1968 Ampeg B15-NF issue - volume controls crossed over

Discussion in 'Amps and Cabs [BG]' started by zoomtoob, Oct 23, 2019.

  1. zoomtoob


    Mar 3, 2014
    I have a 1968 B15-NF. the channel one and two volume controls are both active when I adjust them. plugging in a P-bass to channel one and channel two volume turns it up, even with the chan 1 volume at 0.

    what would make it do that?
  2. beans-on-toast


    Aug 7, 2008
    Start by seeing if there’s an unintentional short somewhere on the PCB, or someone intentionally wired the amp that way. The only way to know for sure is to examine the PCB on both sides, and trace through the wiring for each channel. All it takes is a single errant strand or a splash of solder. Remove V2 and see if adjusting the channel 2 pot has any affect. This could help with the debugging.

    The channels are mixed together by the two 270K resistors that are connected to the input of the phase inverter (Pin-1 of V3). Look at each channel before the mixing resistors.
  3. zoomtoob


    Mar 3, 2014
    Thanks. I'll look at the pcb.

    Any other possible causes?

    Vol 1 max is not as loud as Vol 2. An indicator of something else?
  4. beans-on-toast


    Aug 7, 2008
    One tube could be weaker. Swap V1 and V2 and see if the weaker channel moves. The issue that is causing the shorting problem could be loading the weaker channel.

    Of course there are other possibilities such as an out of spec or faulty component, even a bad solder joint.
    Last edited: Oct 24, 2019
  5. zoomtoob


    Mar 3, 2014
    Tubes are all brand new RCA but a great suggestion. Thanks. Looking at the PCB veeeeeeery closely today also.
    Thanks folks. Anything else is appreciated.
    Now I just hope I don't get banged by a capacitor...!
  6. beans-on-toast


    Aug 7, 2008
    the electrolytic caps need to be discharged before you poke around. How to do this is described in the TalkBass Portaflex Wiki

    Technical - Amplifier | TalkBass.com
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