The PA cage area gets quite hot when in use and some forced air
cooling is desirable. The easiest way of accomplishing this is to
use a small 1 1/2" 12 volt DC fan. Mount the fan on the outside of
the PA cage through the perf holes on the back of the PA cage. Power
the fan from a half wave rectifier filtered with about 100 uF derived from the 12 volt filament supply. Route the fan wires through a chassis hole along back top of the chassis. The rectifier can be
connected between the filament fuse and an insulated standoff. This
mod can be done without drilling any holes or destroying the units
originality. Orient the fan to blow in.
These types of fans are CPU coolers and can be bought for less than
$10. They do not move a lot of air and move even less through the
perf holes. What is necessary is not cooling, but circulation, and
this simple expedient helps a lot.
This mod will greatly extend the service life of the 6JB6 tubes at
the expense of faster accumulating radio dust bunnies.