[Amps] Tube gain vs frequency

[Stirling Schmidt]

Hi all:
    I'll try to keep this short, but have several questions.  First, what is/are the factor(s) that determine a tube's upper frequency response?  I tried thinking (uh-oh!) but only deduced that ceramic tubes, with their much larger plate structure, should only have more stray capacitance to a nearby ground plane than a glass tube, and therefore much less usefullness into the vhf-uhf region - obviously not the case, so what gives?  Second, wouldn't it be beneficial to employ a tube for an HF amp that naturally begins to lose efficiency above HF (seems like the 833 would be a prime candidate)?  Third, if RF flows mainly on the surface of conductors, why don't ceramic tubes have an insulator at the top (it seems as though all the RF flowing around the bottom edge of the plate would concentrate heat exactly where it's least welcome - at the seal - a top insulator would at least divide the current flow in half)?  Bear in mind these are beginners'
 questions - Thanks in advance for your consideration.

73 de kc0nxm