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Paper capacitors were used for coupling and bypass where value, stability, and high frequency response were not so critical. Mica caps were used where initial value and/or long term stability with time and temperature were important (such as in timing circuits). Ceramic disc caps were used mostly where high frequency performance is important, such as IF screen bypass. Of course, there are exceptions to these general guidelines, sometimes for reasons which are not obvious. I suspect the choice of capacitor type was sometimes driven by reasons which had little to do with electrical performance considerations, such as availability, cost, or size.
While one could probably substitute disc or mica caps for paper, I would be hesitant to go the other way. Many versions of disc capacitors have large temperature coefficents, which might make them unsuitable for mica replacements.
Given the performance of modern "mylar" type capacitors, there are probably places where mica caps could be replaced with mylar types. But for most of the less than 1000pf values of micas, mylar typer are not available.
Well, that is my 2 cents worth.
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John Folsom
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