No announcement yet.

A/B box

  • Time
  • Show
Clear All
new posts

  • A/B box

    I have an A/B box that's designed to compare/use two amps.
    You plug your guitar cable into the box and then feed two guitar cables from the A/B box to the two amps.

    Can I use the same A/B box to compare/use two speaker cabs?
    That is, take the speaker cable from the amp, plug it into the A/B box and then feed two speaker cables
    from the A/B box to the two speaker cabs?
    If so, could both speaker cabs be listened to at the same time?

  • #2
    I wouldn't advise it.

    First the guitar pickups put out less then 1 volt, and have and ultra low current.
    Speaker outputs are very high voltage and you have high current levels.

    The switches in the box are only designed for those low voltages and currents unless it just happens to be over engineered. Some of the old stomp switches used to be rated for 5 amp 120VAC or 2.5A 220V which could handle the power.

    I don't advise trying it. You're playing with fire or better yet you're applying lightning to the power amp which leads to a fire.

    Speakers consist of magnetic coils. They retain a charge like a cap or battery. If its a Tube amp, you also have the transformer doing that. As you disconnect them these devices the fields collapse and force an electrical charge back into the circuit. Switches also arc causing huge spikes. They also have a momentary point where neither speaker is connected. This surge between switching from one cab to the other is like zapping the head with a lightening bolts. As you disconnect them these fields collapse and generate an electrical charge back into the circuit as strong as what they might be fed.

    Do you trust every component in that head to be able to handle that kind of abuse? As an electronic tech for nearly 50 years and I sure as hell don't. Your head may be able to handle that kind of abuse. Personally I wouldn't take the gamble. I've seen too many people do dumb things like that in my business and advise against it. Tube amps especially should never have the loads removed while running and SS are highly susceptible to any kind of electrical arcs

    You should connect the speaker cable before you power up and you disconnect it after its fully powered down.

    With all of that said, they do make some cab switching devices that are supposed to be safe to use. They likely use some components that maintain the load and do not arc like a mechanical switch would. I haven't studied the schematics to them nor owned one so I cant vouch for they're safety. I do know the best way to avoid power amp problems is not to jack with it. Always use heavy gauge, unshielded wire and high quality connectors.

    Never use guitar cables to connect speakers. They have the same problem being designed to handle low volt and current from a guitar pickup only and you'll literally fry the guitar cable from the inside out.

    Best advice, Don't abuse your amp. Use it exactly as the MANUAL advises. You may get away with doing stupid things but they'll get the best of you eventually. If I had a dollar for every time a customer told me they've abused it doing dumb things and think they should be able to run it that way because it worked in the past and got away with it, I'd be a millionaire and retired.

    Just because the amp was made well and can resist a good deal of abuse doesn't mean stupidity wasn't the cause of the failure. Thats like saying I robbed the store 5 times before and shouldn't be found guilty when caught robbing the 6th or even the 100th time.

    If you want to use a different cab, power it down and connect the other cab, then power it back up again. Always make sure the impedances match the head too.
    Last edited by WRGKMC; 07-26-2017, 02:08 PM.


    • #3
      OK thanks WRG, I am glad I asked before doing something stupid.
      Everything you said makes sense.
      It is a tube amp by the way.


      • WRGKMC
        WRGKMC commented
        Editing a comment
        Definitely. Think of speaker cables a high wattage. High wattage comes from high volts and high amps. You want ratings on cables and switches high enough to handle a wall outlet's power. If it was some low powered junk amp it wouldn't be a huge deal.

        You could check the switch ratings. If they are high amp Carlton switches you could use it to switch cabs as a convenience with the amp powered down or at least on standby but I wouldn't try it powered.

        Plus you're inviting more problems when you place more cables, jacks and switches between the head and cab. You increase fidelity losses in the form of heat associated with longer cables and connections. Keep it simple and less loss and less things which can go wrong.

        The easiest fix is to simply get another head to drive the extra cab. switching inputs is fine.