has an 8 ohm output, that's it. (if you want to keep the 4 ohm output)
     
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  3. robtking

    robtking TDPRI Member

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    Perfect! Thank you very much. I thought I better ask first before making a mistake that could fry something!
     
  4. James Knox

    James Knox Tele-Holic Silver Supporter

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    Just hook up the 8ohm tap where the 4 ohm was and tap off the 4ohm. You’ll probably Never use it again, lol.
     
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  5. sudogeek

    sudogeek Tele-Meister

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    On most Fenders, the primary speaker jack is a shorting jack (like Switchcraft 12A) and the external speaker jack is non-shorting (like Switchcraft 11). Or, you can use your existing jack and wire a SPDT On-On switch to change the impedance depending on what speaker you connect.
     
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  6. awasson

    awasson Poster Extraordinaire Gold Supporter

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    This isn’t a champ but it’s got 4, 8, 16 taps and I put a jack for each one. I’ve used all three since I built it so it’s well worth having the option.

    I like @sudogeek’s suggestion of a switch. I’d put in a 3-way rotary switch for all 3 taps.

    50DB9636-B8D3-4736-8CF0-A0BAA1E1FA1E.jpeg
     
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  7. tubegeek

    tubegeek Friend of Leo's

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    Are you using a negative feedback connection? You'd want to EITHER leave it as-is connected to the 4 ohm tap, OR (if you're getting rid of the 4 ohm option entirely) remember to use the correct resistor value with the new 8 ohm configuration.
     
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  8. Paul G.

    Paul G. Friend of Leo's

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    Not if you want to connect each jack to different taps. If you decide you want to do your original scheme (4 ohm jack, 8 ohm jack), both need to be non shorting type. Just remember to never turn your amp on without a speaker plugged in.
     
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  9. jmp81sc

    jmp81sc Tele-Meister

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    I use a three way rotary switch in my builds, and use 4/8/16 ohm multitap ot, so I can plug in to different cabs easily.
     
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  10. chas.wahl

    chas.wahl Tele-Meister

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    In case you're looking for a reasonably small (25 mm diameter) 3-position rotary switch marked "4 8 16" here's one. Schurter 0033.4032, rated at 10 A 250 VAC:
    https://www.mouser.com/ProductDetail/Schurter/00334032?qs=j1qng3XoT/nmBB89ILqcJQ==
    Not really cheap, though -- over $16 -- but multiple jacks add up too, and require more real estate. It has a "coin or screwdriver" slot rather than a knob. I think that Marshall may have used this, or something very like it.

    EDIT: it appears that AES sells the "Marshall" one ($27), somewhat larger, but with the same electrical rating
    https://www.tubesandmore.com/products/switch-rotary-impedance-selector-replacement-marshall
    and also the Schurter, or a clone of it ($13):
    https://www.tubesandmore.com/products/switch-rotary-impedance-selector-3-position

    AES also has a rotary switch with "pot-like" construction ($11), but with only 6A 125 V rating:
    https://www.tubesandmore.com/products/switch-rotary-1-pole-3-position
     
    Last edited: Oct 19, 2020 at 11:25 AM
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  11. robtking

    robtking TDPRI Member

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    I'm checking for on/on switches now - thanks!
     
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