But it is not fixed. You can use any type of valves for that purpose. Audio amplifier is constructed with normal circuit.
Power amplifier is made by pentode.
Therefore power output is about 100mw. But it is enough to hear with speaker. The gain of it was 8dB. Triode of 6AN8 is middle amplitude type. Therefore this amplifier has only 20dB gain. In normal super heterodyne, pentode is used for IF amplifier.