Electrolytic capacitors don't like heat and those above chassis capacitors
are close to the tubes, a good reason for fan cooling.
This is another perennial semi-wives' tale that constantly gets retold about these Fisher tube receivers every decade or so, to justify blowing fans on them.
Yes, the big filters in these do have a temperature rating, just like any other electrolytic design.
But these large can filter electrolytics are among the hardiest I've encountered. Extremely well made General Instrument models in most cases. They are effectively mounted in a way to minimize evaporative electrolytic losses for many, many years, marching into decades of service.
For many years of working on these, they nearly always reform up just fine, unless they have been abused in the past. And they do not need any special cooling provision made for them other than the passive ventilation manufacturer recommendations.
I remained an advocate of leaving the original big cans in up until just a year or so ago, as they are now reaching 60 years of age. Some are now starting to more heavily fade in capacitance. In the 800B mentioned above, we left the originals in until only a few months ago, and we have had and played this unit for many decades, fanless, in an open-back cabinet arrangement. And the 800B has the biggest tube complement of them all.
It is the smaller under chassis electrolytic capacitors that dry out on these, as the seals are less enduring. Those are only good for about 25-30 years maximum. But heat is not what kills them. Seal failure does.
Again,
there is no need to provide active cooling on these tubed receivers, unless you are mounting one vertically or in a wall or other space contrary to the passive clearances. On top of the electrical noise these fans impose, all I've found from the ones running under fans is a lot more dust and crud to clean up (and it even gets into the tuner front end mechanicals and the dial).
These are extremely sturdy tubed instruments as far as consumer grade equipment goes. After performing any necessary internal refreshes, adjustments and recaps, just plug them in and enjoy them as the original designers intended. They don't run all that hot as tube gear goes. They are not transmitters. And even the old-time high-power ham operators that I cut my teeth with never put these kind of fans on their outputs. These home receivers will run for another 50 years just fine without fans.