How hot are they ?

I can't think of a tube that runs hotter than a 211/845. A single 300B will not run that much hotter than a pair of el34.

A pair of 300B in a typical class A circuit will draw about 100w of wall power, therefore, they will dissipate a similar amount of power in the form of heat. A push pull quad of el34 in class AB can require up to 100-150w of wall power, therefore, the heat output is not too far off the 300B. I would not be too concerned with a 300B and heat.

Very good Brf , much appreciated .

Everyone please excuse this particular post and respond to my other one with the same title .
I apologize for the duplicity here . It took several days for the Audiogon gods to post the first one which made me think that it was lost .

Thank you to all .