"Phil Anderson" wrote in message
news:WMgZa.74397$Oz4.17285@rwcrnsc54

I need advice on replacing the cooling fan on my GAS Ampzilla.
The current fan was too loud, so I tried slowing it down with an
inline resistor. I got the voltage so low that the fan will not
even spin unless I reach inside and give the blade a spin. That
is both easy to forget and annoying to do.


Can I replace this with a computer cooling fan?

Yes.

What size do I need?

Start out with a fan the same size as the existing fan.

How about a speed control?

Computer cooling fans typically are rated for use with 12 volt DC power at
something like 0.1 amp.

Therefore you need to arrange for an appropriate voltage source in your
ampzilla. There's plenty of power DC there, but you'll need to drop the
voltage. An ideal way to accomplish this would be to build a variable
voltage DC regulator whose output voltage is controlled by a simple
potentiometer. Then adjust this potentiometer for the desired fan speed.
Most PC-style fans operate pretty stably down to 7 or 8 volts. Much below
that, stalling at power up can be a problem.

My Ampzilla needs very little
cooling, but if the fan is all the way off it will overheat after
a few hours.

If a fan the size of the existing fan is too noisy and pushes too much air,
replace it with a smaller fan mounted in a baffle plate. Your goal is to use
a larger fan spinning at the slowest speed that moves enough air and avoids
stalling at power up.

One way to avoid stalling at start-up is to drive the fan through a resistor
with a fairly large electrolytic cap in parallel. The cap will provide a
kick to get the fan rolling, and the resistor will run it at a slow steady
speed.