[Arran]

In my most recent acquisition, Phonola (Electrohome) from 1936, they practiced a bit of tube stuffing, they used a 6F5 for a 1st audio, and used a #76 triode connected as a diode for a detector, whereas they could have used a 6Q7 or a 6R7 for both. It's seems to have been a common practice in the mid 1930s to do this, but usually they used a 6F5 or 6J5, in combination with a 6H6 double diode. Other then wasting more current on extra heaters I don't know what the electrical advantage was of using separate detector, and audio tubes. 
I have heard of some people wiring in gemanium detector diodes in place of the plate detectors many early 1930s radios used, and rewiring the existing tube as a 1st audio stage, but making an adapter might be a better way. 
Regards
Arran