This is the amp in question
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Yellow ring stands in for the 4 inch fan removed (position marked with Fs )along with metal sheet that encloses the heatsink. The view of this amp is upside down as the fan is at the base. So drawing air horizonatally theough a "designed" 5/8 inch space under the amp. Air then goes through the1 in of fan vertically and then has to sharply divert horizontally to flow either way through the 3 ducts in each direction. The central duct contains no active ducts and as far as I can see only interferes with air flow as they are right up against the fan housing. Air then vents mainly at either end when the metal closure in place with not enough space to add a fan or 2 at those points and nowhere near the mains transformer and main air outlet vent . A lot of venting must go up and out the mixer pannel slider holes etc as well. The 2 TO3 mounted thermal switches are marked S and restrict the air flow through the uppermost duct in the pic, the leads and spades as much as the switch body
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
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