Plate to plate impedance

[W4RFM]

Another question I "should" know the answer...  Is their a formula for finding the impedance of the final section of a modulator? Do we have to only use the info given in tube spec sheets  (which are only good for the specified criteria - I assume)... Can't the voltage, current, and tube characteristics be formulated into an answer?   Example:  a pair of 811a's in Class B running 1500 volts at 313 mA give a P to P load of 12.4K.  What about 1000 volts at 250 mA?  Or any other combination...

[W7TFO]

Tube handbooks have loadlines to plot answers to your questions.

73DG

[K9MB]

These two articles from Ham Tips are very helpful. All you need are the data sheets for your modulator tubes to get the curves and set up the load lines following the articles steps.

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0801.pdf

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0903.pdf

[W4RFM]

Thank you Dennis and Michael, this is great information. See I knew that I should know this stuff. 
I am going to print off the RCA bulletins and study them.  This breaks down some walls for me.

Again, there is a wealth of knowledge on this forum.