Ampeg B-15 circuit differences

Discussion in 'Amps and Cabs [BG]' started by yaryaryaryar, Jul 2, 2017.

  1. yaryaryaryar


    Mar 27, 2016
    I'm trying to find info on the differences between the different B-15 circuits. So far I'm having trouble finding an exact answer, but I'm sure this must have been discussed here at some point. If anyone can point me in the right direction, much obliged.

    I've been able to find some info about the difference between B-15NC and B-15NF. But I'm more interested in the difference between 60s and 70s models. I understand the cabs got a little bigger in the 70s, which makes for a bit more room for low notes. But what about the circuit itself? I do know the 70s versions are 30 watts as opposed to the 25 watts for the 60s. Did that alter the sound much? Or is the sound generally the same across decades?
  2. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    In the portalex wiki page (top of the pages under the wiki heading) are sections on the various schematics and amplifier technical information.

    The jump from 25 to 30 watts was with the change from cathode (NC revision) to fixed (NF revision) biasing in 1965. The cabinets changed from double baffle (~1960-66) to single baffle (~1966-69) to a thiele design (~1969-80). The square panel "70s" models change the input layout and added the ultra hi and ultra low switching, but were largely similar to the 60s counterparts.
    beans-on-toast likes this.
  3. beans-on-toast


    Aug 7, 2008
    Follow the links at the top of the page to the TalkBass Portaflex Wiki. The are some cabinet detail and schematics.

    Some people like the early 60's revisions with the double baffle cabinet, others prefer the late 60's/70's revisions with the thiele cabinet. Sonically, the difference is in the mids. The 25w versions were cathode biased, the 30W ones were fixed bias. There is a small difference in how the amps perform. It really comes down to personal taste.

    Also, check out the Ampeg Portaflex Club thread: Official Ampeg Portaflex Club
    coreyfyfe likes this.