Hi,
One of my cathode-biased Eiclone monoblocks has taken to going into very low frequency oscillation lately. The oscillation usually starts about a minute after the amp is turned on. I noticed the woofer on that channel moving in and out, probably around 5hz. It isn't a huge cone excursion, but enough to be clearly noticeable - probably a few volts P-P. The amp still sounds normal, but clearly something is not right.
I built these amps point-to-point, not PCB. I need to crack them open and compare wiring layouts for obvious descrepancies. Otherwise I'm looking for likely causes of LF oscillation in an amp. Anyone?
The amps both worked normally for a few years. This is a recent development. I recently replaced the output tubes, but the oscillation occurs with both old and new tubes.
Cheers,
Greg