Amplifier caps, what they do, and where they are...

[Folsom]

The tube is a triode and pentode in one.  The C2 couples the output of the triode plate to the grid of the pentode for output.

I think for this to mean something, he'd need to know the significance of that. (just a guess that it may not be clear)

[S Clark]

I'll do some quick research on triode/pentode.  You're right.  I'm aware of the terms but not really the relevance. 

Ok, single heater,  single cathode,  two sets of (differing) grids, two anodes. 

The tube is a triode and pentode in one.  The C2 couples the output of the triode plate to the grid of the pentode for output.

AHA!!  I can see that!  Thanks, Rich! 

So, is coupling a term for caps that connect portions of the circuit where something is actively altering things... signal coming in, signal being amplified... but not simply filtering DC or frequency? 

[HAL]

The coupling is of the music signal from the preamp triode section to the output stage pentode driver section, but not the DC bias from the input to output.  Since the output is a gain stage, trying to amplify the DC output voltage on the triode would saturate the output tube, so the coupling cap keeps the DC out of the grid of the output pentode.  This lets its cathode bias circuit work correctly and get the music signal into the output stage.