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Ralph Mowery

In article ,
says...

Transmitter tubes do tend to run hot. Similar to larger mercury rectifier tubes. In some cases, those had to be shielded (enclosed) due to UV emissions. Back in the day, I ran a 35mm projection set-up that used carbon-arc lights. They were driven by mercury rectifier tubes about 10" tall and in metal enclosures against the UV.

But standard receiving tubes, not so much. And modern microwave oven magnetrons, not at all.

Peter Wieck
Melrose Park, PA

As most transmitting tubes are only about 75% or less efficent, they
will run hot. A 1000 watt input tube will send out maybe 750 watts best
case, usually less, that is 250 watts or more heating the tube not
counting the filiment power.

If you do not think a receiving tube is hot, grab a power output tube of
an old All American 5 AM receiver, or for that mater, a 6aq5 or similar
in other receivers audio output stages with your fingers.