Fender amp polarity help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Benny, Dec 11, 2005.

  1. Benny

    Benny Member

    Mar 10, 2004
    Charlotte, NC
    For a series of shows coming up, I'd like to use my bassman and the reverb channel of my BFSR. My plan is to have both amps on at the same time, no need for switching between amps, nothing exotic going on with effects. Will I need to flip the polarity of one of the amps to have the amps play in phase with each other? I recently had a 3 prong plug installed on the SR, so now both amps have 3 prong plugs, and I think the polarity switches are effectively disabled. Both amps have modern speakers wired such that positive voltage at the speaker plug moves the speaker forward.

    If the reverb channel of the SR is out of phase with the bassman, is it better/safer to switch the phase at the speaker plug of one of the amps, and if so, does it really matter which one? Or, is it wiser to use one of the ABY switches or signal splitters on the market that allow for phase reversal before reaching the input of the amp?

    Thanks for your help.
  2. Shea

    Shea Member

    Sep 7, 2004
    It's good idea to get a good A/B/Y box with isolated transformer outputs anyway, so you won't have ground loop hum. But some of them don't have phase switches on them. If the phasing is that important to you, get one that does. I like the Voodoo Labs amp switcher, but it doesn't have phase reversal switches.

    I'm not sure if you'd be able to hear any phase cancellation between the two amps anyway. I think they would have to be positioned so that the speakers are perfectly lined up in the same plane for that to happen.

    Just as an experiment, set up the two amps and play through them. Then switch the speaker leads on the Deluxe Reverb (assuming they're not soldered on) and see whether you can hear any difference.

  3. donnyjaguar

    donnyjaguar Member

    Nov 4, 2005
    Interesting. I noticed on my Fender the #2 channel inverts the signal. I never gave this much thought until I recently designed a smaller tube amp and I did take care to keep the signal the right-side-up going through.
  4. VintageJon

    VintageJon Member

    Nov 3, 2005
    Austin TX
    The factory "POLARITY" switch refers to the AC Line connection.
    NOTHING to do with signal phasing...

    (Man, it's weird to see the confusion caused by multiple uses of the same term, but understandable...)

  5. bob-i

    bob-i Member

    Oct 16, 2005
    Central NJ
    Way back in the 70's I used to play with a Bassman and a Super Reverb. I'd lay down the bassman's 2x12, put the super on that, and put the bassman head on top. I never had any issues with phase cancelation and it was an amazing huge sound.

    Just plug in and try it, I' bet it'll sound great.
  6. gtrnstuff

    gtrnstuff Member

    Oct 19, 2005
    Middle TN
    You know every time the signal goes through a tube gain stage, the polarity reverses. That's how the Normal and Vibrato channels get "out-of-phase", one more half of a 12AX7 for reverb return. A Bassman head's Bass channel usually has one more gain stage than it's Normal, and probably is in phase with a Vibrato channel in a reverb amp.

    You can use one speaker cable that has the tip and sleeve backwards at one end, just mark the cable so you don't get confused later.

    If you notice ground loop hum when your guitar is plugged into both amps, then you need a buffered isolation transformer to one input, like many newer A/B boxes have. Do Not just use a 3-to2 prong AC adaptor; it kills the ground loop, but risky; could kill you.

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