[NJARC] Six volt vs five volt rectifiers

[NICHOLAS SENKER]

I am working on a 1940 Philco radio which uses a six volt rectifier (type 84).  This is neat because it simplifies the circuit and eliminates a winding on the transformer.  My question is why isn't this used more?  It seems that any heavy duty circuit such as a power amp or television uses a 5 volt rectifier, eg 5U4.  Is it because of the possibility of a cathode to heater short?  I imagine this would be disasterous for a six volt rectifier on the rest of the filament string.  Comments?
Nick Senker