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