[Amps] Doorknob capacitors

[Larry Benko]

Is there any reference someone could point me to concerning general 
ratings for doorknob type capacitors.  I have several that are about 
1.25 inches in diameter and 1 inch long with values of 270pF and 400pF 
and DC ratings of 12KV and 20KV.   I assume at RF the most important 
rating would be the rated RF current which based on the dissapation 
factor of the cap. would be basically how much the capacitor would heat 
up.   What would be typical RF current ratings for capacitors such as 
these be?  I was planning on using a couple as extra fixed capacitance 
in a couple of L network tuners I am building.  One reason to minimize 
much heating is that the capacitors hava a fairly significant negative 
temperature coefficient.

Thanks and 73,
Larry, W0QE