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fender s/f ab668 pro reverb


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  • fender s/f ab668 pro reverb

    HELP!!! I have some limited amplifier experience, mostly building, and with very little problems. I have a Fender s/f ab668 on my bench, and I cannot figure out what the problem is. I have done a complete cab job from the power supply to the bypass caps on the input tubes, and the bias supply filter. I have the circuit for this amp and I don't understand why  there is a duel bias supply. There is the normal bias suppy which is -52 volts, and then there is a 150 ohm resister on each cathode of the o/t connected to ground, with a 25 m/f non polarized cap connected to each of those cathodes. The output is oscillating like crazy. Fender also 2x2000 p/f caps from the grids to ground on the outout tubes, I am thinking to get rid of any parasitics in the output. 

    I am really curious why fender(cbs) would have made all these changes to a perfectly good working circuit.

    I am thinking of blackfacing the amp and seeing how that goes, but I won't learn anything by doing that. I may wind-up doing that anyway, but I would like to get this thing running first. Anyone got some ideas.....PLEASE.   bingo boy

  • #2

    this place is probably the least tech savvy amp forum in existance. that may change now that the girl gang is gone, maybe some tech-builders will return or emerge.

    try Plexi Palace or the Metro Amp forum (if it still exists).


    • bingo boy
      bingo boy commented
      Editing a comment
      OK, thanks for that. I was looking for some help with this issue, but now that you have tuned me in, I am going elsewhere. Thanks for the suggestions.