From what i've read and heard from numerous sources over the past 5 decades, (and some fairly knowledgable EE's, Navy Sonarmen, radarmen and radiomen, I.C. Electricians and Fire Controlmen) ( Note: The radarman rate was disestablished in the early 60's!) tube equipment is designed to run in a hot environment. Having said that the human nervous system is a POOR thermometer. 120*F (which is now a mandated max temp setting for water heaters, and other equipment in the house) is hot enough to scald you, and in some cases 1st or 2nd degree burns. 150* (which dishwashers raise the water temp to on "HOT WASH"), will give you 2nd or 3rd degree burns. children are especially susceptible. Humans usually will pull away from a temp of 120*F after 1-2 seconds as "HOT"! Therapeutic whirlpools are used at 105* to 110*F as hot hydro-therapy. And they are plenty hot.
Transformers are designed for a SOA usually between 150*F and 190*F. Tube envelopes for the most part, will usually run between 200*F and 300*F depending on the tube. These are Normal operating temps. Fans and other means of providing cooling air is for the lower temp rated caps, resistors, and diodes inside the chassis. Not the tubes or transformers. Transformer temps are fairly stable due to their mass. As long as the temps of the transformers and tubes are stable, any cooling of the unit should be concentrated on minimizing the radiant heat given off by transformers and the tubes at the capacitors, resistors,diodes and other small parts INSIDE the unit. UNEVEN Cooling of tubes has caused fractures of the glass envelope resulting in a total fialure of the tube, and possible upstream and downstream damage of associated parts. Tubes are also designed to operate at a specific temperature range. The holes in the chassis around the tubes aren't for decoration, they are there to vent warmed/hot air up and around the tubes to 1.) vent off excess heat from the resistors, caps, diodes and other parts. 2.) to provide a steady, stable airflow up and around the tubes and transformers to bleed off excess heat from them. Not to cool them so they can be handled.
If you look at old tube based military gear, any cooling fans are situated in/on the case and blow cooling air INSIDE the unit, not on or at the tubes/transformers.
Personally I don''t use any fans on my gear. If I feel that it's running too hot, i'll take it's temp. If the transformer temps are on the highside of the 150-190 range, I can hook up a variac and run it @ 110-117VAC. The other alternative is to introduce a small blower / fan to the underside of the chassis to cool down the insides. But I never blow air on the tubes, or transformers directly. 1.) the transformers won't cool down unless the load is lowered or mains voltge is lowered. 2.) even unplugged a standard mains transformer takes 1-2 hours to cool off to room temp, so it's not going to cool down fast. 3.) I don't want any type of thermal shock to disrupt my tubes. The warmed air from underneath is enough to cool down the tubes somewhat or even enough to keep them between 200-300*F. which is optimal temp for most tubes. But in any event they are too hot to touch while operating and for a time afterward. Again this is NORMAL.
