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Extension jack/impedance question

Discussion in 'Shock Brother's DIY Amps' started by TelZilla, May 21, 2019.

  1. TelZilla

    TelZilla Friend of Leo's

    Jan 21, 2007
    Cleveburg, USA
    I'm a little confused about how impedance from the output transformer relates to the use of an extension speaker.

    Take the following situation:
    • Combo amp
    • OT with 2,4,and 8 ohm taps
    • Combo cabinet with 2 8 ohm speakers in parallel (4 ohm load)
    • Main speaker jack wired to the 4 ohm tap
    If you wanted to use an extension cab with two additional speakers wired in parallel, What ohm rating should the speakers be? Would you want to use one of the other taps and a switch?

    The fender schematics for, say, an AB763, just show an additional jack wired to the main one. I'm assuming you just plug an extra cab into the jack, but that seems like it would eventually cause problems for the OT. The OT wants to see some particular impedance (in this example, 4 ohms). Doesn't plugging in another speaker (or set of speakers) cause an impedance mismatch? Is the extension jack just intended to be used when the main jack is unplugged?

    I'm kind of thinking the two separate pairs of speakers wired in parallel would function as two impedance loads in series. So if you have two pairs of 8 ohm speakers each wired in series, it would be equivalent to two four ohm loads in series, for a total impedance of 8 ohms, so when you're using the extension jack you should use the 8 ohm tap.

  2. Wyatt

    Wyatt Tele-Afflicted

    Nov 3, 2004

    Leo didn't seem to care. He wired it in parallel to make sure the total load stayed somewhere between optimum (4 ohms in this case) and half (2 ohms) depending on which extension cab you bought (he only sold 4-ohm or 8-ohm cabs)

    In old Fenders, you HAVE to use the main jack first, it's designed to short to Ground when nothing is plugged in.
    Last edited: May 22, 2019
    robrob likes this.
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