[Arny Krueger]

"Ken" wrote in message

On 28 Mar 2010 18:11:03 GMT, Charles Packer
wrote:

you don't say what kind of noise - sometimes the fans
pick up a lot of crud

The hum is louder than it has ever been before. The amp
is quiet when first turned on, but as it warms up, and
depending on the volume level of the program material,
the hum of the fans emerge. I assume this is because the
fans are thermostatically controlled. Oddly enough, the
frequency of the hum is constant as it gets louder. If
the fans' speed increased, I would expect the sound
to increase in frequency as well, but they sound the same
tone, which I'm guessing is 120 Hz. Anyway, on a recent
warm evening after playing a symphony, the fan sound
was irritatingly evident.

Bad capacitors in the power supply?

Bias setting shifting, and current drain growing?

There is a failure mode in fans where the bearing friction increases, and
the current draw goes way up. Waaay up! I've seen a 12v CPU fan motor take
out the 12 volt supply on a desktop PC. I saw this happen last week. Most
of those PC power supplies can put out 12 amps or more at 12 volts.