Members bingo boy Posted February 1, 2013 Members Share Posted February 1, 2013 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 Link to comment Share on other sites More sharing options...
Members KuntfacedKunt Posted February 1, 2013 Members Share Posted February 1, 2013 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). Link to comment Share on other sites More sharing options...
Members bingo boy Posted February 1, 2013 Author Members Share Posted February 1, 2013 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. Link to comment Share on other sites More sharing options...
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