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B25B ext amp jack

Discussion in 'Amps and Cabs [BG]' started by Persuader, Dec 1, 2002.


  1. Persuader

    Persuader

    Feb 7, 2001
    upstate,ny
    I have a newer b25b and it has an ' ext amp ' jack on the back, what is this for? Another ?,how do you run an 8 ohm load safely on the amp?
     
  2. The external amp jack is a line level send so you can plug that into another amp to use as a slave. You can see in the schematic that it takes the signal that is being fed into the power amp section and allows it to be fed to another amp.

    The B25B has 2 taps on the output transformer - 8 and 16 ohms. In its stock form, there is an attached speaker cord hanging out from the back of the amp, and it is 16 ohms, although I think it has a weird connector on it to go into the 16 ohm B25B cabinet. The "Ext Speaker" jack is a switching jack that automatically connects the combined load to the 8 ohm jack when something is plugged into it.

    So I think a way to trick this whole deal is to plug your 8 ohm cab into the "Ext Speaker" jack, but don't plug the attached cable into anything. That throws the switching jack so the output transformer goes to the 8 ohm tap.

    What I did is permanently remove the attached speaker cable, put in a regular shorting 1/4" jack in the "Ext Speaker" spot wired up so it shorts to ground if nothing is plugged in (safety precaution to prevent running with no load), and use the unused cable hole to put in a Single Pole, Double Throw switch (SPDT) and hardwire the 2 output taps to that switch, so now I plug in a cabinet using a regular 1/4" cord and select either 8 or 16 ohms using the toggle switch. It's more standard that way and easier to configure for other cabs.

    I think you already know you can't run a 4 ohm cab with this amp since it doesn't have a 4 ohm tap.

    http://members.aol.com/portaflex/schems/b-25.gif
    [​IMG]

    Chris