Depends. Some run whether needed or not. If not then simply disconnect it. My Dynaco was like that, a simple socket mod let me leave it unplugged most of the time. Usually need to be driving them pretty hard to really need a fan. If it really is needed then remove it. Hopefully find something with a part number on it. Because unless you are lucky enough to find its a simple clean and lube then its gonna be replacement time.
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If the junction temperature exceeds 90 degrees C, your output transistors will have very short lives. You could monitor their case temperature with an inexpensive (read somewhat inaccurate but close enough for this purpose) IR non-contact thermometer. Refit with better heat sinks and use computer grade thermal compound.
You may be able to replace the fan with a fan exactly the same size that is quieter. Digikey and Parts Express have hundreds of them. Measure the voltage on the fan leads it is usually 110 VAC or 25 VDC. If 25 VDC you can wire a pot to it to adjust the speed then as Sleepwalker suggests track the temp on the heat sinks. I have had several amps like the Krell KMA 100 that had fans you couldn't hear.
I’ve got a 5" side discharge on my tube pre-amp and it keeps it very cool. It has three speeds. On low you can hear it when you are next to it, but not across the room.
My SS amp has enormous heat sinks but I might put one of these on top of each one anyway.
The company makes thermostats for their fans as well.
Another idea to consider: take two of the largest diameter DC fans you can find, the larger the better. Wire them in series to a large enough DC wall-wart of the same voltage as just one of the fans and go for 50~100% over the current draw of just one. Pad the bottom (intake) sides of the fans with soft weatherstripping foam to isolate vibration from the fans, and lay them over the top of your amplifier above the heat sinks.
I did the above with an old receiver many years ago, and it kept the amplifier section from overheating, even when driving hard loads at high volume levels.
@mike_in_nc : "The noise reduces the S/N ratio tremendously."
Ah, it had to come out. How can those who obsess about power scrubbers, springs, ceramic feet, power cables and aluminum foil hats not be appalled by whirring electric motors with blades next to, on top of or inside their multi-thousand dollar components that can't keep themselves cool enough to survive in typical climate controlled homes?
I set one of these on top of my AR LS-16 tube pre-amp and it really keeps things cool even on the lowest fan speed:
So, I bought two of them to set on top of my solid state Proceed HPA2 amp. It has two huge heat sink openings that draw from underneath.
With the two fans on low the aluminum heat sink hardly even feels warm.
So the question is this: is there any chance of a SS or tube component running too cool?
I chose that particular model because my components are in an open front console without enough clearance for an upward venting fan.
You can also get a thermostat module very inexpensively if you feel like you need it. Mine do great on the lowest setting so I don't see much of a need.
I probably don't need them at all but my listening room is a converted sun porch and the HVAC is not as effective in here and it runs 4-5 degrees warmer than the rest of the house in summer.