Coupling caps pass the AC while blocking DC.
Best example is an audio out tube.
USUALLY the plate of the audio amp or detector tube goes to a cap going
to the 1st grid of the output tube. If it opens little or no sound.
If it shorts the output tube goes into full conduction & will overheat &
in some cases actually melt the tube. Tubes are forgiving so you do
have a little time to trouble shoot.
Decoupling caps are much like the big 'lytic caps. They ground out unwanted
signals to keep them out of the rest of the set & keep the voltage
fairly constant. You often see them on the G-2 or G-3 of a tube.
For a recap I always recommend doing a few then check the set so it is
a good idea to add extensions or a test tube to your arsenal. Nothing
harder than finding a self induced problem.
73 Zeno
LFOD !