[Boatanchors] Why Are Tube AVC and DET Diodes Always Bad?

[David Stinson]

Ever notice when you test the tubes from a rig,
the Detector / AVC diodes in any used tube 
are nearly always bad?  Is this because of the relatively
small area of cathode dedicated to them, or is there something
in the design of tube-type detectors/AVC circuits that is
hard on these elements?

Has someone devised simple, non-destructive ways
to bypass bad tube detector diodes and do these functions 
with, say, a silicon signal diode, until one can get a NOS tube?

Please- I'm not at all interested in building a 
"Multi-dimenstional Time-defused Quadralateraly
 Galactic Pi-R-round Cornbread-R-Square 
 Phase-Potentulated Product Mega-Demod"
 to do the job of simple diode detector.
 I just want to detect an AM signal, 
 not create anti-matter   ;-)

Thanks,
D.S.