Probably equivalent to coalfire chimney design.
Recently put in a cab top vent for a valve amp in desperate need of one.
The convection air coming out was 40 degree C higher than ambient so metal
of grill got fairly warm, better out than in. I was thinking next time
instead of 2 x 3 inches would be double that and reduce the grill
temperature. But would it, and would the convective effect be better, ie
the output valves running cooler with a larger vent.

Restricting the aperature in a fireplace increases the draught, to make a
lack lustre fire come to life, me was thinking.
No actual chimney , here, just a vent directly over the output bottles,
introduced milled holes in the pcb around the sockets and existing anular
gap around the bottles, through the steel chassis.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/